W.3F UNIVERSITY OF CALIFORNIA. Mrs. SAfeAH P. WALS\VORTH. Received October^ 1894. Accessions No. &*7 %. Class No. HIGH-SCHOOL ASTRONOMY IN WHICn THB DESCRIPTIVE, PHYSICAL, AND PRACTICAL ARE COMBINED, WITH BPECIAL REFERENCE. TO THE WANTS OV ACADEMIES AND SEMINARIES OF LEARNING. BY HIRAM MATTISON, A. M., LATE PROFESSOR OF NATURAL PHILOSOPHY AND ASTRONOMY IN THE FAU,ST SEMINARY ; AUTHOR OF THE PRIMALiY ASTRONOMY ; ASTRONOMICAL MAPS; EDITOR OF BURRITT'S GEOGRAPHY OF THE UEAVENS, ETC., ETC. % 506 J3ROA LOSTOX: 154 TEEMONT ST. CHICAGO: CEO. & C. W. SHEEY70GD. Enterod according to Act of Congress, In tho year 1858. BY HIPvAM MATT1SON, In the Clerk's Office of the District Court of the United States for the Southern District of New York. PREFACE. THE design of this work is to furnish a suitable text-book of Astronomy for academies and seminaries of learning. For juvenile learners, the "Primary Astronomy" is all that can be desired ; and for advanced classes, who wish to study the Constellations, in connection with Mythology, the "Geography of the Heavens " should be chosen in preference to all others ; but for all ordinary students, this intermediate work will be found sufficiently elementary on the one hand, and sufficiently extended on the other. The work is now divided into three parts. After an introduc- tion, which consists of Preliminary Observations and Definitions, and occupies twenty pages, Part First is devoted to the Solar System the sun, planets, comets, eclipses, tides, &c. ; Part Second relates to the Sidereal Heavens the fixed stars, con- stellations, clusters, and nebulae ; and Part Third to Practical Astronomy the structure and use of instruments, refraction, parallax, &c. This department, so seldom introduced into text- books for schools, will be found especially interesting and valu- able. Besides embracing all the late discoveries in astronomy, under a strictly philosophical classification, the work is thoroughly illustrated, by the introduction of diagrams into its pages, in con- nection with the text; and the adaptation throughout to the use of the black-board, during recitation, cannot fail to be appreciated by every practical teacher. The variety of type affords an agreeable relief to the eye of the student, and at the same time distinguishes the main text from the less important matter, the more careful study of which may be left for a review. The suggestive topical questions at the bot- tom of the page complete the design. On the whole, the work is believed to be a decided improve- ment upon the works heretofore in use in this department of study ; and as such it is offered to the professional teachers of the country. H. MATTISOIT. New York, August, 1866. ASTRONOMICAL WORKS In the Author' 9 JAbrary, and more or less consulted in the compilation of the following pages : A Cycle of Celestial Objects, for the use of Naval, Military, and Private Astronomer^ &c. By CAPT. WM. HENRY SMYTH, & bridge. 2 vols. 4to. Cambridge (Eng.), 1742. Astronomia Carolina, &c., by THOMAS STREET; and A Series of Observations on tha Planets, chiefly the Moon, &c., by Du. EDMUND HALLEY. 1 vol. 4to. London, 1716. Astronomical Lectures, read in the Public School at Cambridge (Eng). By WILLIAM WHISTON, M. A., Professor of Mathematics, &c. 1 vol. Svo. London, 1728. The Wonders of tJie Heavens, ; a popular view of Astronomy, &c. By DCNGAN BSAD- FORD. 1 vol. royal 4to. New York, 1843. Popular Lectures on Science and Art, &c. By DIONYSIUS LARDNKB, F. E. S., &c^ Ac. 2 vols. Svo. New York, 1846. Outlines of Astronomy. By SIK JOHN F. "W. HEP.SCHEL, Bart, K. H., &c. 1 vol. Svo. Philadelphia, 1849. Pheiwmena and Order of the Solar System, and Views f the Architecture of ffie Heavens. By J. P. NICHOL, F. E. S. E., &c. 2 vols. 12mo. New York, 1842. TJie Practical Astronomer, &c. By THOMAS DICK, LL.D. 1 vol. 12mo. New York, 1846. Also, " Celestial Scenery, 1 ' and " The Sidereal Heavens," by the same author. TJie Planetary and Stellar Worlds. By PKOF. O. M. MITCHEL. 1 vol. 12mo. New York, 1S49. An Elementary Treatise on Astronomy, &c. By WILLIAM A. NOP.TON, A. M. 1 vol. Svo. New York, 1845. An Introduction to Astronomy, &c. By DENISON OLMSTED, A. M. 1 vol. Svo, New York, 1844. Also, Letters on Astronomy, and Life and Writings of JSbenezer Por- ter Ma-son, by the same author. 2 vols. 12mo. 77*5 Solar System; or, the Sun, Moon, and Stars. By J. E. HINP, Director of Mr. Bishop's Observatory, Eegcnt's Park, London. 1 vol. 12mo. London, 1S52. A Pictorial Display of the Astronomical Phenomena of the Universe, &c. By C. F. BLOPNT. 4to. New York, 1844. The Recent Progress of Astronomy, &c. By ELIAS Loosiis, Professor of Mathematics, &c. 1 vol. 12mo. New York, 1850. dnnual of Scientific Discovery, &c. By DAVID A. WELLS, A. M. 1 vol. 12mo. Bos- ton, 1852. T!ie Sidereal Messenger ; a Monthly Journal, devoted to Astronomical Hcienee. By O. M. MITCBEL, A. M. (Now discontinued.) Also, Astronomical Lectures by ARAGO, LARDNEK, MITCHEL, and NIOHOI.; and Ele- mentary Treatises by BURRITT, KENDAL, UAKTLFT, M.!!NTIRK. AKBOTT. OSTRANDF.R, BLAKE, HASLKK, SMITU, CJLAKK, v osK,TvuiK, COU&TOCK, HASKIN^, UYAJS, KEATU, CONTENTS. IITBODUGTIOI^ PRELIMINARY OBSERVATIONS AND DEFINITIONS. CHAP. I. ORIGIN AND HISTORY OF THE SCIENCE. Ptolemaic Theory of the Structure of the Universe ..... 12 The Copernican System 13 II. DEFINITIONS. Solids, Surfaces, Ac 16 Spheres, Hemispheres, and Spheroids 17 Lines and Angles 19 Of Triangles 20 Circles and Ellipses 21 The Terrestrial Sphere 23 The Celestial Sphere ' 25 First Grand Divisions of the Universe. . 28 PART FIRST. THE SOLAR SYSTEM. CHAP. I. THE PRIMARY PLANETS. Classification of the Solar Bodies 29 Names of tho Primary Planets 31 Explanation of Mythological Signs 32 Distances of the Planets 36 Light and Heat of the Planets 38 Magnitude of the Planets. 40 Density - 41 Gravitation 42 Periodic Revolutions of the Planets. ... . . ^ 1 8 CONTENTS. PAG* CHAP. I. Hourly Motion of the Planets in their Orbits 45 Centripetal and Centrifugal Forces 45 Laws of Planetary Motion 46 Aspects of the Planets 48 Sidereal and Synodic Revolution. 49 The Ecliptic, Zodiac, Signs, Ac. . . . : 50 Celestial Latitude and Longitude 53 Mean and True Places of a Planet 54 Direct and Retrograde Motions 55 Morning and Evening Stars 67 Deviation of the Orbits of the Planets from the Ecliptic 58 Philosophy of Transits 60 IL PRIMARY PLANETS CONTINUED. Inclination of the Axis of the Planets, and its Effects. . 65 Rotation of the Planets upon their axes 69 Time 70 Equation of Time 72 Time, as affected by Longitude 76 True Figure of the Planets 77 Precession of the Equinoxes 80 HL TELESCOPIC VIEWS OF THE PLANETS. Mercury Phases, Mountains, o\ver, goodness, ar.tl superintendencv of the SUPREME BEING!" Ferguson. 3. So remarkably does this science exhibit the glory and majesty of God, by its astounding revelations of liis works, that it almost necessarily tends to fill the rniiid with awe and reverence. It was in view of this tendency that the poet Young said, " An undevout astronomer i mad." 4. To the moral influence of the contemplation of the heavers, we have frequent reference in the sacred Scriptures. "The heavens declare the gto.y of God; and the firmament showeth his handy-work." (Psalm xix. 1.) ""When I consider thy heavens, the work of thy fingers; the moon and stars, which tliou hast ordained ; what is man, that thou art mindful of- him? and the son of man, that thou visitest him?" (Psalm viii. 8, 4.) 5. Astronomy is probably the most ancient of all the sciences. Some of the Chaldean observations date as far back as 2,250 years before Christ, or only 98 years after the Flood ! Laplace speaks confidently of Chinese observations 1,100 r>. c. ; and Mr. Bailly, an English astronomer, fixes the time of a conjunction of Mars, Jupiter, Saturn, and Mercury, mentioned in Chinese records, at 2,M9 years before Christ. 1. The ancient Chinese astronomers and mathematicians were held to a fearful re- fponsibility for the correctness of their calculations. In the reign of the Emperor (Jhou- UTig, his two chief astronomers, ITo and //?, were condemned to death for neglecting to> announce the precise time of a solar eclipse, which took place B. C. 2,169. 2. The Holy Scriptures, some parts of which are very ancient, contain several allusions to the science of astronomy. In the first chapter of Genesis we have an account of the crfation of tlie Sun, Moon, and Stars. "And God said, Let there be lights in the firma- rrent of the. heaven, to divide the day from the night, and let them be for signs, and for seasons, and for days and years. And let them be for lights in the firmament of tlie heaven, to give light upon the earth: and it was so. And God made TWO great lights; the greater light to rule the day, and the lesser light to rule the night: he made the et.irs also." Verses 14-16. 3. In the book of Job, written 1,50 r - years before Christ, we read of several constella- tions that bear tlie same names now tn'at they did three thousand years ago. "Which maketh Arcturns. Orion, and Pleiades, and the chambers of the south." (ixT 9.) Again . ' Canst thou bind tlie sweet influences of Pleiades, or loose the bands of Orion ? Canst ti'ou bring forth Mazzaroth in his season? or canst thou guide Areturus with his sons?" xxx viii. 81, 82.) 4. How astronomy regarded ? (Smyth? Ferguson? Young? Scriptures?) o. What of antiquity of astronomy'? Chaldean and Chinese observations i (Responsibility of Chinese astronomers ? Ancient Scriptural allusions $) EARLY ASTRONOMERS. 11 6. The first astronomers were shepherds and herds7/ien^ who were led to this study by observing the movements of the sun, moon, and stars, while watching their flocks from year to year in the open fields. ANCIENT ASTRONOMERS OBSERVING THE IIEAVEM5. 7. TJiales, one of the seven wise men of Greece, was the first regular teacher of Astronomy, B. c. 600. The next was Anaximander, a disciple of Thales, who suc- ceeded him as head of the school at Miletus, B. c. 548. lie asserted the true figure of the earth, and seems to have had some idea of its daily re-volution. Anaximanrler is supposed to have been the first who constructed globes and mape. He taught that the moon shines by reflection, and in several other respects advanci-d beyond the knowledge imparted by his distinguished tutor. 8. Pythagoras, another Greek philosopher, who founded the school of Croton, B. c. 500, greatly enlarged the science. He first gave form to the vague ideas that the sun was in the center of the planetary orbits, that the earth floated unsupported in space, and that the dis- tant stars were worlds, and probably inhabited. . jtcturos of a sagacious mind, not possessed of the evidence requisite to gi it,5 opinions," Pythagoras is said lo nave perished from hunger, in his old " It was Pythagoras, 1 " says Smyth, " who taught, in fact, the system which now im- mortalizes the name of Copernicus." But lie adds that li is teach ings were but "the con- quisite to give stability to age. 6. Who were the first astronomers ? How led to this study ? 7. Who first regular teacher of this science ? How early? Who next ? and when ? What correct notions did lie seem to entertain ? (For what elso didting'ushed ?) 8. "Who next after Anaximandcr ? -What advances did he make in this Study ? (What does Smyth sav of his teachings? What said of his death f 12 AbTRONOMY. 9. Ptolemy, an Egyptian philosopher, taught astronomy in the second century of the Christian era. He adopted the theory that the earth was located in the center of the universe, that it was perfectly at rest, and that the sun, moon, and stars actually revolved around it, from east to west, as they appear to do, every twenty-four hours. This system is called, after its author, the Ptolemaic Theory. PTOLEMAIC THEORY OF THE STRUCTURE OF THE UNIVERSE. 1. Ptolemy supposed the earth to be in the center of a system of crystalline arches, or hollow spheres, arranged one within the other, as represented in the cut. It is thought by some that he understood the spherical figure of the earth, and the cut is constructed upon this supposition. Ptolemy further supposed that the sun, moon, and stars were fixed in these crystalline spheres, at different distances from our globe ; that the Moon was in the first, Mercury in the second, Venus in the third- the Sun in the fourth. Mars. 9. Who was Ptolemy? and when did he flourish? Describe his theory, (ilow lorvite sun, moon, &e. ? What absurdity did it involve, ;w* it respects COPEENICAN SYSTEM. 13 Jupiter, and Saturn in the next three, and the fixed stars in the eighth. The ancicnta had no knowledge of Uranus or Neptune. This ponderous machinery was supposed to revolve from eat-t to west around the earth, carrying with it the sun^ moon, and stars, every twenty-four hours; and the spheres being crystal, the distant stars were visible through them. 2. If the sun was designed to enlighten and warm the different sides of our globe, the Ptolemaic method of effecting this object is most unreasonable. To carry the sun around the earth, to warm and enlighten its different sides, instead of having the earth turn first one side and then the other to the sun, by a revolution on its axis, would be like carrying a fire around a person who was cold, and wished to be warmed, instead of his turning himself to the fire as he pleased. 3. The Ptolemaic theory would require a motion inconceivably rapid in all the heavenly bodies. As the sun is ninety-five millions of miles from the earth, the entire diameter of his sphere would be one hundred and ninety millions of miles, and its cir- cumference about six hundred millions. Divide this distance by twenty -four the num- ber of hours in a day and it gives twenty-five 'million miles an hour, or sixty -nine thousand four hundred and forty-four miles per second, as the velocity of the sun ! This theory would require a still more rapid motion in the fixed stars. It would require the nearest of these to move at the rate of nearly fourteen thousand millions of miles per second, or seventy thousand times as swift as light, in order to accomplish their daily course. But with all these difliculties in its way, the Ptolemaic theory was generally believed till about the middle of the sixteenth century, or three hundred years ago. THE COPERNICAN SYSTEM. 10. About the year 1510, the ancient theory of Pythag- oras was revived and improved by Copernicus, a Prus- sian astronomer, and has since been called, after him, the Copernican System. 1. The investigations of Copernicus were conducted between the years 1507 and 1530. In the latter year he finished his tables of the planets, and his great work, The Revolu- tion of the Celestial Orbs ; but he did not venture to publish his views till thirteen years after, or 1543, when he recer- " :., chaotic, confused, unorganized], and darkness dwelt upon the face of the deep; and the spirit of God moved upon the face of the wnters. * * * And Go* said. Let the waters under the heavens be gathered together unto one placf, and let ....vj dry land appear: and it was so. And God called the dry land earth,, and the gathering together of the waters called he SKIS.*' Genesis i. 2, 9. 10. Up to this time there was no " earth," either as continents or islands, neither were there any "seas," but all the elements were mingled together; and a mass of Uuid thus dropped iiito space, from the hand of the Creator, would be as certain to assume the form of a globe, as the melted lead from the shot-tower, or the water from the passing cloud. 3. The apparent elevation and depression of the North Star, as we approach toward or recede from it, shows that the surface of the earth is convex, or that the earth is a globe. 4. The fact that the tops of mountains are last seen as we recede from, or first as wo approach, the sea-shore, proves that the surface of the water upon which we sail is con- vex ; so when a ship is approaching the shore, the topmasts are always seen first, and the hull or body last And when seamen wish to survey the horizon' at sea to R great distance, in search of whale or other shipping, they " go to the mast-head," as they call it from which point they can often discover objects that are entirely invisible from tho deck, of the ships. 5. If an aqueduct is to be constructed a mile lonsr, so as to be filled with water to tho brim at every point, it must be about eight inches higher in the middle than at the ends, so as to allow the surface of the water to conform to the convex figure of the globe. We eay higher, not that it needs to be higher as determined by a water level, for a water level is convex, but higher as determined by a straight line drawn from one end of tho aqueduct to the other. This definite knowledge of the curvature of water, even for small distances, shows that the earth's surface is convex or, in other words, that the earth ia spherical. (The curvature Irom a tangent line is 8 inches tor one mile, trom the point of contact; 32 inches for two miles; 72 inches for three miles, &c.) 6. When the moon falls into the shadow of the earth and is eclipsed, or, in othei words, the earth gets into her sunlight, and throws its shadow upon her, the shadow is seen to be convex. We must either conclude, therefore, that the earth, which casts tho shadow, is in the form of a dinner-pi ate, and is .always kept sidewise, and the same side toward the sun (which we know is not the case) ; or that it is a globe, and casts a coni- cal shadow, whatever its position. 7. The earth is known to be a globe, from the fact tint, ships are constantly sailing around it. 8. It is not certain whether Ptolemy admitted the earth to be a sphere or not. Some writers maintain that he rejected this doctrine, and others that he admitted it In the " PRIMARY ASTRONOMY," page 8, the author has inserted a cut representing the Ptole- maic theory, with the earthy?a; but in this work (page 12), where the same theory is represented, the earth is shown as a globe. In all other respects, the theory represented is the same in both works; and this is only a minor point in the system. 12. A second leading feature of the Copernican theory is, that the apparent revolution of the sun, moon, and stars westward every day, is caused by the revolution of the earth around its own axis, from west to east, every twenty-four hours. That the heavenly bodies appear to revolve westward, is no proof that they are acta- ttlly in motion. We often transfer our own motion, in imagination, to bodies that are at rest; especially when carried swiftly forward without any apparent cause, as when 0:10 travels in a steamboat or railway car, and when for a time he forgets his own motion. " Copernicus tells us that he was first led to think that the apparent motions of .the heav- enly bodies, in their diurnal revolution, were owing to the real motion of the earth in the opposite direction, from observing instances f the same kind among terrestrial ob- ects ; as when the shore seems to the mariner to recede as he rapidly sails from it, and as trees and other objects seem to glide by us, when, on riding swiftly past them, we lose the consciousness of our own motion." This remark would go to show that tue revolu- tion of the earth on its own axis was an original discovery with Copernicus. 12. State the second leading feature of the Coperuican system. (Do not our own senses furnish proof that the heavenly bodies revolve westward daily ? Why not ? What remark from Copernicus? What does it seeui to uiii'ly 3) COPEENICAN SYSTEM. 15 13. A third feature of the Copernican theory is, that the sun is the grand center around which the earth and all the other planets revoLyA:, THK COPHRNICAN SYSTEM. 1. The above cut is a representation of the Copernicam. Theory of the Solar System. In the center is seen the sun. in a state of rest. Around him, at unequal distances, are the planets and fixed stars the former revolving about him irom -west to east, or from the right over to the left. The white circles represent the orbits, or paths, in which t!io planets move around the sun. On the right is seen a comet plunging down into the sys- tem around the sun, and then departing. This is the Copernican Tiieory of the Solar System. " Ohow unlike the complex works of man, Heaven's easy, artless, uneucumber'd plan !" 2. The truth of the Copernican theory is established by the most conclusive and satis- factory evidence. Eclipses of the sun and moon are calculated upon this theory, and astronomers are able to predict thereby their commencement, duration, Ac., to a minute, even hundreds of years before they occur. "We shall therefore assume the truth of this system without further proof, as we proceed hereafter to the study of the heavenly bodies. 18. State the third prominent feature of the theory of Copernicus. (De- scribe the cut. AY hat additional evidence of the truth of tliia theory, as a whole 16 ASTRONOMY. CHAPTER II. DEFINITIONS.* 14. SOLIDS, SURFACES, &c. A Solid, or Body, is a figure having length, breadth, and thickness. A Surface is the outside or exterior of a body, and has length and breadth only. Surfaces are of three kinds Plane, Concave, and Cw^ vex. A surface may also be rough or smooth, hard or soft ; the above definition having reference only to the general jig ur of bodies. A Plane Surface is one that is perfectly flat or even, 1'ke the floor of a building, or the sides of a room. 1. We may imagine what is called a plane, to extend off beyond the plane surface* as far as we please ; or, in other woi ds, to be indefinitely extended. When a plane or ft line is extended in this way, it is said to be produced. 2. An imaginary plane may exist where there is no body having a plane surface; or between two lines, like the plane of a circle. A sheet of tin, laid across a small wire hoop, would represent the plane of that circle, in whatever position it might be held. whether horizontally, perpendicularly, or otherwise; and the place which the tin woul&/, or body a surface. How many kinds of surfaces! (Any other distinctions?) What is a plane surface? (Maya plane extend beyond the plane surface ? May a plane exist where there is no body ? II- lustrate. What is a plane produced ?) What are parallel planes ? DEFINITIONS. 17 PERPENDICULAR PLANES. Perpendicular Planes are such as stand exactly upright upon each other, or cross each oilier at right angles. In the figure, one plane is placed horizontally, hml the other perpendicular to it They arc therefore perpendicular to each other, however they may stand in relation to the observer. Inclined Planes are such as are inclined toward, and cut each other obliquely. The Angle of Inclination is the angle contained between the two surl'aces of the planes near- est each other. Trie spaces A and B in the adjoining cut repre- sent the Angle of Inclination. The Area of a plane figure is the amount of surface contained therein. CONVEX A1TD CONCAVE SCT-FACEO. A Convex Surface is one that is swollen out like the outside of a bowl. A Concave Surface is one that is hollowed out like the inside of a bowl. 15. SPHERES, HEMISPHERES, and SPHEROIDS. A Sphere is a globe or ~ball, every part of the surface of which is equidis- tant from a point within, called its center. This is the ordinary definition; but in Astronomy, the terra is applied to the apparent concave of tho heavens, as if it were the actual concave surface of a hollow sphere. dicular I Inclined ? What is meant by the angle of inclination ? The ar&i of a plane surface ? Describe a convex surface a concave. 15. Describe a sphere hemisphere spheroH. (Derivation of spheroid?) 18 ASTRONOMY. A HEMISFHEB AN OBLATE SPHEROID. A Hemisp7iere is the Jialf of a sphere or globe, or of the apparent concave of the heav- ens. In Geography we often read of the Eastern and Western, and North- ern and teouthern hemispheres, but in Astronomy the term is only ap- plied to the Northern and Southern portions of the heavens. A Spheroid is a body resembling a sphere, but yet not perfectly round or spherical. The term spheroid is from the Greek sphaira, a sphere, and eidos, form, and signi fies sphere-like. Spheroids are of two kinds Oblate, and Oblong or Prolate. An Oblate Spheroid is a globe slightly flattened, as if pressed on oppo- site sides. This is a difficult figure to represent upon paper. Should the pupil fail to obtain a correct idea, the Teacher will be at no loss for an illustration. A Prolate sphere. This figure, like an Oblate Spheroid, admits of various degrees of departure from the spherical form. It may be much or but slightly elongated, and the ends may be alike or other\vise. A common egg is an Oblong Spheroid. or Oblong Spheroid is an elongated AXIS OF A SPHERE. The Axis of a sphere is the line, real or imaginary, around which it revolves. The Poles of a sphere are the extremities of its axis, or the points where the axis cuts the two op- posite surfaces. The Equator of a sphere is an imaginary circle upon its surface, midway between its poles, the plane of which cuts the axis perpendicularly, and divides the sphere Into two equal parts or hemispheres. Kinds of spheroids? Describe each. What is the axis of a sphere ? What the pofa / The equator t By what other name culled ? What a Less Cir-jle ? licndieiue ? DEFINITIONS. 19 The equator of a sphere is sometimes called a Great Circle, because no larger circle can be drawn upon its surface. A Less Circle is one that divides a sphere into two unequal parts. In the cut, the circles are re ve. The represented in perspecti iddle of the sphere, whe diameter is included; while the Less Circle passes around it between the Equator and the Poles, and ia consequently " less" than the Equator. Meridians of a sphere are lines drawn from pole to pole upon its surface. 16. LINES and ANGLES. A Point is that which has no magnitude or extension, but simply position. " The common notion of a point is derived from the extremity of some slender body, such as the extremity of a common sewing-needle. This being perceptible to the senses, is a physical point, and not a mathematical point; SOT, by the definition, a point has no magnitude." PROFESSOR PERKINS. A Right Line is the shortest distance between two points. A Curved Line is one that departs con- tinually from a direct course. Parallel Lines are such as remain at the same distance from each other through- out their whole extent. Oblique Lines are such as are not paral- lel, but incline toward or approach each other. When two lines intersect or cut each other, the space included between them is called an Angle. A RIGHT LINE. CUEVliD LINE. PARALLEL LINES. OBLIQTTE LINES. 16. What is a point ? (Physical ? Mathematical ?) A right line ? a curved line ? parallel lines ? an angle ? kind of angles f Describe u rigl \ angle an acute an obtuse 20 ASTRONOMY. ACUTE AND OBTUbC A-NULKS. Angles are of three kinds namely, the Right Angle, the Acute Angle, and the Obtuse Angle. Right Angles are formed when one right line intersects another perpendicu- larly, and the angles on each side are equal. An Acute angle is one that is less, and an Obtuse angle one that is greater, than a right angle. 17. OF TRIANGLES. A Triangle is a plane figure, bounded by straight lines, and having only three sides. Triangles are of six kinds viz., Right-angled, Obtuse- angled, Acute-angled, Equilateral, Isosceles, and Scalene. A Right-angled Triangle is one having one right angle. The parts of a Bight-angled Triangle are the Base, the Perpendicular, and the Hypothenuse. BIGHT- ANGLED TRIANGLE. BASE Ilypotlienuse, from a Greek word, which signifies to subtend or stretch a line sub- tended from the base to the perpendicular. OBTUSE- ANGLED TRIANGLE. An Obtuse angled Triangle is one having an obtuse angle. An Acute-angled Triangle is one having three acute angles. An Equilateral Triangle has all three of its sides equal. Equilateral, from the Latin ceqtvus, equal, and lateralis, from latus, side. AN EQUILATERAL TRIANGLE 17. What is a triangle? How many kinds ? Describe (or draw) a right- triangle. Describe its parts. (IIypotlienu.se Au obtuse ? Acute? DEFINITIONS. 21 An Isosceles Triangle has only two of its sides equal. The term Isosceles is from a Greek word, signifying equal legs; hence a triangle with two equal legs is called an isosceles Triangle. A Scalene Triangle is one having no "two sides equal. The term Scalene is from the Greek skalenos, and signifies obliquo, unequal. (See obtuse and acute angled.) A CIKCLK. 18. CIRCLES AND ELLIPSES. A Circle is a plane figure, bounded by a curved line, every part of which is equally distant from a point within called the center. Concentric Circles are such as are drawn around a common center. The Circumference of a circle is the curved line which bounds it. The Diameter of a circle is a right line passing through its center, and ter minating each way in the circumfer- ence. The Radius of a circle is a right line drawn fom its center to any point in the circumference. The plural of radius is radii ; and as radii proceed from a common center, light, which proceeds from a luminous point in all directions, is said to radiate ; and the 'ight thus dispersed is sometimes called radiations or radiance. All circles, whether great or small, are supposed to be divided into 360 equal parts, called degrees; each degree into 60 equal parts, called minutes y and each minute into 60 equal parts, called seconds. They are marked respectively thus : Degrees (), minutes ('), seconds ("). Equilateral? (Derivation?) Isosceles? (Derivation?) Scalene? (Derivjk- tion ?) 18. What is a circle? Concentric circl es ? The Circumference ? Dimeter* Rudris2 (I'lurai, &c.?) How all circles divided ? (What is a j.wtractor* DIAMETER, CIRCUMFKlt- ENCE, ETC. ASTRONOMY. A PROTRACTOR. PARTS OF A CIRCLE. To feave the trouble of dividing a circle into 860, in order to measure the degrees of an angle, we make use of an instrument called a Protractor. It consists of a semi- circle of silver or brass, divided into de- grees, as represented in the inclosed figure. To measure an angle, as A B C, the straight edge of the protractor is placed upon the line B C, so that the center around which it is drawn will be exactly at the intersection of the lines, or point of the angle, as at B ; then the number of de- grees included between the lines on the protractor will represent the quantity or amount of the angle. From this it will be seen that the amount of the angle does not depend upon the length of the lines which form it, nor upon the magnitude of the circle on which the degrees are marked by which it is measured, but simply upon tho width of the opening between the lines, as compared with the whole circumference &round the point B. A circle marked off into degrees, minutes, and seconds, is called a graduated circle. Circles are also divided into Semicircles, Quadrant*, Sextants, Signs, and Arcs. A Semicircle is the half of a cir- cle, or 186. A Quadrant is one quarter of a circle, or 90. The term Quadrant is applied to a nautical instru- ment, of the form of a quarter of a circle, which is much used by navigators in determining the altitude or appa- rent hight of the sun, moon, and stars. A Sextant is the sixth part of a circle, and contains 60. The word Sextant also denotes an instrument similar to a Quadrant, and is used for similar purposes. The main difference is, that one represents 60, and the other 90, of a circle. The Octant, or eighth part of a circle, is also used for similar purposes. A Sign is the twelfth part of a circle, or 30. An Arc is any indefinite portion of a circle. The word Arc is from the Latin a-rcus, a bow, vault, or arch. By associating the woru cr with arch, the student may always remember its meaning. A Chard is a right line, joining the extremities of an arc. The Chord of an Arc is said to be subtended (from #M&, under, and teno, to stretch), because it seems stretched under the arc like the string of a bow. In the cut, there are four arcs, and as many chords. The lower arc is a large one, \\-liile the arc and chord, A C, are quite small. Still each division of the circle, whether great or small, is an arc, and the line joining the extremities of each arc, respectively, iti a clwrd. ABO AND CHORD. Describe. A graduated circle ?) "What larger divisions of a circle? What is a semicircle ? A quadrant? (Note.) A sextant ? (Note.) A sign ? An urc ? fDerivatiou of term ?) Define a chord. (Why said to be subtended \) DEFINITIONS. 23 FOCI OF AN ELLIPSE. ECCENTRICITY OF AN ELLJPSK. An Ellipse is an oblong figure like an oblique view of a circle, having two points called its foci, around which, as centers, the figure is described. Foci is the plural of focus. The longer diameter of an ellipse is called its Major Axis, and the shorter its Minor Axis. Axes is the plural of axis. The longer is some- times called the Transverse, and the shorter the Conjugate, Axis ; but major and minor are more sim- ple and perspicuous, and therefore preferable. The Eccentricity of an ellipse is the distance between its cen- ter and either focus. Eccentric B, from, and centrum, center. Hence a circle that varies in its distance from the center is eccentric. So, also, persons who depart from the usual round of thought and custom are called eccentric persons. 19. THE TERRESTRIAL SPHERE. The Terrestrial Sphere is the earth or globe we in- habit. 1. Though the earth is not, strictly speaking, a sphere, as that figure is denned (14), but rather an oblate spheroid (14), still it is usually called a sphere on account of its near approach to that figure, and as a matter of convenience. 2. Terrestrial, Latin terrestris, from terra, the earth. "There are also celestial bodies, and bodies terrestrial ; but the glory of the celestial is one, and the glory of tho terrestrial is another." 1 Cor. xv. 40. The Axis of the earth is the imaginary line about which it makes its daily revolution. The Poles of the earth are the extremities of her axis where they cut or pass through the earth's surface. The wire upon which an artificial globe turns represents tho earth's axis, and too extremities, the North and South Polos. The Equator of the earth is an imaginary circle drawn around it, from east to west, at an equal distance from the poles, and dividing it into two equal parts, called Hemispheres. See illustration, page 18. An ellipse? Its foci? (Plural and singular?) Major and minor axes? (Singular and plural ?) Eccentricity of un ellipse ? (Derivation ?) 19. The teriestrial sphere? (Is the earth a sphere? Derivation of term terrestrial ?) A xis of the earth ? Polos ? Equator ? Latitude ? Purallcki i ASTRONOMY. Latitude upon the earth is distance either North or South of the Equator, and is reckoned each way toward the Poles in Degrees, Minutes, and Seconds. As the distance from the Equator to the Pole cannot be more than a quarter of a circle, or 90, it is obvious that no place can have more than 90 of latitude ; or, iu other words, all p! ws upon the earth's surlace must be between the Equator and 90 of latitude, either north or south. Parallels of Latitude are circles either North or South of the Equator, and running parallel to it. "We may imagine any conceivable number of parallels between the Equator and the Poles, though upon most maps and globes they are drawn only once for every ten THE TROPICS AND POLAB CIRCLE. The Tropics are two parallels of latitude, each 23 28' from the Equator. The Northern is called the Tropic of Cancer, and the Southern the Tropic of Capricorn. 1. The Tropical Circles are shown at E E in the an- nexed figure. 2. The sun never shines perpendicularly npon any points on the earth further from the Equator than the Tropics. Between these he seems to travel regularly, leaving the Southern Tropic on the 23(1 of December, crossing the Equator northwa'rd on the 20th of March, midiing the Northern Tropic on the 21st of June, crossing the Equator southward on the 23d of September, and reaching the Southern Tropic again on the 23d of December. In this manner he seems to cross and recross the Equator, and vibrate between the Tropics from year to year. The cause of this apparent motion of the sun will be explained hereafter. The Polar Circles are two parallels of latitude, 23 28' from the Poles. (See F F in the last cut.) The Northern is called the Arctic, and the Southern the Antarctic, Circle. The Tropics and Polar Circles divide the globe into five principal parts, called Zones, namely, one Torrid, two Temperate, and two Frigid. A zone properly signifies a girdle ; but the term is here used in an accommodnte.1 sense, as only three of these five divisions at all resemble a girdle. The parts cut oflf by the polar circles are mere convex segments of the earth's surface. The tropics ? Names? Polar circles ? Names? Zones? Names? (Aro fiere in reality any frigid zones?) Situation of the several zones? Men 1- lan? ? Longitude on the earth ? First meridian ? (Kuropean and Ameri ouu charts and globes ?) How longitude reckoned ? Its greatest o,.M.eut * DEFINITIONS. 25 The Torrid Zone is situated between the Tropics ; the Temperate, between the Tropics and the Polar Circles ; and the Frigid, between the Polar Circles and the Poles. Meridians are imaginary lines drawn from pole to pole over the earth's sur- face. Meridians cross the Equator at right angles ; and the plane of any two Meridians directly opposite each other would divide the eartli into Eastern and Western Hemispheres, as the Equator divides it into Northern THS FIVE ZONES. and Southern. We rnay imagine Meridians to pass ivable point upo face. They meet at the Poles, and are furthest apart point upon the earth's sur- through every concei face. They me at the Equator. Longitude upon the earth is dis- tance either East or "West of any given meridian. A degree of longitude at the Equator comprises about 69 miles, but is less and less as the meridians approach the Poles, at which points it is nothing. A degree of latitude is about 69 miles on all parts of the globe. The First Meridian is that from which the reckoning of Longitude is commenced. On European charts and globes, longitude is usually reckoned from the Eoyal Ob- servatory at Greenwich, near London; "but in this country it is often reckoned from the Meridian of Washington. It would be better for science, however, if all nations reckoned longitude from the same Meridian, and all charts and globes were constructed accordingly. As Longitude is reckoned both East and West, the great- est longitude that any place can have is 180. 20. THE CELESTIAL SPHERE. The Celestial Sphere is the appa- rent concave sur- face of the hea- vens, surrounding the earth in all di- rections. The relation of tho Terrestrial to the Oclestia! Sphere may be understood by the *.bove diagram, in "which the stars surround the earth in all directions, as they seem to Ail the whole celestial vault. Q ASTRONOMY. IQTTATOE Or TUB KZATESS, OR IQlTINOCTIAlfe The Axis of the Heavens is the axis of the earth pro duced or extended both ways to the concave surface oi the heavens. The Equator of the heavens, or Equinoc- tial, is the plane of the Earth's equator extended to the starry heavens. Declination is dis- tance either north or south of the Equinoc- tial. Declination is to the heavens precisely what latitude is upon the earth. It is reckoned from the celestial equator, both North and South, to 90, or to the poles of the heavens. Celestial Lati- tude can be explained better hereafter, and so with the terms Moniptio, Zodiac, ective spheres. But while longitude is reckoned both east and west of the first meridian, and can only amount tt> 1800, Right Ascension is reckoned only eastward, and consequently may amount to 860, or the whole circle of the heavens. The principal difference between Eight As- cension and Celestial Longitude is> that the former is reckoned on the Equinoctial, and the latter on the Ecliptic. The /Sensible Horizon is that circle which terminates our view, or where the earth and sky seem to meet. The Rational Horizon is an imaginary plane, below the visible horizon, and parallel to it, which, passing through the earth's cen- ter, divides it into upper and lower hemispheres. 1. These hemispheres are distinguished as upper and lower with reference te the ob aervcr only. SENSIBLE -I- HORIZON 20. Celestial sphere ? (Kelation to terrestrial ?) Axis of the heavens ? Equator of the heavens ? Declination ? (How illustrated by terrestrial latitude ? How reckoned ? Its limits ?; Eight ascension ? (How resemM What difference ?} Sensible horizon ? Rational I Explain bj DEFINITIONS. 2. The sensible horizon is half the diameter of the earth, or about 4,000 miles from the rational ; and yet so distant are the stars, that both these planes seein to cut the celestial arch at the same point; and we see the same hemisphere of stars above th? sensible horizon of any place that we should if the upper half of tLe earth wero re- moved, and we stood on the rational horizon of that place. Tlie Poles of the Horizon are two opposite points- one directly above, and the other directly beneath , us. The first is called the Zenith, and the latter the" Nadir. The points Tip and Down, East and West, are not positive and permanent directions, but merely rela- tive. TTP AND DOWN, AND EAST AND WEST. 1. As the earth is a sphere, inhabited on all sides, the Zenith point is merely opposite its center, and the Nadir toicard its center. So with the directions Up and Down: one is from the center, and the other toward it ; and the same direction which is up to one, is down to another. This fact should not merely be acknowledged, but should be dwelt upon until the mind has become familiarized to the conception of it, and di- vested, as far as possible, of the notion of an absolute up and down in space. We should remember that we are bound to the earth's surface by attraction, as so many needles would be bound to the surface of a spher- ical loadstone. 2. East and West also are not absolute, but merely relative, directions. East is that direction in which the sun appears to rise, and West is the opposite direc- tion ; and yet, so far as absolute direction is concerned, what is East to one, as to the observer at A, is West to B, and so with C and D. And as the earth revolves upon its axis every twenty-four hours, it is obvious that East and West upon its surface must, in that time, change to every point in the whole circle of the heavens. The same is true of the Zenith and Nadir, or of up and down. Space, in Astronomy, is that boundless interval or void in which the earth and the heavenly bodies are situated, and extending infinitely beyond them all, in every direc- tion. Space has no limits or, in other words, is boundless, or infinite. Suppose six persons were to start from as many different points upon the earth's surface as, for instance, one from each polo, and one from each of the positions occupied by observers in the next figure. Let them ascend or diverge from the earth in straight, lines, perpen- dicularly, to its surface, and though they were to proceed onward, separating from each other, with the speed of lightning, for millions of ages, none of these celestial voyagers would find an end to space, or any effectual barrier to hinder their advance- ment. Should they chance to meet another world in the line of their flight, it would soon be passed, like a ship met by a mariner upon the ocean, and beyond it space Mould still invite them onward to explore its immeasurable depths. And thus they might go on forerer, without changing their position in respect to the center or lonn- ddt-iesot immensity : for as eternity has no beginning, middle, or end, so space is with- out center or circumference an ethereal ocean, without bottom or shore. diagram. Poles of the horizon ? Names? Up and down positive or rela- tive points? (Illustrate by diagram; also east and west.) Term space in astronomy? (Has it any limits ? Illustration.) 28 ASTRONOMY. THE 60LAR SYSTEM. SOLAR 8YRTKM ANT) SIDEREAL HEAVENS. 21. FIRST GRAND DIVISIONS OF THE UNIVERSE. The visibls uni- verse may be con- sidered under two grand divisions viz., the SOLAR SYS- TEM and the SIDE- REAL HEAVENS. The Solar System consists of the sun and all the planets and comets that re- volve around him. The Sidereal Hea- vens include all those bodies that lie around and beyond the Solar System, in the region of the Fixed Stars. 1. The word Sidereal is from the Latin sideralis, and signifies pertaining to the shirs. The Sidereal Heavens are, therefore, the heavens of the fixed stars. 2. The relation of the Solar System to the Sidereal Hea- vens is shown in the annexed cut, where the sun appears only as a star, at a distance from all others.and surrounded by his own retinue of worlds. The Solar System is drawn upon a small scale, and the Sidereal Heavens are seen wound and at a distance from it in every direction. In considering the general subject of Astronomy, we shall proceed according to the foregoing classification, treating first of the SOLAR SYSTEM, and, secondly, of the SIDEREAL HEAVENS. 21. How visible universe divided? Define each? (Derivation of term "side-real ? Relation of solar system to the sidereal heavens ? Illustrate by drawing.) Of which division does the author first treat ? PART I. THE SOLAR SYSTEM CHAPTER I. THE PRIMARY PLANETS. .. 22. The Solar System derives its name from the Latin term sol, the sun. It signifies, therefore, the System of the Sun. It includes that great luminary, and all the planets and comets that revolve around him. 23. The Sun is the center of the system, around which all the solar bodies revolve, and from which they receive their light and heat. 24. The Planets are those spherical bodies or worlds that revolve statedly around the sun, and receive their light and heat from him. The term planet signifies a wanderer, and was applied to the solar bodies because they seemed to move or wander about among the stars. The Orbit of a planet is the path it pursues in its revo- lution around the sun. 25. The planets are divided into Primary and Secon- dary planets. The Primary Planets are those larger bodies of the system that revolve around the sun only, as their center of motion. The Secondary Planets are a smaller class of bodies, 22. Of what does Part II. treat ? What meant by the Solar System ? In eludes what ? 23. What is the sun ? 24. Describe the planets. (The term ?) The orbit of a planet ? 25. I low planets divided? Describe each. (What other names for secon- daries *) SO ASTRONOMY. that revolve not only around the sun, but also around the primary planets, as their attendants, or moons. The secondary planets are also called Moons or Satellites. A satellite is a follower or attendant upon another. OF THE SOLAR SYSTEM. In this cut, the sun may be seen in the center. The white circles are the Orbits of the primary planets. The planets may be seen in those orbits at various distances from the sun. The numerous orbits so close together are those of the Asteroids. The Bicindary planets may be seen near their respective primaries, revolving around them, while they all go on together around the sun. On the right is seen a Comet plunging Into the system, with his long and fiery train. His orbit is seen to be very elliptical. All these bodies are opake, the sun excepted. Even the blazing comet shines only by reflection. 26. The planets are again divided into Interior and Exterior planets. The Interior Planets are those whose orbits lie within the orbit of the earth, or between it and the sun. 26. What meant by interior and exterior planets ? (Why not inferior and uperior ?) PRIMARY PLANETS. 31 The Exterior Planets are those whose orbits lie out the orbit of the earth. Some Astronomers speak of these two classes respectively as Inferior and Superior. The reason seems to be, that as those nearer the sun than the earth are lower than she Is that re, nearer the great center of the system they arc, in this respect, inferior to her; while, on the other hand, tiiose that are above, or beyond her, are her superiors. But as the distinction is founded upon, and is intended to denote, the position of the planets with respect to tie eartlfs orbit, it is obvious that interior and exterior are the more appropriate terms. It seems hardly allowable to call the Asteroids superior plan- ets, and Mercury and Venus, which are much larger, inferior. 27. Comets are a singular class of objects, belonging to the solar system, distinguished for their long trains of light, their various shapes, and the great eccentricity of their orbits. NUMBER AND NAMES OF THE PRIMARY PLANETS. 28. The principal Primary Planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Five of these, including the Earth, were known to the ancients ; but Uranus and Neptune have both been discovered during the last hundred years. Besides the eight larger planets, there are now known to exist eighty-live small planets, called Asteroids, all revolving between the orbits of Mars and Jupiter. Four of these, namely, Ceres, Pallas, Juno, and Vesta, have been known to exist since 1807. The remaining eighty-one have all been discovered since 184:5, and most of them between 1852 and 1865. (For a complete list of the Asteroids, see page 247.) The terra Asteroid signifies star-like, and is applied to these small planets because of their comparative minuteness. They are never seen except through telescopes, and through ordinary instru meats are awt always readily distinguished from the fixed stars. 29. The Primary Planets are denoted in astronomi- cal works by certain signs or symbols; and as their names are derived from Mythology, their symbols usually relate to the imaginary divinities after whom they are named. 27. What are comets ? How distinguished ? 28. Number of the principal planets ? How long known ? What other planets ? How long known ? What said of their discovery ? Meaning of tlwj term Asteroid ? 2C. How are the planets designated in astronomical works ? (Describe the preceding cut. Where the' sun ? Primaries ? Secondaries ? Aste- roids? Orbits? Couiet and orbit? Which self-luminous^ and which opake.) ASTRONOMY. ROD OF MERCURT 1. In the preceding cut, the planets are placed at their respective distances from the sun, as nearly as can be represented in so small a drawing; The orbits of tho asteroids are represented by a few whito circles only, located between the orbits of Mars and Jupiter. 2. The mythological history and symbols of a few of the planets will now be given as samples of the whole, many of the asteroids not having any signs attached to their names as yet in astronomical works. MYTHOLOGICAL HISTORY AND SYMBOLS. 30. MERCURY was the messenger of the gods, and the patron of thieves and dishon- est persons. His symbol denotes his cadw- ceus, or rod, with serpents twined around it x).* 1. Mercury was represented as very eloquent, and skillful in in- terpreting and explaining as the god of rhetoricians and orators. Hence, when Paul and Barnabas visited Lystra, addressed the^peo- ple, and wrought a miracle, they said, " The gods have corne down to us in the likeness of men. And they called Barnabas Jupiter, and Paul Mercurius, because he icas the chief speaker," 2. "The caduceus of Mercury was a sort of wand or scepter, borne by Mercury as an ensign of quality and office. On medals, it is a symbol of good conduct, peace, and prosperity. The rod represents power; the serpents, wisdom,; and the two icing, diligence and activity.'" ENCYCLOPAEDIA. 8. The original form of this sign may be understood by the preceding cut, to which the present astronomical symbol ( Q ) bears but a slight resemblance. 31. YENUS was the goddess of love and beauty, and her sign is an ancient mirror or looking-glass ( 9 ), which she is represented as carrying in her hand. Anciently, mirrors were made of brass or silver, highly pol- fshed, so as to reflect the image of whatever was brought before them. Hence it is said in the Book of Exodus, written fifteen centuries before Christ, that Moses "made the laver of brass, and the foot of it of brass, of the looking-glasses of the women," &c. For convenience, the ancient mirrors had a handle at- tached, as represented in the cut, which very much resembles the sign of the planet. 32. THE EARTH (called by the Greeks Ore, and by the Latins Terra) has two sym- bols one representing a sphere and its equator (0), and the other () the four quarters of the globe. * All these symbols should be drawn in rotation upon the Blackboard, during recita- tion, by the Teacher, or some member of tho class. It will be well, therefore, for t?ie student to observe each sign carefully, that he may be prepared to draw and explain it, if called upon. 80. Who was Mercury, in Mythology, and what does his symbol denote (How was he represented ? What Scriptural allusion ? Pescribe his cadu- cous. The meaning of its parts ?) MIBROR OF VENUS. MYTHOLOGICAL HISTORY AND SYMBOLS. 33 33. MAES was the god of war, and his sign ( $ ) represents an ancient shield or buckler. SPBAR ATn> SHIKI - D It OF MARA crossed by a spear. Gunpowder was not known to the ancients, consequently they had no pistols, muskets, or cannon. They fought with short swords and spears, and defended themselves with the shield, carried on the left arm. A shield and spear were, therefore, very appropriate emblems of war. The original form of the sign of Mars is pre- sented in the cut 3. FLOKA was the " queen of all the flowers," and her symbol (5??) is a flower. the "Kose of England." 35. CLIO was one of the Muses. Her sign star, with a sprig of laurel over it. 36. VESTA was the goddess of fire, and her sign () is an altar, with &fire blazing upon it. 37. IRIS was the beautiful waiting-maid of Juno, the queen of heaven. Her symbol (&*) is composed of a semicircle, representing the rainbow, with an interior star, and a base line for the horizon. "As an attendant upon Juno," says Prof. Hind, "the name was not inappropriate at the time of discovery, when Juno was traversing the 18th hour of right ascension, and was followed by Iris in the 19th." 38. METIS was the first wife of Jupiter, and the god- dess of prudence and sagacity. Her symbol (,) is an eye (denoting wisdom) and a star. 39. HEBE presided over children and youth, and was cup-bearer to Jupiter. Her sign (2) is a cup. Hebe was celebrated for her beauty, but happening one day to stumble and spill the nectur, as she was serving Jupiter, she was turned into an footer, and doomed to haruesa and drive the peacocks of the queen of heaven. 40. PARTHENOPE w T as one of the three Syrens a sea nymph of rare beauty. They were all admirable singers; hence a lyre (l) is her appropriate sign. 1. The three Syrens Parthenope, Ligeia, and Leucosia were represented as dwoll- 81. Venus and symbol ? (Ancient mirrors ? Scripture allusion ?) 32. The Earth anci-ut name and symbols ? 33. Mars and symbol i (Ancient mode of warfare ? > 34. Flora and sign ? 35. Clio and symbol ? 86. Vesta and her symbol ? 87. Iris and her sign ? (Prof. Hind's remark ?* 88. Metis and her sign ? 89. Hebe and her sign I (Incident mentioned in note ?) 40. Parthenope and sign f (What said of 'Jia Syrens 3 Of the appro priateness of the name ?) 2* 34: ASTRONOMY. ing upon the coast of Sicily, and luring mariners upon the rocks of destruction by their enchanting songs. Hence whatever tends to entice or seduce to ruin is often called a 1 syren soi As this planet was discovered at the Naples Observatory, in Italy, it was quite ap- f ropriate to name it after one of the Syrens, that Mythology located on the coast of a neighboring island. 41. EGERIA was the counsellor of Kuma Pompilius. Symbol not yet agreed upon by astronomers. 42. ASTRJEA was the goddess of Justice, and her sign (ill) is a balance. Mythology teaches that Justice left heaven, during the golden age, to reside on the earth ; but becoming weary with the iniquities of men, she returned to heaven, and commenced a constellation of stars. The constellations Virgo and Libra in the zodiac are representations of Astnea and her golden scales. So the female figure, holding a pair of scales, in the coat of arms of several of the United States, is a representation of Astrtea, and denotes Justice. 43. IRENE was one of the Seasons. The planet was so named by Sir John Herschel, in honor of the peace pre- vailing in Europe at the time of its discovery (May, 1851). Its symbol (3^) is a dove, with an olive branch in her mouth, and a star upon her head. 44. EUNOMIA was another of the Seasons a sister cf Irene. (Symbol not ascertained.) 45. JUNO was the reputed queen of heaven, and her sign ( $ ) is an ancient mirror, crowned with a star an emblem of beauty and power. 46. CERES was the goddess of grain and harvests, and her sign (?) is a sickle. 47. PALLAS (or Minerva) was the goddess of wisdom and of war. Her symbol ( $ ) is the head of a spear. 1. The ancient Palladium was an image of Pallas, preserved in the castle of the city of Troy ; for while the castle of the city of Minerva was building, they say this image fell from heaven into it, before it was covered with a roof. Tooke's Pantheon. 2. To a similar fable, respecting an image falling from heaven, the Town-Clerk al- ludes, Acts xix. 35: "Ye men of Ephesus, what man is there that knoweth n)t how that the city of Ephesus is a worshiper of the great goddess Diana, and of the imago which fell down from Jupiter ?" 41. Egeria and her symbol ? 42. Astraea and sign ? (Mythological legend ? Virgo and Libra ? Where else found ?) 43. Irene by whom named, and why ? Symbol ? 44. Eunomia and symbol ? 45. Juno and symbol? 46. Ceres and her symbol? 47. Pallas and her symbol ? (Ancient Palladvum f Keputed origin t Scrip 4 iral allusion to it ?) MYTHOLOGICAL HISTORY AND SYMBOLS. 35 SATURN, OR CHRONOS. 8. EYGEIA was the goddess of health, and the daugh- ter of Esculapius, the father of the healing art. (Symbo) not ascertained.) Our TtT'^ern word Hygtian, which signifies the laws of health, Is derived from the goddess Hyg Utt. 49. JWiTER was the reputed father of the gods the king of heaven. His symbol (^) was originally the Greek letter , zeta, (the same as our Z) the initial of the Greek word Zeus, the name for Jupiter. 50. SATURN called by the Greeks Chronos presided over time and chronology. His sign (*?) represents a scythe. 1, Saturn was represented in Mythology as an old man, with wings, bald excepting a fore- lock, with a scytlie in one hand, and an hour- glass in the other. The same figure is now used to represent time. 2. Our modern word chronology, from chronoa, time, and logos, discourse, signifies the science of keeping time, dates, &c, 51. URANUS was the father of Saturn, and presided over astronomy. The symbol of this planet ( r #) consists of the letter II, with a planet suspended' from the cross-bar, in honor of Sir William Herschel, its discoverer. This planet is popularly known by the name of Herschel, hut astronomers now almost universally call it Uranus. It bears this name in the British Nautical Alman-ao for 1 651, with the full consent of Sir John Herschel, the son of the great discoverer. It was first called Georgium Sidus, by Dr. Herschel, iu honor of his royal patron, George HI. 52. NEPTUNE was the god of the seas, but the symbol of the planet (IP) is composed of an L and a Y united, with a planet suspended from the hair-line of the V, in honor of Le Yerrier, its discoverer. This planet was first called Le Verrier, but is more generally known by the name of Neptune. 53. The MOON was called Luna by the Romans, and 48. Hy^eia and symbol ? (Term Tiygeian ?) 49. Jupiter and his symbol ? 50. Saturn ? Greek name ? Symbol ? (How represented in Mythology Word chi'&rwlogy f) 51. Uranus and symbol ? (What other names, and why?) 52. Neptune and his symbol ? (Former name ?) 36 ASTRONOMY . Selene by the Greeks. She is known by various sym- bols, according as she is new, half-grown, or fall, thus: , , O. . 1. From Luna, we have OUT modern terms lunar and Itvnctcy ; the former of which signifies pertaining to the moon, and the 'alter a disease anciently supposed to be caused by the moon. 2. Selene, in Mythology, was the daughter of Helio*, the Son. Our English word selenography a description of the moon's surface is from Selene, her ancient tame, aud grapho, to describe. 54. The SUN called Sol by the Romans, and Helios by the Greeks is represented by a shield or buckler, thus : 0, , . As the large and polished bucklers of the ancients dazzled the eyes of their enemies, this in- strument was selected as an appropriate emblem of the sun. DISTANCES OF THE PLANETS. 55. The orbits of all the planets being more or less elliptic, they must vary in their distances from the sun in proportion to the ellipticity of their respective orbits, and their position in their orbits. The following table exhibits the mean or average distances of the several planets from the sun, commencing with Mercury and proceeding outward. Mercury ... 37 millions, or 37,890,000 Yenus .... 69 " 68,770,000 Earth . . . . T 95 " 95,298,260 Mars .... 145 " 145,205,000 The Asteroids, from 210 " to 300,000,000 Jupiter ... 496 " or 495,817,000 Saturn .... 909 " 909,028,000 Uranus . . . 1,828 " 1,828,071,000 Neptune . . . 2,862 2,862,457,000 1. The first column of round numbers only should be committed to memory by the student These should be well fixed in the mind, as it will greatly facilitate the pro- gress of the student hereafter. The family of Asteroids being less important, their distances need not be learned in detail. It is impossible for the human mind to form any adequate conception of the dis- tance represented by the phrase u a million of miles." It is only by conceiving aright in regard to short distances, and then using illustrations and instituting comparisons, that we can form any distinct idea of these really inconceivable spaces. 53. The Moon Latin and Greek names ? Symbols ? (Words lunar anr lunacy ? Who was Selene in Mythology ? Selenography ? Derivation ?) 54. The Sun Latin and Greek names? Symbol, and why? 55. Tiehearse, in round numbers, the distances of the planets from the (Substance of note 1st I Object of note 2d ? Note 3d < Note 4th ?) DISTANCES OF THE PLANETS. 37 2. The comparative distances of the planets are represented in the cut, page 15 and *Iso in the following : OF THE PLANETS. /+ I I 3. To assist his conception of these vast distances, the student may imagine a rail- road laid down from the sun to the orbit of Neptune. Now if the train proceed from the sun at the rate of thirty miles an hour, without intermission, it will reach Mercury in 152 years; the Earth in 361 years: Jupiter in 1.884 years; Saturn in 3.493 years,- Uranux in 6,938: and -Neptune in 10.800 years i Such a journey would be equal to riding 900,000 times across the continent, from Boston to Oregon ! 4. It is now about 5.870 years since the creation of man. Had a train of cars started from the sun at that time toward the orbit of Neptune, and traveled day and night ever since, it would still be 284 millions of miles within the orbit of Uranus about where the head of the locomotive stands, as shown in the cut! To reach even that planet would require over 1.000 years longer; and to arrive at Neptune, nearly 6,000 years to come ! Such is the vast area embraced within the orbits of the planets. and the spaces over which the sunlight travels, to warm and enlighten its attendant worlds. 56. The apparent magnitude of the heavenly bodies depends much upon the distance from which they are viewed ; the magnitude increasing as the distance is diminished, and diminishing as the distance is increased. NEAR AND REMOTE VIEWS OF THE SAME OBJECT. Let A represent the position of an observer upon the earth, to whom the sun appears 32'. or about half n degree in diameter. Now it is obvious that if the observer advance to B (half way), the object will fill an angle in his eye ttcice as large a? it filled when viewed from A. Again: if he recede from A to C, the object will appear but half as large. Hence the rule, that the apparent magnitude is increased ^s the distance is diminished, and diminished as the distance is increased. 57. Could a beholder leave the earth, and, descending toward the sun, station himself upon Mercury, he would find the apparent magnitude of the sun vastly increased. Should he then return, and pass outward to Mars or Jupiter, he would observe a corresponding diminution in the sun's magnitude, in proportion as the distance was increased. Hence the apparent magnitude must vary 56. How apparent magnitudes of heavenly bodies modified 1 (Illustrate hy diagram.) *57. Suppose a person to go to Mercury what effect upon apparent size of the sun 1 38 ASTRONO^IY. exceedingly, as viewed from different points in the solar system. THE SUN, AS SEEN FROM THE DIFFERENT PLANETS. From N. H. S. Jupiter. Mars. The above cut represents the relative apparent magni- tude of the sun, as seen from the different planets. In angular measurements, its diameter would be as follows : From Mercury . 82^' " Yemis . . 44^' " Earth . . 32' " Mars . . 21' The Asteroids, say 12' From Jupiter . . 6' " Saturn ... 3^ " Uranus . . If " Neptune . . 50" Let us continue our imaginary journey outward, be- yond Neptune, toward the fixed stars, and in a short time the glorious sun, so resplendent and dazzling to our view, will appear only as a sparkling star and the fixed stars will expand to view as we approach them, till they assume all the magnitude and splendor of the sun him- self. LIGHT AND HEAT OF THE PLANETS. 58. As the distances of the planets, respectively, affect the apparent magnitude of the sun, as viewed from their surfaces, so it must affect the relative amount of light and heat which they respectively receive from this great luminary. 59. The amount of light and heat received from the sun, by the several planets, is in inverse proportion to the square of their respective distances. 58. What effect lias the distances of the planets from the sun, respectively upon their relative "tght and heat ? 69. What rule governs the diffusion of light ? (Illustrate ly a diagram.) LIGHT AND HEAT OF THE PLANETS. 39 PHILOSOPHY OF THE DIFFUSION OF LIGHT. 1. Here the light is *en passing in right lines, from the sun on the left towarJ the several planets on therijht. It is rfiso shown that the surfaces A, B, and C receive "\. State the diameters of the several planets ? (Why blanks in the table ? What diameters are given polar, equatorial, or neither \) *>2. Give the magnitude of the principal planets, as compared with the .f,arth. (How ascertain relative magnitudes ? How possible that a mere star can be such an iinaieiise wond !) DENSITY. 41 COMPARATIVE MAGNITUDE OF THE SUN AND PLANETS. 2. It may seem almost incredible that what appear only as small stars in tho heavens should be larger than the mighty globe upon which we dwell. But when we consider their immense distance, and the effect this must have upon their apparent magnitude, as illustrated at 55, it is evident that the planets could not be seen at all were they not very large bodies. The above cut will give some idea of the magnitude of the several planets, as compared with each other, and also with the sun. 63. The Sun is 1,400,000 times as large as our globe, and 500 times as large as all the other bodies of the solar system put together. It would take one hundred and twelve such worlds as our earth, if laid side by side, to reach across his vast diameter. DENSITY. 64. The planets differ greatly in their density, or in tho compactness of the substances of which they are com- posed. Mercury is about three times as dense as oui globe, or equal to lead. Venus and Mars are about the same as the earth ; while Jupiter and Uranus are only -Jtli as dense, or about equal to water. Saturn has only j^th the density of our globe, answering pretty nearly to cork-. 63. State the magnitude of the sun as compared with the earth. With the rest of the system. Illustration ? 64. What meant by density f Do the planets differ in this respect ? State and ill-ostrate. (How masses of planets ascertained ? How with M.ercury ?) 42 ASTKONOMY. The masses of the planets are determined by the revolution of their respective satel- lites : but as Mercury has no satellite, the determination of his mass and density be- comes a very difficult and uncertain matter. " But it fortunately happens," says Prof Hind, "that we have a curious method of approximating to this element, viz.. by the perturbations produced by the planet in the movements of a comet known as Encke's, which revolves around the sun in little more than three years, and occasionally ap- proaches very near Mercury, &c. From computations based upon these perturbations, I'rof. II. concludes that Mercury is only about y^- more dense than our globe a result widely different from that arrived at by his predecessors. GRAVITATION. 65. Attraction, or Gravitation, is the tendency of bodies toward each other. It is that tendency which causes bodies raised from the earth, and left without sup- port, to fall to its surface. ATTRACTION or TH , EJLRTH All substances fall toward th-s earth's center from every part of the globe, as a spherical loadstone would attract parti- cles of steel to its surface in every di- rection. Hence when these four men, etandingon different fides of the globe, drop each a stone, tney all fall toward the same point, because the earth at- tracts them all to herself 66. Gravitation is what constitutes the weight of bodies, and depends upon the quantity of matter in the bodies attracting, and their distances from each other. The reason why a cubic for,',, of cork weighs much less than the same bulk of lead, is, that being less deusc, ie contains much less matter to be attracted. 67. From the above law of attraction, it follows that large bodies attract much more strongly than small ones, provided their densities are equal, and their distances the same ; and as the force of attraction constitutes the weight of a body, it follows that a body weighing a given num- ber of pounds on the earth, would weigh much more on Jupiter or Saturn, and much less on Mercury or the Asteroids. 65. Define gravitation. (What illustration given ?) 66. What relation lias gravitation to weight? Upon what does it depend for its degree of force \ (Why is a cubic foot of cork lighter than the same bulk of lead?) 67. What effect have the balk and density of the planets upon the tceigJit of bodies on their surfaces? (State comparative weights. IHustration? Why not attractive force or weight in exact proportion to bulk * How nm*t i/xlie be weighed to ascertain difference, aud why ?) GRAVITATION. 43 1 The following table shows the relative attractive force of the sun and planets. A body weighing one pound on the earth, would weigh, . oz. | lb. on. On Mercury 1 lj j On Saturn 1 Venus 15 " Mars 8 " Jupiter 2 8 Uranus 12 " Neptune unknown. " The Sun 28 5* 2. A person weighing 150 ibs. on the earth would consequently weigh but 74 ibs. upon Mars ; while upon Jupiter, his weight would be 375 Ibs. ; and upon the sun, 4,250 Ibs. 1 The attractive force of the Asteroids is so slight, that, if a man of ordinary muscular strength were transported to one of them, he might probably lift a hogshead of lead from its surface without difficulty. S. But the learner will notice that the attractive force, as shown in the above table, is not in strict proportion to the bulk of the planets respectively. This difference will be accounted for by considering the difference in their density (64). From the principles there laid down, it will be seen at once, that though one planet be as large again as another, still, if it were but half as dense, it would contain no more matter than the smaller one, and their attractive force would be equal. If Jupiter, for instance, were as dense as the earth, his attractive force would be four times what it now is; and if the density of all the solar bodies were precisely the same, their attractive force, or the weight of bo'dies m. their surfaces, would be in exact proportion to their bulk. 4. It must be remembered, however, that if a body were actually weighed upon the surface of each planet, by scales, it would weigh the same on all, because the force of attraction upon the w.iyhts would be just equal to that of the body to be weighed, whether it were more or less. With a steelyard it would be the same. A spring" and hook, therefore, is the only instrument with which we could weigh objects accurately on the different planets. 68. If the earth were only one-half as dense as she now is, it would reduce the weight of bodies at her surface one-half. So if a body were taken from the earth's sur- face half way down to her center, the weight would be reduced one-half. At her center it would be nothing, because the attractive force would be the same in all directions. In this cut, the diameter of the earth is divided into four equal parts C. D, E, and F. At A, the whole attraction amounts to four pounds. When the stone reaches B, the part C attracts as strongly upward as D does downward, and their forces balance each other. Then as C and D mutually neutralize each other, we have only the parts E and F, or one- half the globe, to attract the stone downward; consequently the attractive force would be only half as great at B as at A, and the stone would weigh only two pounds. 69. The force with which bodies gravi- tate toward each other is in direct pro- portion to their respective masses, and in inverse propor- tion to the squares of their distances. A man carried upward in a balloon weishs less and less as his distance from the earth is increased. The same law holds good in regard to the planetary worlds. The nearei a planet is te the sun, or to any other body, the stronger the mutual gravitation. 68. How effect weight of bodies on the earth to reduce her density one- naif? How to take down half way to center 2 Quite to center ? (Illustrate by diagram.) G'.>. Give the exact law of gravitation ? (What said of a man ascending m a balloon ? Of more distant planets ?) 44 ASTRONOMY. 69. This great law was disco vered by Sir Isaac New- ton, in. 1666. He was then only twenty-four years of age. The inquiry which led to the discovery is said to have been suggested to the mind of this youthful philosopher by seeing an apple fall from the limb of a tree. " What dre* these two globes (the apple and the earth) together ?" PERIODIC REVOLUTIONS OF THE PLANETS. TO. The planets all revolve around the sun from west to east, or toward that part of the heavens in which the sun appears to rise. To assist his conception of the direction in which the planets revolve, the student may suppose that if the earth was in her orbit beyond the sun, at 12 o'clock, she would go what we should call eastward, which would be the same direction that we should call westward on the earth, at the same time ; as bodies revolving in a circle move in opposite directions on opposite sides of the circle. 71. The passage of a planet from any particular point in its orbit, around to the same point again, is called its periodic revolution; and the time occupied in making such revolution is called its period, or periodic time The periodic times of the principal planets are as fol lows : Tears. Days. Mercury ... 88 Yenus ... 225 Earth .... 1 -~ Mars 1 322 Tears. Davs. Jupiter ... 11 317 Saturn ... 29 175 Uranus . . ^ 84 27 Neptune . T164: 226 72. The periodic times of the Asteroids vary from 1,200 to 2,000 days, the average being about 1,600 days, or four and a-half years. This resemblance in the time of their revolutions is due to the fact that they vary but slightly in their distance from the sun, a cir- cumstance which governs the time of the revolution of all the planets. 69. When and by whom were the Laws of Gravitation discovered ? How old ? (What led to this discovery ?) 10. In what direction do the planets revolve in their orhits ? (Give illus- tration.) 71 . What meant by the periodic revolution of a planet ? Its period or peri- odi.c tim-f ? Give the periods of the principal planets. 72. Periodic times of the Asteroids ? Cause of agreement ? (What constitutes the year of a planet? Compare years.) CENTRIPETAL AND CENTRIFUGAL FORCES. 45 HOURLY MOTION OF THE PLANETS IN THEIR ORBITS. 73. The velocity with which the planets fly through space, in performing their periodical journeys around the sun, varies from 11,000 to 110,000 miles an hour. The hourly motion of the earth amounts to 68,000 miles ! 1. The hourly motion of the planets is, approximately, as follows: Miles Mcrcnry 110,000 Venus 75,000 Earth 68,000 Mars 55,000 Miles. Jupiter 30,000 Saturn 22,000 Uranus 15,000 Neptune 11,000 Here, instead of finding the swiftest planets performing the longest periodic journeys, this order is reversed, and they are found revolving in the smallest orbits. The nearer a planet is to the sun, the more rapid its motion, and the shorter its periodic time. The reasons for this difference in the velocities and periodic times of the planets, will appear in a subsequent paragraph. 2. It may seem incredible to the student that the ponderous globe is flying through space at the rate of 68,000 miles an hour, or some 80 times as swift as a bullet; but^. like many other astonishing facts in Astronomy, its truth can easily be demonstrated. The diameter of a circle is to its circumference as 7 is to 22 nearly. The earth's dis- tance from the sun being 95,000,000 miles, it is obvious that the whole diameter of her orbit is twice that distance, or 190,000,000; then, as 7: 22:: 190.000,000 : 597,142,857 miles, the circumference of the earth's orbit. Divide this sum by 8,766, the number of iiours in a year, and we have 68,108 miles as the hourly velocity of the earth. 3. As the earth is not propelled by machinery like a steamboat, or borne upon wheels like a railroad car, it is not strange that we are insensible of its rapid motion, especially as every thing upon its surface, and the atmosphere by which it is surrounded, move 9nward with ft in its rapid flight. CENTRIPETAL AND CENTRIFUGAL FORCES. 74. The mutual attractive force of the sun and planets is called th# centripetal force ; while the tendency of the planets to fly off from the sun, as they revolve around him, is called the centrifugal force. 1. The term centripetal is from centrum, center, and peto t to move toward ; and centrifugal, from centrum, and fugio, to fly from the center. 2. The centrifugal force is generated by the revolution of the planet, and is in pro- portion to its velocity the more rapid the revolution, the stronger the tendency to fly olf from the sun. 3. If the centrifugal force were suspended, the planets would at once fall to the sun; and if the centripetal force were destroyed, the planets would fly off in straight lines, and leave the solar system forever. Then might be realized the chaos and confusion of the poet : " Let Earth unbalanced from her orbit fly, Planets and sunsrnn lawless through the sky; Let ruling angels from their spheres be hurled, Being OB being wreck'd, and world on world." 75. It has already been stated (65), that the force of attraction depends somewhat upon the distances of the attracting bodies those nearest together being mutually V3. What said of the velocity of the planets ? Of the earth ? (Table ? Ktimarks upon it ? How is the hourly velocity of the earth ascertained ? Why not sensible of this rapid motion?) 74. Centripetal and centrifugal forces ? (Derivation of terms ? How txm- force generated ? Suppose either suspended ?) 46 ASTRONOMY. attracted most. It follows, therefore, that Mercury ha* the strongest tendency toward the sun, Yenus next, this Earth next, &c., till we get through to Neptune ; and as the centrifugal force which is to balance the centripetal is created by the velocity or projectile force of the planets, that velocity must needs be in proportion to their dis- tances, respectively, from the sun ; the nearest revolving the most rapidly. This we find to be the actual state of things in the solar system. The mechanism of the solar system strikingly displays the wisdom of the great Creator. The centrifugal force depends, of course, upon the rapidity of the revolution ; and in order that these forces might be exactly balanced, God has imparted to each planet a velocity just sufficient to produce a centrifugal force equal to that of its gravita- tion. Thus they neither fall to the. sun on the one hand, nor fly off beyond the reach of his beams on the other, but remain balanced in their orbits between these two great forces, and steadily revolving from age to age. " How manifold are thy works ! in wis- dom hast thou made ttieni all." LAWS OF PLANETARY MOTION. 76. Three very important laws, or principles, governing the movements of the planets, were discovered by Kep- ler, a German astronomer, in 1609. In honor of their discoverer, they are called Kepler's Laws. Kepler was a disciple of Tycho Brahe, a noted astronomer of Denmark, and WHS equally celebrated with his renowned tutor. His residence and observatory were in Wittenberg, Germany. 77. Tiie first of these laws is, that the orbits of all the planets are elliptical, having the sun in the common focus. The point in a planet's orbit nearest the sun is called the perihelion point, and the point most remote the aphelion point. Perihelion is from peri, about or near, and lielios, the sun ; and aphelion, from apo, from, and Tielios, the sun. From this first law of Kepler, it results that the plan- ets move with different velocities, in different parts of their orbits. From the aphelion to the perihelion points, the centripetal force combines with the centrifugal to accelerate the planet's motion ; while from perihelion to 75. Why the planets nearest the sun revolve most rapidly in their orbits . (Remark ?) 76. Laws of planetary motion ? (Who was Kepler ?} 77. State the first of Kepler's laws. Perihelion? Aphelion* LAWS OF PLANETARY MOTION. RADIUS VICTOR. -e- aphelion points, the centripetal acts against the centrifugal force, and retards it. 1. From A to B in the diagram, the centrifugal force, represented by the line C, acts with the tendency to revolve, and the planet's motion is accelerated ; but from B to A the same force, shown by the line D, acts against the tendency to advance, and the planet is retarded. Hence it comes to. Aphelion with its least velocity, and to Perihelion with its greatest. 2. In the statement of velocities on page 45, the mean or average velocity is given. 78. The second law is, that the radius vector of a planet describes equal areas in equal times. The radius is an imaginary line joining the center of the sun and the center of the planet, in any part of its orbit. Vector is from veho, to carry ; hence the radius vector is a radius carried round. By the statement that it describes equal areas in equal times, is meant that it sweeps over the same surface in an hour, when a planet is near the sun, and moves swiftly, as, when furthest from the sun, it moves most slowly. The nearer a planet is to the sun, the more rapid its motion. It follows, therefore, that if the orbit of a planet is an ellipse, with the sun in one of the foci, its rate of motion will be unequal in different parts of its orbit swiftest at perihelion, and slowest at aphe- lion. From perihelion to aphelion the centripetal more directly counteracts the cen- trifugal force, and the planet is retarded. On the other hand, from the aphelion to the perihelion point, the centripetal and centrifugal forces are united, or act in a similar direction. They consequently hasten the planet onward, and its rate of motion is con- stantly accelerated. Now suppose, when the planet is at a certain point near its peri- helion, we draw a line from its center to the center of the sun. This line is the radius vector. At the end of one day, for instance, after the planet has advanced considerably in its orbit, we draw another line in the same manner to the sun's center, and estimate the area between the two lines. At another time, when the planet is near its aphelion, we note the space over which the radius vector travels in one day, and estimate its area. On comparison, it will be found, that notwithstanding the unequal velocity of the planet, and consequently of the radius vector, at the two ends of the ellipse, the area over which the radius vector has traveled is the same in both cases. The same principle ob- tains in every part of the planetary orbits, whatever may be their ellipticity or the mean distance of the planet from the sun; hence the rule, that the radius vector describe* equal areas in equal times. In the preceding cut, the twelve triangles, numbered 1, 2, 8, fcc., over each of which the radius vector sweeps in equal times, are equal. 79. The third law of Kepler is, that the squares of th& 78. State the second law of planetary motion. Define radius vector, plain this serond law.) ASTRONOMY. periodic times of any two planets are proportioned to the cubes of their mean distances from the sun. 1. Take, for example, the earth and Mars, whose periods are S35'25fi4 and 6S6-9796 days, and whose distances from t'.ie sun are in the proportion of 1 to 1-52369, and it will be found that (365-2564)* : (6S6-9796)* ::(!)': (1-52369) 3 . 2. According to these laws, which are known to prevail throughout the solar system, many of the facts of astronomy are deduced from other facts "previously ascertained. They are, therefore, of great importance, and should be studied till they are, at least, thoroughly understood, if not committed to memory. The MARS IN CONJUNCTION e 6 ASPECTS OF THE PLANETS. 80. By the aspects of the planets is meant their ppsi tions in their orbits with respect to each other, principal aspects are conjunction, quadrature, and op- position. Two bod- ies are in conjunc- tion when in the same longitude ; that is, on the same north and south line in the heavens. The sign for con- junction is 6 . When 90 apart, bodies are said to be in quadrature, with the sign n ; and when 180 apart, or in opposite parts of the heavens, they are in opposition, and the sign is . Conjunctions are of two kinds. An inferior conjunction is when the planet is between the earth and the sun ; and a superior conjunction, when it is beyond the sun. 1. Let the student imagine himself stationed upon the earth in the cut Then the eun and three planets above are in conjunction. The inferior and superior are distin- guished : while at A, a planet is shown in quadrature, and at the bottom of the cut the iianet Mars in opposition with the sun and interior planet 79. State the third law. (Illustration ? What said of the importance of a knowledge of these laws ?) 80. What meant by the aspects of the planets* State principal aspects? "Define each. "Signs ? I low many kinds of conjunctions ? Define each. (Explain by diagram. When is Venus nearest ? What difference at superior taiid inferior conjunctions ?) SIDEREAL AND SYNODIC REVOLUTIONS. 49 2. "When at her superior conjunction, Venus is 154 millions of miles from the earth but when at her inferior conjunction, she is only 26 millions of miles distant, or the whole diameter of her orbit nearer. SIDEKEAL AND SYNODIC REVOLUTIONS. 81. The sidereal revolution of a planet is a complete revolution from any given point in its orbit around to the same point again. Sidereal, from sideralis a revolution as measured by the stars. See page 28, note 1. The periodic revolutions of the planets, given at Art. 71, are sidereal revolutions. 82. A synodic revolution is from one conjunction to the same conjunction again. 1. The term synod signifies a meeting or convention ; and the synodic revolution of a planet is a meeting revolution : that is, from one meeting or conjunction to another. 2. The difference between a sidereal and synodic revolution may be illustrated by the motion of the hands of a clock or watch. At twelve o'clock, the hour and minute hands are together ; but at one o'clock, when the minute-hand has made a complete revolu- tion, and points to XII. again, the hour-hand has gone forward to I., and the minute- hand will not overtake it till about five minutes afterward. The revolution of the minute-hand from XII. to XII. again, represents the sidereal revolution of a planet; and when it overtakes tha hour-hand, it becomes a synodic revolution. 3. The sidereal and synodic periods of the principal planets are as follows : Sidereal. Synodic. Mercury 88 days 115 days. Venus 225 " 594 Mars 1 year, 822 T80 Jupiter 11 " 307 " 399 Saturn 29 " 175 " 378 Uranus 84 " 27 367 Neptune 164 " 226 " 867 From this table it is seen that the synodic periods of the more distant planets corre- spond very nearly with the periodic time of the earth. Being remote from the sun, they move very slowly, and the earth coon overtakes them, after performing her periodic revolution. SYNODIC PERIODS OF THE EXTEKIOE PLANETS, Suppose the earth and Uranus to be in conjunction, as shown at A B. In 865J days, the earth performs her sidereal or periodic revolution, and return 1 } to the point A again. La the mean time Uranus, whose periodic time is 84 years, has passed 81. What meant by the sidereal revolution of a planet ? (Derivation of term? Are the periods of the planets sidereal revolutions ?) 82. Synodic revolution? (Illustrate difference by clock. What fact re- specting synodic periods of distant planets ? How explained ? Illustrate by diagram.) 50 ASTRONOMY. through only ^ th part of his orbit, or about 4p to the point C ; and in 4^ days ihe earth overtakes him on the line I). It is on this account that the synodic period o' Uranus is only 307^ days, or 4^ days longer than the periodic time of the earth. THE ECLIPTIC, ZODIAC, SIGNS, ETC. 83. The Ecliptic is the plane of the earttts orbit, or the path in which the sun appears to revolve in the heavens. 1. In the above cut, an attempt is made to represent the ecliptic, or plane of th* earth's orbit. It is an oblique view, which makes the orbit appear elliptical. It shows one-half of the snn and half the earth on one side, ami half on the other. The circle projecting beyond the orbit is to represent the plane of the ecliptic, indefinitely ex- tended. 2. If the student has any difficulty in getting a correct idea respecting the ecliptic, let him suppose the orbit of the earth to be a hoop of small wire laid upon a table: the Burface of the table, both within and without the hoop, would then represent the plane of the ecliptic. From the above definition and description, it will be seen that the eclip- tic passes through the center of the earth, and the center of the sun; consequently the ecliptic and the apparent path of the sun through the heavens are in the same plane. It will be easy, therefore, to ascertain the true position of the ecliptic in the heavens, and to imagine its course among the stars. 3. The plane of the earth's orbit is called the ecliptic, because eclipses of the sun nd moon never take place except when the imma is in or near this plane. 84. The position of the ecliptic to persons north of the equator is south of us. It runs east and west, cutting the centers of the sun and earth. North of the ecliptic is called above it; and south of it, "below it. The student should again be reminded that there is no absolute up nr down In th* universe. He must also guard against the idea that the ecliptic may be horizontal. This term has reference only to the earth, and is descriptive of a plane depending altogether for its own position upon that of the observer, as shown and illustrated at 20. Though the ecliptic is a permanent plane, and cuts the starry heavens around us at the same points from age to age, it has no absoJute up or down, unless it should be tlw direction to and from the sun. The distinction of above &T\<\ beltncis, merely arbitrary, and grows out of onr position north of the equate r, which aiakes the south side of the- ecliptic ap- pear down to u& 83. What the ecliptic? (How cnt the earth and snn ? Point out its course in the heavens. Why called the ecliptic?) ft4. What meant by abov* and below the ecliptic ? (Remarks in note. > ECLIPTIC, ZODIAC, SIGNS, ETC. 51 85. The Poles of the Ecliptic are the extremities of an imaginary axis upon which the ecliptic seems to revolve. As the ecliptic and equinoctial are not in the same plane, their poles do not coincide, or are not in the same points in the heavens. The cause of this variation will be ex- plained hereafter. 86. The Zodiac is an imaginary belt 16 wide, viz., 8 on each side of the ecliptic, and extending from west to east quite around the heavens. In the heavens, it in- cludes the sun's apparent path, and a space of eight de- grees south, and eight degrees north of it. THE KCLTPTIO AND ZODIACS. In this cut, the interior dotted circle represents the earth's orbit; the exterior the ple understood by the pupil. In reciting, however, it is only necessary to give the first names as Aries, Taurus, Gemini, &c. By carefully observing these symbols, the stu- dent will detect a resemblance between several of them and the objects they represent For instance, the sign for Aries represents his horns ; so also with Taurus, &c. 89. The ancients pretended to predict future events by the signs, aspects, &c. This art, as it was called, was denominated Astrology. Astrology was either natural or judicial. Natural ^ Astrology aimed at predicting re- markable occurrences in the natural world, as earthquakes, volcanoes, tempests, and pestilential diseases. Judicial Astrology aimed at foretelling the fates of individuals or of em- pires. "This science," says Webster, "was formerly in great request, as men ignorantly supposed the heaven- ly bodies to have a ruling influence over the physical and moral world ; but it is now universally exploded by true science and philosophy." A fragment of this ancient superstition, like the adjoining figure, may still be met with occasionally in the pages of an al- manac ; and there are still persons to be found in al- most every community who think certain "signs" govern certain portions of the human body, and that it is very important to do everything "when the sign is right" Impostors, also, are still taking advan- tage of this credulity; and, professing to "tell for- tunes," as they call it, by the stars, impose upon and defraud the ignorant The stars have no more to do with our " destiny" than we have with theirs. 90. The order of the signs is from west to east around the heavens. Thus Aries, Taurus, Gemini, &c., around to Pisces. 88. Names of the signs ? Symbols on blackboard. &. "What is astrology? How divided? Define each. (Remark of Web- ster? Of the author ?) 90: The order of the signs ? (Describe the cut. What said of Taurus 'h CELESTIAL LATITUDE AND LONGITUDE. 53 PKllPNIHCei.AK VIEW OF THE ECLIPTIC. OU 081 On pages 50 and 51. we presented dbliqiie views of the ecliptic. The above is a p peniUcidar view. The sun is seen in the center, and the earth revolving around hin ; and in the distance is shown the circle of the starry heavens. This circle is divider! into twelve equal parts, representing the twelve signs ; while the object which the stars in each sign were supposed to resemble is placed in that sign, and the m/mJ>ol iinme* diately opposite and within the sign. But the head of Taurus should point east instead of west. CELESTIAL LATITUDE AND LONGITUDE. 91. Celestial Longitude is distance east of a given point in the heavens, reckoned on the ecliptic. Begin- ning at the Vernal .Equinox, it is reckoned eastward to 360, or to the point whence we staHed. The pupil will consult the preceding cut, in which the longitude is marked for every ten degrees. By holding the book up to the south of him, the surface of the page will represent the plane of the ecliptic ; and the reckoning of 10, 20, 30, &c., from the top of the cut eastward, will answer to the manner in which celestial longitude is reckoned eastward around the heavens. 92. Celestial Longitude is either Heliocentric QY Geo- centric. The heliocentric longitude of a planet is its longitude as viewed from the sun ; and the geocentric, its longitude as viewed from the earth. Geocentric is from ge, the earth, and kentron, center; and fitMoesntric from heftos, the sun, and kentron, center. 01. Celestial longitude ? Where begin to reckon ? Ulustxt^ by book. Point out order of reckoning in the heavens. 92. What is heliocentric longitude ? Geocentric* (l>onvation if terms? Illustrate by diagram.; ASTRONOISIY. GEOCENTRIC AND HELIOCENTRIC LONGITtlUB. fn this cut, the planet B, when viewed from the earth at A, seems to be ;n the sign C2 ; but when viewed from the sun, it appears to be in n. Again : when at C, her apparent longitude from the earth is in il], ; when from the sun, she appears to be in t . The learner will not only perceive the difference between geocentric, and 7u>lioeent-ria longitude, but will see why the latter more than the former indicates the true position of the planet It is an easy thing, however, if one is known, to deduce the other from it. MEAN AND TRUE PLACES OF A PLANET. 93. The mean place of a planet is the place it would have occupied had it revolved in a cir- cular orbit, and with uniform velocity. The true place is that which it really occupies, revolving as it does in M, an elliptical orbit, and with unequal velocity. 1. In the cut, the dotted ellipse represents the orbit of the planet, and the points T T T, &c., its true place. In the circle or hypothetical orbit, the points M M, Arc., indicate the mean place of the planet. MEAN AND TRUE PLACES OF A PLANET T M What is meant by the mean place of a planet ? The true place f (When DIRECT AND RETROGRADE MOTIONS. 55 DIKECT AND RETROGRADE MOTIONS. 2. From the perihrtion to the aphelion, it will be seen that the true pbre is in ad- vmice, or eattimrd, of the mean place ; white from aphelion to perihelion again, th mean place is in advance of the true. But at the perihelion and aphelion points, the uiean and true places coincide. 3. In one respect, the cut conveys an erroneous impression, as it represents the planet s passing over an equal distance in its orbit in equal times. This is not the fact The difference in its velocity in different parts of its orbit could not well be represented here ; but the student will find it beautifully illustrated by the second cut on page 47, and in feexpbuutorjr note accompanying it 94:. Celestial Latitude is distance north or south of the ecliptic, and is reckoned to the pole of the ecliptic, or to 90. DIRECT AND RETROGRADE MOTIONS. 95. The apparent motion of a planet is said to be direct when it is eastward among the stars, and retrograde when it seems to go back or westward in the ecliptic. When it seems to move neither east nor west, it is said to be stationary. 96. The cause of the appa- rent retrogression of the in- terior planets is the fact that they revolve much more rapidly than the earth, from which we view them ; causing their direct motion to appear to be retrograde. Suppose the earth to be at A, and Venus at B, she would appear to be at 0, among the stars. If the earth remained at A while Ve- nus was passing from B to D, she would seem to retrograde from C to E ; but as the earth passes from A to F while Vnus goes from B to D, Venus will appear to be at (i ; arid the amount of her apparent westward motion will only be from to Gr. 97. The apparent retro- grade motions of the exterior planets is due to the rapidity with which the position from which we view them is is the true in advance of the mean? When the reverse? When do they coincide ? Wherein is the cut defective ? Where have we a true rcpreseu tiition ?) 94. Cetestisl'latifadt f How reckoned? 9;".. When is a planet's apparent motion direct? Retrograde? When is a planet said to be stationary? %. State the cause of the apparent retrogression of an interior planet, (illustrate by diagram,) y7. The cause of retrogression of exterior planets. (Illustrate by diagram- What feet, show u 56 ASTRONOMY. changed, as we are carried rapidly through space with the earth, in her annual journey around the sun. 98. The portion of the ecliptic through which a planet seems to retrograde is called the Arc of Retrogradation. The more remote the planet the less the arc, and the longer the time of its retrogression. RETROGRADE MOTION OF THB BXTKEIOB PLATTETS. :B i : 1. Suppose the earth at A, and the planet Neptune at B, he would then appear to be at C, among the stars ; but as Neptune moves but a little from B toward F, while the earth is passing from A to D, Neptune will appear to retrograde from C to E. What- ever Neptune may have moved, however, from B toward F, will go to reduce the amount of apparent retrogression. 2. It is obvious from this figure, that the more distant an exterior planet is, and the lower it moves, the less will be its arc of retrogradation, and the longer will it be retro- grading. Neptune appears to retrograde 180 days, or nearly half the year. The following table exhibits the amount of arc and the time of the retrogradation of the principal planets: Arc. Day Mercury 13^ 23 Venus *. 16 42 Mars 16 73 Jupiter 10 121 Saturn 6 139 Uranus 4 151 Neptune 1 180 99. The greatest elongation of an interior planet is the greatest apparent distance east or west of the sun at which it is ever found. In the second cut back, the point B would represent the greatest eastern, and D the greatest western, elongation of the planet At these two points she would appear to be stationary. 100. The greatest elongations of Yenus vary from 45 to 48 . The fact that she never departs more than 48 from the sun proves that her orbit is within that of the earth ; and the variation in her elongations shows that her orbit is not an exact circle. 98. What meant by the Arc of Retrogradation? 99. Greatest elongation ? 100. Greatest elongation of Venus ! What does it prove ? VENUS AS MORNING AND EVENING STAE. 57 101. When Venus is west of the sun, and risp.s before him, she is morning xtar and when east of the s\u\ ! e is evening star. VENUS AS HORNING AND EVENING STAR. M r \\/ \ ' ' /> X J! C - 1. Let the student hold the book up south of him, and he will at once see why Venus is alternately morning and evening star. Let the plane A B represent the sensible or visible horizon, C D the apparent daily path of the sun through the heavens, and E ?ition. The sun is shown at three different po namely, rising in the east, o'n the meridian, and setting in the west; while Venus is seen revolving around him from west to east, or in the direction of the arrows. Now it is obvious that when Venus is at F, or went of the sun, she sets before him as at G, and rises before him as at H. She must, therefore, be morning star. On the other hand, when she is east of the sun, as at J, she lingers iu the west after the sun has gone down, as at K, and is consequently evening star. ' 2. In this cut, Yenus would be at her greatest elongation eastward at J, and ^cest~ ward at F, and in both cases would be " stationary." At L and M she would be in conjunction with the sun. 3. Were the earth to suspend her daily rotation, with the sun on the meridian of the observer, as represented at L, we might readily watch Venus through her whole circuit around the sun. 4. Venus may sometimes be seen at mid-day, either east or west of the sun, and Dr. Dick considers the day-time most favorable for observing her with a telescope. 102. Venus is morning and evening star, alternately, for about 292 days, or from one conjunction to another. Appearing first east and then west of the sun, she was regarded by the ancients as two different stars, which they called Phosphor and Hesperus. When Venus is near her greatest elongation from the sun, she is one of the most beau- tiful stars in the heavens. She is very easily found, either just before sunrise, or just after sundown; and we earnestly recommend the class to ascertain where she is, at the time of learning this lesson, and to watch her movements for a few months, and see if tliey do not correspond with the description here given. The knowledge acquired will thus be located in the ueavens. 101. When morning and when evening star 1 102. How long is Venus alternately morning and evening star 1 How re ?arded by the ancients \ (Remark in note 58 ASTRONOMY. 103. The greatest elongations of Mercury vary from 1G to 29 degrees from the sun, which proves his orbit to be elliptical, and to be within that of Venus. 1. As Mercury never departs more than 29 from the sun. when at his greatest elonga- lion, and Venus is never nearer than about 45, when at her greatest elongation, it is evident that his orbit is inside that of Venus. 2. When at perihelion, Mercury is only 29,305,000 miles from the sun's center ; while in the opposite part of his orbit, or in aphelion, he reaches to 44,474,000 making a vari- ation of distance, arising from the ellipticity of his orbit, of more than 15,169,000 miles, which is nearly five times as great as in the case of the earth. 104. In consequence of the nearness of Mercury to the sun, he is very rarely seen ; and if seen at all, it must be in strong twilight, either morning or evening. He never appears conspicuous, even under the most favorable cir- cumstances, but twinkles like a star of the third magni- tude, with a pale rosy light. By consulting an almanac, the student can ascertain -when Mercury is at his greatest elongation, and if it is eastward, look out for him low down in the west, just after sun- set If his elongation is westward, he must be looked for in t/te east, before sunrise. It will be worth rising early to see him. DEVIATION OF THE ORBITS OF THE PLANETS FROM THE PLANE OF THE ECLIPTIC. 105. Although the sun is the great center around which all the planets revolve, it should be borne in mind that no two <,f them revolve in the same plane. Taking the plane of the earth's orbit or ecliptic as the standard, the orbits of the other planets all depart from that plane, some more and some less. As a consequence, they all pass through or cut the plane of the ecliptic twice at every revolution. VENUS PASSING AND BEPASStNG THE PLANK OF THB EARTIl'S DEBIT. In this cut, the space included within the orbit of the earth is tinted to represent plane. Within her orbit, and part above, and part below it, may be seen the orbit of 103. Greatest elongation of Mercury ? Proves what ? (Show how demon- strated. What said of the eccentricity of the orbit of Mercury?) 104. Is Mercury often seen? When, if at all? Appearance? (How find ?) 105. Are all the planetary orbits in the same plane ? "What consequence follows ? DEVIATION OF THE ORBITS OF THE PLANETS. 7("'is. the arrows showing her direction. Her orbit goes out of sight when it oassea the plane of the ecliptic. 106. The points where a planet passes the plane of the ecliptic are called the Nodes of its orbit. They are in opposite sides"of the ecliptic, and of course 180 apart. The point where they pass south of the ecliptic is called the descending node, and marked 3 ; and that through which they pass north of the ecliptic is called the ascend- ing node, and marked &. The Line of the Nodes is a ]ine drawn from one node to the other across the ecliptic. The nodes, ascending and descending, and their symbols, acdalso the line of the nodes, marked L N, are all well represented in the preceding cut. INCLINATION OF THE ORBITS OF THK PLANETS TO THE PLANE OF THE ECLIPTIC. 107. The nodes of the planetary orbits are not all in the same longitude, but are distributed all around the ecliptic. In astronomical works and calculations, the longitude of the ascending node only is noted, as the opposite node is always just 180 from it. The longitude of the ascending nodes of the planets, respectively, is as follows : 75 Hebe . 188 Pallas 173 Earth Parthenope Hygeia . . Mars . . 48 Jupiter 93 Flora 110 141 112 Clio Irene Uranus . . 72 Testa 103 130 Iris 2GO 171 106. What arc the Nodes of a planet's orbit ? How situated with respect to each other ? What called respectively, and why ? What meant by the lint if the nodes ? 107. Are the nodes of all the planetary orbits in the same longitude ? How distributed ? Which node usually mentioned and located ? Why not both ? 60 ASTRONOMY. 108. The deviation of the planets, respectively, from the ecliptic varies from 1 18" to 34. The orbits of the larger planets are all near the ecliptic, while some of the asteroids depart widely from it. On this account they are sometimes called ultra-zodiacal planets. The preceding cut may help the student to fonn an idea of the inclination of the planetary orbits; but we must guard against the impression it may make that all the planetary nodes are in the same part oftfie ecliptic, as we were obliged to represent in the cut. Instead of tliis, they are distributed all about the ecliptic. Again : the cut shows the several planets at about the same distance from the sun, contrary to the fact, as stated and illustrated on page 30. The dotted line represents the earth's orbit, or plane of the ecliptic, and the other lines the planes of the orbits of several of the plan- ets, and their departure from the ecliptic. The inclination of the several orbits 's, ia round numbers, as follows: Mercury 7 Venus 3023' Earth Mars lost' Flora 5053' Clio 7 08' Vesta 7008' Iris 5023' Metis 5034' Hebe 14047' Parthenope . . . Egcria Astraea 5 19' Irene Eunomia Juiio 130 3' Ceres 10 87 1 Pallas 34037' Hygeia Jupiter 1 18" Saturn SP W Uranus 1 46' Neptune 1 46' OF TRANSITS. 109. The passage of a heavenly body across the me- ridian of any place, or across the disk of the sun, is called a transit. A planet will seem to pass over the disk of the sun when it passes directly between us and him ; and as none but the interior planets can ever get between us and the sun, it is obvious that no others can ever make a transit over his disk. The te"/s 'transit is sometimes used with reference to terrestrial objects, as when we speak of the transit or passage of goods through a country. The words transition, transitive, transitory, &c., are derived from the primitive word transit. 110. Mercury and Venus are the only planets that can appear to cross over the sun's disk, as viewed from oui globe. "Were we stationed upon one of the remote exterior planets, we might see the earth, and Mars, and Jupiter transit the sun ; but as it is, we shall never witness such phe- nomena, or, at least, till we leave the present world. 111. "Were the orbits of Mercury and Yenus in the same plane with that of the earth, they would transit the 108. To what extent do the planetary orbits depart from the ecliptic? What said of the larger planets ? Of the smaller ? (Remarks upon the cut. State the inclination of Mercury, Venus, Mars, &c.) 109. What is a transit ? When do planets transit the sun ? What planets do this ? Why not the exterior ? (Remarks upon term transit.) 110. What planets make transits across the sun's disk? (Remarky \K aote. ) TRANSITS. 61 sun at every synodic revolution ; but as one-half of eac"b of their orbits is above, and the other half below the ecliptic, they generally appear to pass either above 01 below the sun. B ..--> -A*- ...... Let the right line A, joining the earth and the sun in the above diagram, represent the plane of the ecliptic. Now when an interior planet is in this plane, as shown at A, it may appear to be upon the sun's disk; but if it is either above or below the ecliptic, as shown at B and C, it will appear to pass either above or below the sun, as shown at D and E. 112. A transit can never occur except when the inte- rior planet is in or very near the ecliptic. The earth and the planet must be on the same side of the ecliptic ; the planet being at one of its nodes, and the earth on the line of its nodes. PHILOSOPHY OP TBAXSrrS. lliis cut represents the ecliptic and zodiac, with the orbit of an interior planet, h* nodes, &c. The line of his nodes is, as shown, in the 16 of b and the 16 of fll Now if the earth is in , on the line L N, as shown in the cut, when Mercury is at his ascending node (Q), he will seem to pass upward over the sun's face, like a dark spot, as represented in the figure. On the other hand, if Mercury is at his y when the earth is in the 16 of m,, the former will seem to pass downward across the disk ol the sun. 113. As the nodes of the planetary orbits are in oppo- 111. Why not transits every revolution of Mercury and Venus? (Illus- trate by diagram.) 112. When must transits occur, if at all ? Where must the earth und planet be ? (Illustrate by diagram.) ASTRONOMY. site sides of the ecliptic, it follows that the earth must pass the line of the nodes of the interior planets, re- spectively, in opposite months- of the year. These months are called the node months of the planet, and are the months in which all its transits must occur. 114. In making transits across the sun's disk, the planets seem to pass from east to west, and to ascend or descend, as respects the ecliptic, according as the planet is at the ascending or descending node. This variation in the direction of the planets, during different transits, is well repre- sented in the next cut. 115. The node months of Mercury are May and No- vember. All the transits of Mercury ever noticed have occurred in one or the other of thes* months, and for the reason already assigned. The first ever observed took place November 6, 1631; since which time there have been 29 others by the same plaiiet iu all 308 In May, and 22 in November. 116. The last tran- sit of Mercury oc- curred November 11, 1861; and the next will take placo November 4, 1868. Besides this, there will be four more dur- ing the present cen- tury two in May, and two in Nov'r. The accompanying cut is a de- lineation of all the transits of Mercury from 1802 to the close of the present century. The dark line running east and west across the sun's center represents SOUTH the plane of the ecliptic, and tho dotted lines the apparent paths of Mercury in the several transits. The planet is shown at its nearest point to the sun's center. His path in the last transit and in the next will easily be found. 2. The last transit of Mercury was observed in this country by Professor Mitchel. at the Cincinnati Observatory, and by many others both in America and in Europe. TRANSITS OF MERCURY. &ORTH 113. What are the node months f (Explain by diagram.) 114. In what direction do planets cross the sun in transits, and why? 115. Which are the node months of Mercury ? 116. When did the last transit of Mercury occur ? When will the next tftke place ? (What represented in the cut. ? "Describe. Where is the planet Itown ? What said of last transit of Mercury ?) TRANSITS. 63 Che writer had made all necessary f reparation for observing the phenomenon at his residence, near Oswego, New York ; but, unfortunately, his sky was overhung with clouds, which hid the sun from his view, and disappointed all his hopes. 117. The node months of Venus are December and June. The line of her nodes lies in Gemini (IE) and Sagittarius (^); and as the earth always passes those points in the months named, it follows that all transits of V enus must occur in those months for ages to come. This proposition will be well understood by consulting the cut on page 61 ; for as the ineof Venus's nodes is only one sign ahead of that of Mercury, the earth will reach .hat point in the ecliptic in one month after she pasy.es the line of Mercury's nodes; so .hat if his transits occur in May and November, hers should occur in June and Decem- uer, as is always the case. 118. The last transit of Venus occurred June 3, 1769 ; and the next will take place December 8, 1874. 1. Only three transits of Venus have as yet been observed namely, December 4, 1639 ; June 5, l?bl ; and June 3, 17G9. it is said that Itittenhouse was so interested in view- ing that of 1769. that he actually fainted. In defining the term transit, Dr. Webster says: "I witnessed the transit i>f Venus over the sun's di^k, June 3, ITb'O." (See ' Una- bridged" Dictionary.) The next four will occur December 8, 174; December 5, l&jii; June 7, 2004 ; and June 5, 2012. 2. The first transit ever witnessed was that of December 4, 1639. The observer was a young man named Horrox. living in an obscure village near Liverpool, England. The table of Kepler, constructed upon the observations of Tycho Brahe, indicated a transit of Venus in 1631, but none was observed. Horrox, without much assistance from books and instruments, set himself to inquire into the error of the tables, and found that such a phenomenon might be expected to happen in 1639. lie repeated his calculations during this interval with all the carefulness and enthusiasm of a scholar ambitious of being the first to predict and observe a celestial phenomenon which, from the creation of the world, had never been witnessed. Confident of the result, he communicated his ex- pected triumph to a confidential friend residing in Manchester, and desired him to watch for the event, and to take observations. So anxious was Horrox not to fail of witnessing it himself, that he commenced his observations the day before it was expected, arid re- sumed them at the rising of the sun on the morrow. But the very hour when his cal- culations led him to expect the visible appearance of Venus on the sun's disk, was also t/'te appointed hour for the public worship of God, ontlie Sabbath, The delay of a few minutes might deprive him forever of an opportunity of observing the transit. If its very commencement were not noticed, clouds might intervene, and conceal it until the sun should set ; and nearly a century and a half would elapse before another opportunity would occur. He had been waiting for the event with the most ardent anticipation for eight years, and the result promised much benefit to the science. Notwithstanding all (Att, Horrox twice suspended his observation*, and twice repaired to the house of God, the great Author of the bright works he delighted to contemplate. When his duty wad chus performed, and he had returned to his chamber the second time, his love of scienco was gratified with full success, and he saw what no mortal eye had observed before. If any thing can add interest tp this incident, it is the modesty with which the young astronomer apologizes to the world for suspending his observations at all. " I observed it," says lie, ''from sunrise till nine o'clock, again a little before ten. and lastly at noon, and from on* to two o'clock ; the rest of the day being devoted to higher duties, which might not be neglected for these pastimes." 3. The transit of 1769 was observed with intense interest by astronomers in both hemi- spheres. To secure the advantages of observations at different points, Capt. Cook was 117. Node months of Venus ? Where line of nodes ? Why June and December her node months ? ( Why only one month after those of Mercury ?) 118. When last transit of Venus ? Next ? (How many have been ob- served ? What said of Kittenhous r - ' Webster? When next four transits of Venus ? When first transit notivx , . What said of it? That of 1769 use of observations ?) 64: ASTRONOMY ?ent to the Pacific in the bark "Endeavor," where he perished subsequently by the hands of savages at one of the Sandwich islands. Observations upon these transits fur- nish data for important astronomical calculations. 119. In consequence of the earth's annual revolution around the sun, he appears to travel eastward, through all the signs of the zodiac, every 365J days. It is this eastward motion of the sun that causes the stars to rise and set earlier and earlier every night. BUN'S APPAKENT MOTION AROUND THE ECLIPTIC. i^et a person walk around a tre, for instance, at a short distance from it, and it will appear to sweep around the horizon in an opposite direction. So as the earth revolves annually about the sun, the sun appears to traverse the circle of the heavens in the oppo- site direction. Suppose the earth is at A on the 20th of March"*, the sun will appear to be at B in the opposite side of the ecliptic. As the earth moves on in her orbit from A to C, the sun will appear to move from B to D ; and will seem thus to traverse the whole circle, of the heavens every 365.5: days, or as often as the earth revokes around him. The time of the sun's apparent entrance into the different constellations as he jour- neys eastward, is usually laid down in almanacs. Thus: "Sun enters T (Anes) 20th of March, &c.;" at which time the earth would enter the sign ^ (Libra), and the^ura would seem to enter the opposite sign Aries. 119. What said of sun's apparent motion ? Cause? Time of revolution ? Effect upon the stars? (Illustration from tree? By diagram.) What is n: eaut by the sun's entering Aries ? When 2 Where earth theiV / PRIMARY PLANETS. 65 CHAPTER II. PRIMARY PLANETS CONTINUED. 120. BESIDES the revolution around the sun, the planets all revolve rapidly about their respective axes, as they perform their celestial journeys. This is called their diurnal revolution. The evidences of the earth's revolution have already been considered on pages 13 and 14. That most of the other planets revolve has been ascertained by carefully observing the motions of spots, as they seemed to pass periodically over their disks. 121. The axis of the earth is inclined to the plane o^ the ecliptic 23 28'. It is always parallel to itself that is, it always inclines the same way, and to the same amount. INCLINATION OF THE EABTH'B AXIS TO THE PLANE OF THE ECLIPTIC. PLANE OP ^5!>k THE ECLIPTIC. /<5fi 1. The inclination of the earth's axis, and its parallelism to itself; are exhibited in the above cut, as also in the cuts, pages 50, 51, and &4, to which the student will do well to turn. 2. The author is aware that the poles of the earth have a slow motion around the pole of the ecliptic, requiring 25,000 years for a single revolution, but prefers to consider this point hereafter, in connection with the precession of the equinoxes. 122. The axes of all the planets are inclined more or less to the planes of their respective orbits. This incli- nation, so far as known, is as follows : Venus ... 75 Mars . 28 42 Jupiter ... 3 05' Saturn 26 50' 120. What revolution have the planets besides around the sun ? What called ? (What proof of the earth's revolution ? Of the other planets ?) 121. What said of the axis of the earth ? Of the stability of its inclina- tion ? (Is there no variation ?) 122. Are the axes of the other planets inclined ? To what extent, respect- ively? (Substance of note 1 ? Illustrate by diagram. Note 2 ?) ASTRONOMY. 1. The student will bear in mind that the above inclination is not to the eclipti<\ or plane of the earth's orbit, but to the plane of the orliits of the several planets respect- ively. Take the case of Venus, for instance : The orbit of Venus departs from the ecliptic 8, as stated at 108, while her axis is in- clined to the plane of her orbit 75, as shown in the above figures. This distinction should be kept definitely in view by the student. 2. The inclination of the axes of the several planets, each to the plane of its own or- bit, is represented in the following cut : INCLINATION OF THE AXES OF THE SEVERAL PLANETS TO THB PLANES OF THEIE CEBITS. , 123. The inclination of the earth's axis to the plane of the ecliptic causes the equinoctial to depart 23 28' from the ecliptic. This angle made by the equinoctial and the elliptic is called the Obliquity of the Ecliptic. OBLIQUITY OF THE ECLIPTIC. A jLe< the line A A represent the axis of the earth, and B B the poles or axis of the ecTip- Mc.. Now if the line A A inclines toward the plane of the ecliptic, or. in other words, departs from the line B B to the anvtm.i of 23 28', it is obvious that the plane of the 123. What effect has the inclination of the earth's axis upon the equinoc- tial ? What is the obliquity of the ecliptic ? (Illustrate by diagram.) EQUINOCTIAL AND SOLSTITIAL POINTS. 67 equator, or equinoctial, will depart from the ecliptic to the same amount This depart- ure, shown by the angles OC, constitute the obliquity of the ecliptic. ' 124. The permanent inclination of the earth's axis, and her revolution around the sun, cause first one pole to be enlightened and then the other, thus producing the sea- sons. The same inclination and revolution cause the sun to appear to oscillate from north to south, crossing the equator twice every year. This is called the surfs decli- nation. (See page 26.) This subject of the seasons will be sufficiently understood by examining the cuts on pages 64 and 65. 125. The equinoctial points in the earth's orbit are two points in opposite sides of the ecliptic, at which the sun is exactly in the equinoctial ; or, in other words, the plane of the equinoctial exactly cuts the sun's center. The first of these is passed on the 20th of March (the sun beginning then to decline northward), on account of which it is called the vernal equinox / and the other on the 23d of September, on account of which it is called the autumnal equinox. (See the earth at A and B, in the cut, page 64.) If the sun is vertical at the equator, be will, of course, shine to both poles, as repre- sented in the cut, and the days and nights will be equal all over the world. Hence the name equinoctial, from the Latin cequus, equal, and nvx, night. 126. The solstitial points are those points in the earth's orbit where the sun ceases to decline from the equinoc- tial, and begins again to return toward it. They are respectively 90 from the equinoctial points. The Summer Solstice is reached on the 21st of June, when the sun has the greatest northern declination, and it is summer in the northern hemisphere. The Winter Solstice is reached on the 23d of Decem- ber, when the sun has the greatest southern declination, and it is summer in the southern hemisphere, and winter m the northern. (See the earth at E F, cut, page 64.) 124. "What other effects from the inclination of the earth's axis ? Sun's declination ? 125. What are the equinoctial points f How distinguished, and why ! When passed ? (Substance of note ?) 126. The solstitial points ? How far frcm the equinoctial points ? How distinguished ? When passed ? 68 ASTRONOMY. 127. The amount of the sun's declination north and south of the equinoctial is 23 28' ; answering to the in- clination of the earth's axis, by which it is caused, and marking the limits of the tropics upon the earth's surface. 1. On the 21st of June the sun reaches his greatest northern declination, or Summer Solstice, and is vertical on the Tropic of Cancer. From this tim^ he approaches the equator of the heavens till the 20th of September, when he crosses it, and begins to de- cline southward. On the 23d of December he has reached his greatest southern declination, or Winter SHADOWS AT THE EQUATOR. Solstice, and begins to return toward the, equinoctial, which he passes on the 20th of March, and reaches his Summer Solstice again on the. 21st of June. In this manner he continues to decline, first north and then south of the equator, from year to year. But it should not be forgotten that the sun does not really move, ^ first north and then south, but that the apparent mo- g tion is caused simply by the inclination of the earth's ^/' axis and her revolution around the sun. s / 2. The sun's declination may be easily measured by the shadow of a suitable object upon the earth's surface. Suppose the flag-staff in the cut to stand / perpendicularly, and exactly on the equator. On ^ the 23d of December the shadow would be thrown C B A northward to A, or 23 28' just as far as the sun has declined south. At 12 o'clock, on the 20th of March, and the 23d of September, there would be no shadow; and on the 21st of June, it would extend southward 23 28' to C. Thus, at the equator, the shadow falls first north and then south of all perpendicular objects, for six mouths alternately. 23-28' MEASURING THE SUN 8 DECLINATION IN NORTHERN LATITUDE. 3. This cut shows how the student may measure the sun's declination wherever lie ttiaybs located north of the equator. The shadows are such as are cast by objects during the year, about 45 north of the equator. On the 23d of December, when the sun has his greatest declination, the shadow of the flag-staff extends north at 12 o'clock to the point C, where two boys are seen, having just driven down a stake. From this time to June 21st the shadow gradually shorten*, till on that day it reaches the point B, where another stake is driven. It then begins to elongate, and in six months is extended to C again. The point A is just half-way from B to C in angular measurement, though the distances on the plain in the picture are very different When the sun is on the equator, March 21st and September 23d, the shadow will reach only to A ; and the angle A B and the top of the statf shows the northern, and A C and the top of the staff the southern declination. It will be found to be 23 2S' each way, as marked in the figure. 127. To what extent does the sun decline from the equinoctial north and south? Why not more? (Substance of note 1 ? Note 2, and explain by liagiam. Note 3, and diagram. What is a gnomon f) ROTATION OF THE PLANETS UPON THEIR AXES. 69 4. The angle formed by the top and bottom of the pole and the point A will exactly correspond with the latitude of the place where the experiment is made. 5. Let the students try this matter for themselves. Select a level spot, and put up a stake, say ten feet high. Get an exact "noon mark," or north and south line, whore the stake is driven, and at 12 o'clock, every fair day, put down a small stake at the end of the shadow. In this manner you will soon be able to measure the sun's declination for yourselves, to determine the latitude of the place where >ou live, and to understand how mariners at sea ascertain their latitude by the declination of the sun. 6. The ancients had pillars erected for the purpose of making observations upon their shadows. Such a pillar is called a gnomon. ROTATION OF THE PLANETS UPON THEIR AXES. 128. The time, so far as known, of the revolution of the planets upon their respective axes, or, in other words, the length of their natural days, is as follows : h. m. Mercury ... 24 5 Yenus ... 23 21 Earth .... 24 00 Mars , 24 37 h. m. Jupiter . . . 9 56 Saturn . . . 10 29 Uranus ... 9 30 Neptune . . Unknown These statistics are given upon the authority of Sir John P. W. Herschcl, though he marks Juno and Uranus as doubtful. 129. The revolution of the earth upon its axis is the cause of the agreeable vicissitudes of day and night. PHILOSOPHY OF DAT AND NIGHT. How wisely adapted to the happiness of His creatures are all the works of God ! The night prepares us for the day, and the day in turn prepares us to welcome the night; and in both instances the change ministers to the happiness of man and beast. Andbut for being carried around into the darkness of the earth's shadow, we should never have admired the dazzling firmament, as it declared the glory of God, and showed forth his handiwork. How beautiful the poetic allusion to the revealing power of night I Mysterious Night ! when our first parent knew Thee, from report divine, and heard thy name, Bid he not tremble for this lovely frame, This glorious canopy of light and blue ? Yet, 'neath a curtain of translucent dew, Bathed in the rays of the great setting flame, Hesperus with the host of heaven came ; And lo ! creation widened in men's view. 128. In what time do the other planets rotate on their respective axes ! f^ote?) ' 129. Cause of day and night ? (Substance of note ? Poetic quotation ?) 70 ASTRONOMY. Who could have thought such darkness lay conceal'd Within thy beams, O Sun ! or who could find, Whilst fly, and leaf, and insect stood revealed, That to such countless orbs tliou uiad'st us blind ? Why do we, then, shun death with anxious strife? If light cau thus deceive us, may not life ? 130. The earth and all the other planets revolve eastward upon their axes, or in the same direction in which they revolve in their orbits. This also is determined (with the exception of the earth) "by observing the motion of spots upon their surfaces, by the aid of telescopes. 1. In the cut we have an arc of the earth's orbit, and the earth revolving on her axis as she revolves around the sun. The arrows show the direction in both cases. 2. By holding the book up south of him, and looking at- tentively at the cut, the student will understand why the sun "rises' 1 or first appears in the east. It is because the earth revolves eastward. Thus the observer at A is carried round into the light, ami Bees the sun rise when he reaches B. TIME. 131. Time is duration measured either by natural or artificial means.* The principal natural indicators of the lapse of duration are the revolution of the earth upon its axis, marking a natural day ; the change of the moon, denoting a lunar month ; and the cycle of the seasons, denoting a year. Time is measured artificially by clocks, watches, chronometers, dials, &c. ; the standard being the solar day still, which is divided artificially into 24 parts, called hours, and these again into minutes and seconds. The aboriginal tribes of this country all reckoned time by "moons," or months, r" denoted by the moon's changes. 132. The motion of the earth upon its axis is the most regular of which we have any knowledge. It does not vary one second in a thousand years. To this stability of the earth's motion upon her axis the prophet refers when he says: ' Thus tiaitii the Lord, If ye can break my covenant of the day, and my covenant of the 130. In what direction do the planets rotate on their axes ? How ascer- tained ? (Explain why the sun appears to.rise in the east.) 131. What is timer What natural standards ? Artificial? (How meas- ured by aborigines ?) 132. What said of earth's motior on axis? (What reference to in Scriu- TIME. 71 ar.dthat there should not be day and nieht in their seasons, then may also my cc reliant be broken with David," &c.Jeremiah xxxiii. "20. 133. Time is of two kinds Solar and Sidereal. A solar day is the time elapsing from the sun's crossing the meridian of any place, to his coming to the same me- ridian again. A sidereal day is the time intervening between the transit of a star across the meridian, to its coming to the same meridian again. 134. A solar day consists of 24 hours, at a mean rate, but a sidereal day is accomplished in 23 hours, 56 min- utes, and 4 seconds ; the solar day being nearly 4 minutes longer. This slight difference of about 4 minutes daily, between solar and sidereal time, amounts to one whole day in every 365J days. Owing to the revolution of the earth around the sun, and his apparent annual revo- lution eastward among the stars, it requires 366 revolu- tions of the earth, as measured by the fixed stars, to make 365-J- days, as measured by the sun. 135. The cause of this difference in the apparent revo- lutions of the sun and stars, and consequent difference in the length of a natural day, as measured by the passage of a star or of the sun across the meridian, is this : The earth is constantly advancing in her orbit while she re- volves on her axis, causing the suii to appear to move slowly eastward among the stars ; or, what is the same thing, the stars to appear to rise earlier and earlier every night, and one after another to overtake and pass by the sun. (See Article 119.) When, therefore, the meridian is brought around to that point in the heavens where the sun was 24 hours before, he is not there, but has moved a little eastward. But a star that, 24 hours before, was exactly behind the center of the sun in the distant heavens, will be found west of the sun, and will conse- quently cross the meridian before the sun does. The time required for the meridian to revolve from the star to the sun constitutes the 3 minutes 56 seconds difference between solar and sidereal time. 133. Kinds of time ? Define oncli. 14. Length of solar day ? Sidereal? Difference? Amount in year ? 185. State the cause of the difference in the time of the apparent revolutioo of the sun uud stars. Illustrate hy diagram. 72 ASTKONOMY. BOLAE AND SIDEREAL 7IMK. v Q _______ ___ _____ SIDEREAL DAY SUN ON THE MERIDIAN 1. To the man at A the sun (S) is exactly on the meridian, or it is twelve o'clock, aoon. The earth passes on from B to D, and at the same time revolves on her axis. When she reaches D, the man who has stood on the same meridian has made a complete revolution, as determined by the star G (which was also on his meridian at twelve o'clock the day before) ; but the sun is now east of the meridian, and he must wa.it/our minute* for the earth to roll a little further eastward, and bring the sun again over his north and south line. If the earth were not revolving around the sun, her solar and sidereal days would be the same ; but as it is, she has to perform a little more than one complete revo lution each solar day, to bring the sun on the meridian. EQUATION OF TIME. 136. As the distant stars have no motion, real or ap- parent, around the ecliptic, and the earth's motion upon it is uniform, it results that sidereal time is always exactly the same. A clock that keeps sidereal time is called a sidereal do^Jc. One of these instruments is almost indispensable in the observatory of the astronomer. 137. Solar time is constantly varying. No two suc- cessive solar days are exactly of a length. The 24 hours given as the length of a solar day (134) is the average of all the solar days throughout the year. Hence it is called mean solar time. The time, as indicated by the transit of the sun across the meridian, from day to day, is called apparent time. 138. A well-regulated clock will keep mean solar time, and will vary from the apparent time (as indicated by a noon mark, or dial) to the amount of 16J- minutes one way, and 14J the other. The sun will at one time cross the meridian 16J minutes before it is noon by the clock the apparent time being 16 J minutes faster than mean or clock time ; while at another time it will be noon by the clock 14 J minutes before it is noon by the sun. 136. Is sidereal time always the same ? Why must it be? (What is a nidereal clock ?) 137. What said of the variations of solar time ? What is mean solar time ? Apparent ? 138. What time do ooumion clocks keep ? How much variation from sun ? Cow? EQUATION OF TIME. 73 139. The difference bet ween 'apparent and mean solar time is called the Equation of Time. It is greatest about the 3d of November, when the clock is 16 minutes and 17 seconds behind the sun. Four times a year viz., April 15th, June 15th, September 1st, and December 23d the clock and sun will agree; or, in other words, mean and apparent time will be alike. 140. The inequality of the solar days depends upon two causes the unequal velocity of the earth in her orbit (77, 78), and the inclination of her axis to the plane of her orbit (123). 141. If the earth's crbit were an exact circle, she would move with the same EQirAL 80 LAK DATS. velocity in all parts of it; and if she revolved with regularity upon her axis, * her solar days would be - exactly of a length. ^ Let the circle in the adjoining cut rep- / resent the earth's orbit, and the projec- >^ tions from the earth toward the sun a ; pillar or gnomon standing upon a given )it~- meridian. The cut will then show that ^^ with a circular orbit, and uniform motion isy in it, and a regular rotation upon her 1p axis, the earth would bring the gnomon V around toward the sun at regular inter- ^ viils. both of distance in her orbit, and of time. In that case, all apparent solar days would be equal. 142. As the orbit of the earth is elliptical, it requires more time for the earth to pass from the vernal equinox, through the aphelion, to the autumnal equinox, than it does from the autumnal equinox, through the perihelion, to the vernal equinox. The difference is about eight days the sun being north of the equinoctial about eight days longer than he is south of it. Hence the summers of the northern hemisphere are longer than the winters. 143. As the earth's orbit is an ellipse, and the earth 139. What this difference called ? When greatest? When no difference J 140. What causes the inequality in the length of the solar days ? 141. What necessary in order that they may be equal"? (Illustrate by dia- gr;nn nn90 " Interior do 26"-6(>S = 117,339 " Equatorial diameter of the body 17" -991 = 79,160 " Interval between the planet and interior ring 4" -339 = 19.090 " Interval of the rings 0"'408 = 1,791 " Thickness of the rings not exceeding . ... 250 " 187. The rings of Saturn serve as reflectors to reflect the light of the sun upon, his disk, as our moon reflects the light to the earth. In his nocturnal sky, they must appear like two gorgeous arches of light, bright as the full moon, and spanning the whole heavens like a stupen- dous rainbow. In the annexed cut, the beholder is supposed to be situated some 30 north of the equator of Saturn, and looking directly south. The shad- ow of the planet is seen travelling up the arch as the night advances, while a New Moon is shown in the west, and &FuftA[oon in the east at the same time. 188. The two rings united are nearly 13 times as wide as the diameter of the moon ; and the nearest is only T ^th as far from the planet as the moon is from us. 1. The two rings united are 27,500 miles wide; which -f- 2, 160 the moon's diame- ter 12^. So 240,00u miles, the moon's distance -7- 19,000 the distance of Saturn's in- terior ring = 12|-f. 2. At the distance of only 19,000 miles, our moon would appear some forty times as large as she does at her present distance. How magnificent and inconceivably grand, then, must these vast rings appear, with a thousand times the moon's magnitude, and only one-twelfth part of her distance 1 186. State the distances and dimensions of his rings, beginning at the body of the planet, and passing outward. (What additional statistics from Her- bchel?) 187. What purpose do the rings of Saturn serve? How appear in his evening sky ? 188. Width of two rings, as compared with moon ? Distance ? (Demon- strate both. How would our moon appear at the dist-ince of Saturn's rings 1) NIGHT SCENE UPON SATURN. 94 ASTRONOMY. 189. Besides the magnificent rings already described, the telescope reveals eight satellites or moons, revolving around Saturn. But these are seen only with good in- struments, and under favorable circumstances. On one occasion, the writer saw five of them at once, with a six-inch refractor manu- factured by Mr. Henry Fitz, of New York ; but the remaining three lie has never seen. For a further description of these satellites, see chapters on the Secondary Planets. 190. The periodic time of Saturn being nearly 30 years (72), his motion eastward among the stars must be very slow, amounting to only 12 a year, or one sign in 2^ years. It will be easy, therefore, having once ascer- tained bis position, to watch his slow progress eastward year after year. Saturn is now (October, 1852) about 15 west of the seven stars, and consequently will pass them eastward early in 1854. UKANUS. 191. Uranus is scarcely ever visible except through a telescope ; and even then we see nothing but a small round uniformly illuminated disk, without rings, belts, or discernible spots. His apparent diameter is about 4", from which he never varies much, owing to the small- ness of our orbit in comparison with his own. Sir John Herschel says he is without discernible spots, and j r et in his tables he lays down the time of the planet's rotation (which could only be ascertained by the rotation of spots upon the planet's disk), at 9i hours (1'28). This time is probably given on the authority of Bchroeter, and is marked as doubtful by Dr. Herschel. 192. The motion of Uranus in longitude is still slower than that of Saturn. His periodic time being 84 years 27 days, his eastward motion can amount to only about 4 17' in a whole year. To detect this motion requires instruments and close observations. At this date (1853) Uranus has passed over about - 8 of his orbit, since his discovery in 1781 ; and in 1865 will have traversed the whole circuit of the heavens, and reached the point where Herschel found him 84 years before. 189. What else seen about Saturn? When seen? (Observations of tho author.) 190. Motion of Saturn eastward ? Rate ? 191. How Uranus seen? How appear through telescopes? Apparent diameter ? Why so small, when so much larger than Venus ( Why so little variation ? (Remark respecting spots.) li)2. What said of Uranus' apparent motion? Rate per year? In 1853 how far since discovered 2 When made a complete revolution since 1781 ? THE SOLAR SYSTEM IN MINIATURE. 05 193. Uranus is attended by several satellites four at least, probably five or six. Sif William Herschel reckoned six, though no other observer has confirmed this epinion ; and even his son, Sir John Herschel, seems to consider the existence of six satellites quite doubtful. NEPTUNE. 194. Neptune is a purely telescopic planet, and his immense distance seems to preclude all hope of our coming at much knowledge of his physical state. A single satellite has been discovered in attendance upon him, and the existence of another is suspected ; but ii others exist, they are as yet undetected. 195. On the 3d of October, 1846, Mr. Lassell, cf Liverpool, England, supposed he had discovered a ring about the planet, similar to the rings of Saturn ; but this supposition has not yet been confirmed by th^ observa- tions of other astronomers. 196. The periodic time of Neptune being 164 years 226 days, his motion in longitude amounts to only about 2 10' per year; and yet this slow motion of about 21" per day is easily detected, in a short time, by the rid of the proper instruments. It is by this motion, as w^ll as by the disk which it exhibits under the telescope, that the object was first distinguished from the fixed stars, and recognized as a planet. THE SOLAR SYSTEM IN MINIATURE. 197. Choose any level field or howling-green, and in its center place a globe two feet in diameter, to represent the sun. Mercury may then be represented by a mus- tard-seed, at the distance of 82 feet ; Venus by & pea, at the distance of 142 feet ; the earth aluo by a pea, at the distance of 215 feet. A large pin's head would repre- sent Mars, if placed 327 feet distant; svod the A.steroids may be represented by grains of sand. fr?ra 500 to 600 193. Attendants of Uranus ? How many ? (Remark .n note !) 194. How Neptune seen? What attendant ? SUS-PIUKU t J 9n. Supposition of Lassell ? Is it confirmed ? 196. Motion of Neptune per year '{ Why so slow ? ^ 't hp detected 197. What representation of' the solar system? Size Oi' doo 1 Merc!iry. 96 ASTRONOMY. feet from the center. A moderate sized orange, would rep- resent Jupiter, at the distance of 80 rods, or 1,320 feet ; while a smaller orange would represent Saturn, at the distance of 124 rods, or 2,046 feet. Place a full-sized cherry or small plum three-fourths of a mile distant for Uranus, and another a mile and a quarter distant for Neptune, and you have the solar system in miniature. 198. To imitate the motions of the planets in their orbits, in the above illustration, Mercury must move to the amount of his own diameter in 41 seconds ; Yen us, in 4m. 14s. ; the earth, in 7m. ; Mars, in 4m. 48s. ; Jupi- ter, in 2h. 56m. ; Saturn, in 3h. 13m. ; Uranus, in 2h. 16m. ; and Neptune, in 3h. 30m. CHAPTER IV. SEASONS OF THE DIFFERENT PLANETS, ETC. 199. The general philosophy of the seasons has already been explained (Art. 119 to 125). The inclination of the axis of a planet determines the extent and character of its zones / and the length of its periodic time determines the length of its seasons. Thus the axis of the earth being inclined toward the ecliptic 2-3 28', the tropics fall 23 28' from the equator, and the polar circles 23 2S' from the poles; and the period of the earth's revolution around the sun being 365^ days, it follows that each of the lour seasons must include about three months, or 91 days on an average. It the axis were wore inclined, the tropics would full further from the equator, and the polar circles fur- ther from the poles, so that the torrid and frigid zones would be wider, and the tem- perate narrower; and if the earth's period were longer, her seasons, respectively, would be longer. 200. The general temperature of a planet is probably governed by its distance from the sun (59, 60) ; but the temperature of any particular portion of a planet de- pends mainly upon the directness or obliquity with which and where placed I Venus, what and where ? Earth ? Asteroids ? Mars &c. 198. How imitate the motions of the several planets ? 199. What determines the extent and character of a planet's zones ? What tLe length of its seasoua ? (Illustrate by inclination and period of the earth.) SEASONS OF THE DIFFERENT PLANETS, ETC. 97 the rays of light fall upon it a circumstance that greatly affects the amount of light received by any given por- tion of its surface. Hence we have summer in the northern hemisphere in July, when the earth is farthest from the sun ; and winter in January, when she is near- est the sun (144). Though nearer the sun in January than in July, still, as the northern hemisphere is then inclined from the sun, his rays strike its surface obliquely ; less light falls upon the same space than if its contact w T ere mwe direct, and it is consequently cold. But in July, the rays are more direct the northern hemisphere being inclined toward the sun and it is summer, notwithstanding we are three millions of miles further from the sun than in January. 1. The comparative amount of light received in the northern hemisphere In July and January may be illustrated by the accompany- ing figure, in which the rays of light at dif- BUMMER AND WINTER BATS. ferent seasons are represented to the eye. In January, they are seen to strike the northern hemisphere obliquely, and consequently the same amount of light is spread over a much greater sur- face. In July, the rays fall almost perpendicularly upon us, and are much more intense. Hence the variations of temperature which constitute the seasons. 2. If the student is not perfectly clear as to how . the north pole is turned first toward and then from tlie sun, he will need to be guarded against the vulgar idea that the earth's axis " Wabbles," as it is called. By consulting 119 to 121, and the cuts, it will be seen that the very permanency of a plan- et's axis, combined with its periodic revolution, gives the beautiful and ever welcome changes of the seasons. How simple, and yet how effectual, this Divine mechanism ! 201. As the inclination of the axis of a planet and the length of its periodic time determine the extent and character of its zones, and the length of its seasons, it- follows that where these are known, we have a reliable clew to the seasons of a planet, even though we have neither visited nor heard from it ; and as we do not know the inclination of the axis of Mercury, we have no knowledge of his seasons. 200. What governs the general temperature of the planets ? The tem- perature of particular zones ? What result from this laGB=SS OF THE MOON EASTWARD. motion of the moon east- ward is 13 10' 35". Her average hourly motion is about 32,300 miles. This motion may be detected by watching her for a lew/ hours only ; and by markets t ing her position, with ref- ence to the stars, from night to night, her daily journeys will appear pro- minent and. striking. The estimate of 13 10' 35" is made for a sidereal day of twenty-four hours. In the above cut, the daily progress of the moon may be traced from her conjunction or " change" at A on the right, around to tho suine point again. This being a sidereal revolution, requires only^27 days. 19 * 223. Daily angular motion eastward ? How detected ? (For what day is Ibis estimate made ?) THE MOON. 107 224. In her journey ings eastward, the moon often seems to run over and obscure the distant planets and stars. This phenomenon is called an oc- cidtation. The adjoining cut represents the new moon as just atxuit to obscure a distant star, by passing be- tween us and it. In 1850, she occulted Jupiter for three revolutions in succession viz_, Jan. 3uth, Feb. 27th, and March 26th. Through a telescoi>e, tlie uioon is seen to be constantly obscuring stars that are invisible to the naked eye. They disappear be- hind the moon's eastern iimb, and in a short time reappear from behind her western ; thus distinctly exhibiting her eastward motiwn. 225, Though the moon's orbit is an ellipse, with res- pect to the earth, it is, in reality, an irregular curve, always concave toward the sun, and crossing the earth's orbit every 13 nearly. 1. If the earth stme 46 millions of miles. < near UK) times tlie diameter of the moon's orbit, during a single lu- nation, it is evident that the moon's orbit never can return into itselfj or retrograde, as here rep- resented. THE MOON 8 ORBIT ALWAYS CONCAVE TOWARD THE SUN. 3. That the lunar orbit is always concave toward the sun, may be demonstrated by he above diagram. Let the upper curve line A B represent an arc of the earth's orbit, equal to that passed through by the earth during half a lunation. Now the radius and arc being known, it is found that the chord A B must pass more than 400,000 miles within the earth. But a* the moon departs only 240,000 from the earth, as shown in the figure, it follows that she must describe the curve denoted by the middle line, which is concave toward the sun. 224. What are occit 7 totwn.s ? How produced ? (Are they frequent? Are planets ever occulted? Describe process.) 225. What is the form of tlie moon's orbit with respect to the earth ? The sun ? (How if the eurth were stationary ? If moving slowly ? Demon- strate her orbit to be concave, &c. Draw orbit for complete lunation, an* 3 describe her relative motion.) 108 ASTRONOMY. 4 This subject may be still further illustrated by the following cut, representing THE MOON'S PATH DURING A COMPLETE LUNATION. C - B MOON'S PATH. Here the plain line represent? the earth's orbit, and the dotted one that of the mo>!i. At A the moon crosses the earth's track 240,000 miles behind her. She gains on the cartii, till in seven days she passes her at 1> as a Full Moon. Continuing to gain oa the eartli, she crosses her orbit at 0, 240,000 miles ahead of her, being then at her Third Quarter. From this point the earth gains upon the moon, till seven days afterward she overtakes her at D as a New Moon. From D to E the earth continues to gain, till at E the moon crosses 240,000 Behind the emal! circles in the cut represent the moon's orbit with respect to the earth, which is as regular to ux as if the earth had no revolution around the snn. 226. The moon never retrogrades on the ecliptic, or returns into her own path again ; but is always ad- vancing with the earth, at the rate of not less than 65,700 miles per Mrhe moon's orbitual Telocity, with respect to theearth, is about 2,300 miles per hour. When out- Bide the earth, as at B, in the last figure, she gains 2,300 miles per hour, which, added to the earth's ve- locity, would give 7o,300 miles as the hourly velocity of the moon. When within the earth's orbit, as at I), she loses 2,300 miles per hour, which, subtracted from 63,000 miles (the earth's hourly velocity), would leave 65,700 miles as the slowest motion of the moon in space, even when she is falling behind the earth. 2. Could we look down perpendicularly upon the ecliptic, and see the paths of the earth and moon, we should see the latter pursuing her serpentine course, first within and then outside our globe, somewhat as represented by the dotted line in the annexed figure. Iler path, however, would be concave toward the sun, as shown on the preceding page, and not convex, as we were obliged to represent it hero in so small a diagram. 227. That the moon is opake, like the rest of the plan- ets, and shines only by reflection, is obvious, from the fact that we can see only that part of her upon which the sun shines ; and as the enlightened portion is some- times toward and sometimes from us, the moon is con- stantly varying in her apparent form and brightness. These variations are called her phases. 226. At what rate docs the moon advance with the earth 1 Moon'a or bitual velocity, with respect to the earth \ Slowest motion \ (Illustrate the moon's course.) 227. What proof that the moon is opake 1 What meant by her phases f THE MOON. 109 228. The cause of the moon's phases her waxing and waning is her revolution around the earth, which ena- bles us to see more of her enlightened side at one time than at another. CAUSE OF THE MOON'S PHASES 4 1. This cut represents the moon revolving eastward around the earth. In the outside circle, she is represented as she. would appear, if viewed from a direction at right angles with the plane of her orbit. The side toward the sun is enlightened in every case, and she appears like a half moon at every point. 2. The interior suit represents her as she appears when viewed from the earth. At A it is New Moon ; and if seen at all so near the sun, she would appear like a dark globe. At B she would appear like a crescent, concave toward the east. At 0, more of her enlightened side is visible ; at D. still more ; a.nd at E, the enlightened hemisphere is fully in view. We then call her' a Full Moon. From E around to A again, the dark portion becomes more and more visible, as the luminous part goes out of view, till she comes to her change at A. When at I) and F, the moon is said to be gibbous. 3. If the student will turn his book bottom upward, and hold it south of him, he will pee -ucJiy the crescent of the old moon at II is concave on the west, instead of the east, like the new moon, and why she is seen before sunrise, instead of just after sunset 229. The cusps of the moon are the extremities of the crescent. Her syzygies are two points in her orbit 180 apart, where she is new and full moon. (See positions 1 and 3 in the last cut.) The quadratures are four points 90 apart (like 1, 2, 3, and 4 in cut) ; and her octants eight points 45 apart (like A, B, C, &c., in the cut). 230. The moon is said to change when she comes in conjunction with the sun, and is changed from Old Moon to New Moon. 228. Cause of phases ? (Illustrate.) 229. What are the cnxps of the moon? Her Syzygies ? Quadratures ? Oc- tants? (Illustrate on blackboard.) 230. What meant by the ckan,c 234. What are these rude figures supposed to be? (INot< 235. "What interesting fact established by watching the n lows from it ? (illustrate by sketch, of cut on blackboard.) (Note.) moon ? What fol- 112 ASTRONOMY. 236. As the same side of the moon is always toward us, it follows that the earth is invisible from one-half of the moon. From the other half, our globe would appear like a stationary planet, nearly thirteen times as large as the moon appears to us, and exhibiting all her varying phases. 237. Though the moon always presents nearly the same hemisphere toward the earth, it is not always precisely the same. Owing to the ellipticity of her orbit, and the conseqi ent inequality of her angular velocity, she ap- pears to roll a little on her axis, first one way and then the other thus alternately revealing and hiding new territory, as it were, on her eastern and x western limbs. This rolling motion east and west is called her libration in longitude. The accompanying cut will illustrate the sub- ject of the moon's librations in longitude. 1. From A around to C, the angular motion is slmcer than the average, and the diurnal motion gains upon it, so that the pillar points wcxtof the earth, and we see more of the eastern limb of the moon. 2. From C to A, again, the moon advances faster than a mean rate, and gains upon the V jy diurnal revolution; so that the pillar points ea-fft ^L/-^. / of the earth, and we see more of the moon's "--..*- jrk western limb. Thus she seems to lihrate or roll, laj first one way and then the other, during every periodic revolution. At B, we see most of her eastern limb ; and at D, most of her western. 238. The axis of the moon is inclined to the plane of her orbit only about one and a half degrees (1 30' 1OS"). But this slight inclination enables us to see first one pole and then the other, in her revolution around the earth. These slight rolling motions are called her librations in latitude. As the inclination of the earth's axis brings first one pole and then the other toward the sun, and produces the seasons, so the inclination of the moon's axis brings first one pole and then the other in view from the earth. But as her inclination is only Ij , the libration in latitude is very slight. 236. What other fact follows from the moon's keeping the same side toward us ? How would our globe appear from the moon \ 237. What are the moon's Vibrations f In longitude, and cause? (Illus- trate on blackboard.) 238. In latitude? Cause ? (Illustrate bv the case of the earth.) THE MOON TELESCOPIC VIEW. 113 TELESCOPIC VIEW OF THE MOOW. 239. The moon's year consists of 29 J of our days ; but as she makes but one revolution upon her axis in that time, she can -have but one day and one night in her whole year. And so slight is the inclination of her axis to the plane of her orbit, that the sun's declination from her equator is only about 1-|. She must therefore have perpetual winter at her poles ; while at her equator, her long days are very warm, and her long nights very cold. 240. By the aid of the telescope, the surface of the moon is found Jto be exceedingly rough and uneven, cov- ered with vast plains, deep valleys, and lofty mountains. Several of the latter are from three to four and a half miles high. That they are really mountains is proved by three facts : 1st, the line of the terminator is jagged or uneven, as shown in the cut ; 2d., shadows are seen pro- jecting first to the east and then to the west, showing the existence of elevations of some sort, that intercept the light ; and 3d, from new to full moon, bright spots break out from time to time, just east of the ter- minator, in the dark portion, and grow larger and larger, till they join the illumi- nated portion, show- ing them to be the tops of mountains, which reflect the sun- light before it reaches the intervening val- leys. 1. Specimens of these s?iad- ows may be ^een in the cut. pro- jecting to the leit. Bright points of light, or, in other words, the illuminated tops of mountains, may also be seen near the terminator, in the dark portion. The writer has often watched them, and seen them enlarge more and more, as the sun arose upon the side of the moon toward us, and enlightened the sides of her mountains. 239. Length of moon's year ? Number of natural days ? Sun's declina- tion upon her ? Climate at equator and poles ? 240. How appear through telescopes? What proof of mountains? (Be- n>arks upon cut ? Observations of the author ? Describe shadows* and thei r chui ge& Illustrate, by reference to the Andes and their shadows.) 114: ASTRONOMY. 2. The shadows are always projected in a direction opposite the sun, or toward the dark side of the moon ; and as her eastern limb is dark from tup change to the full, and her western from the full to the change, of course the direction of the shadows must be reversed. 3. Suppose a person stationed t;t a distance directly over the Andes. Before the 6ii n arose, he would see the tallest peaks enlightened; and as he arose, the long shadows of the mountains would extend to the west. At noon, however, little or no shadow would be visible; but at sunset, they would again be seen stretching away to the eaxt. This is precisely the change that is seen to take place with the lunar shadows, except that the time, required is a lunar day, equal to about 15 of our days, instead of one of our days of 12 hours. 241. Some of the lunar mountains are in extensile ranges, like our Alps and Andes; while others are cir- cular, like the craters of huge volcanoes. Great num- bers of the latter may be seen with telescopes of only moderate power. Through such an instrument, the moon will appear of a yellowish hue, and the circular moun- tains like drops of thick oil on the surface of water. Two extensive ranges, and several of the circular elevations, are shown in the last cut. Dr. Scoresby, of Bradford, England, who examined the moon through the monster telescope of Lord Rosse, says he saw a vast number of extinct volcanoes, some of whose craters were several miles in breadth. Her general appearance was that of a vast ruin of nature. Dr. Herschel supposed he saw the light of several active volcanoes upon her surface. 242. In regard to the existence of an atmosphere around the moon, astronomers are divided. From obser- vations during ellipses of the sun, and other phe- nomena, it is thought that if the moon has any atmos- Ehere at all, it must be very limited in extent, and far iss dense than that of the earth. Dr. Scoresby saw no indications of the existence of water, or of an atmos- phere. From observations during several occultations of stars, the writer is of opinion that a refracting medium of some sort exists in the vicinity of the moon. The atm. What use made of the moon in navigation? Explain the process* What called ? What otl:er method for determining longitude ? 116 ASTRONOMY. the Khip is west of Greenwich, as the moon appears east of her Greenwich place. From this difference between her place as laid down in the tables, and her observed place, a3 referred to certain prominent stars, the manner determines how far he is east or west of tlie meridian of Greenwich. The moon's geocentric place (or place, as viewed from the center of the earth) may be given instead of her Greenwich place, and the same conclu- sions arrived at In either case, this is called the lunar method of determining the longitude. It is also ascertained by simple comparison of local and standard time, as explained at 151. 246. The best time for observing the inoon with a tele- scope is from the change to the first quarter, and from the third quarter to the change. Near the first and third quarters, the shadows of objects are seen at right angles with the line of vision, and to the best advantage ; w T hile at full moon, objects cast no shadows visible to us. CHAPTER VI. ECLIPSES OF THE SUN AND MOON. 247. An Eclipse is a partial or total obscuration or darkening of the sun or moon, by the intervention of some opake body. Eclipses are either solar or lunar. A solar eclipse is an eclipse of the sun, and a lunar eclipse is an eclipse of the moon. A solar eclipse is caused by the moon, when she passes between the earth and the sun, in her revolution eastward, and casts her shadow upon the earth. A lunar eclipse takes place when the moon is in opposition to the sun, and passes through a portion of the earth's shadow. The general law of shadows may be illustrated by the following : Here the sun and planet are represented as of the same size and the shadow of the latter is in the form of a cylinder. 246. "When is the best time for viewing the moon with a telescope ? Why ? 247. What is an eclipse? A solar ? Lunar? Cause of solar eclipses ? Of lunai ? When do lunar eclipses take place ? (Illustrate the laws of shadows AY Oiugram on blackboard.) ECLirSES OF THE SUN AND MOON. 117 In this cut, the opake body is the larger, and the shadow projected from it diverges r grows more broad as the distance from the planet increases. Here the luminous "body is tJie larger, and the shadow converges to a point, and takei the form of a cone. Here, also, the luminous body is the larger, and both precisely of the same size as in the cut preceding; but being placed nearer each other, the shadow is shown to be con- siderably shorter. 24:8. All the planets, both primaries and secondaries, cast shadows in a direction opposite the sun (see the adjoining cut) The form and length of these shadows depend upon the comparative magni- tude of the sun and planet, and their dis- tance from each other. If the sun and a planet were of the same size, the shadow of the planet would be in the form of a cylinder, whatever its distance. If the planet were Larger than the sun, the shad- ow would diverge, as we proceed from the planet off into space; and the nearer the sun, the more divergent the shadow would be. 248. What said of the shadows of the planets ? Of their/own and length f [low would it be if the sun and planet were of the same size ? If the planet 118 ASTRONOMY. But as all the planets are much smaller than the sun, their shadows all converge to a point, and take the form of a cone and the nearer to the sun, the shorter the shadow. These principles are partly illustrated in the preceding cut. The planets nearest the sun have comparatively short shadows, while those more remote extend to a great dis- turice. No primary, however, casts a shadow long enough to reach the next exterior planet 249. Eclipses of the sun must always happen at New Moon, and those of the moon at Full Moon. The reason of this is, that the moon can never be between us arid the sun, to eclipse him, except at the time of her change, or new moon ; and she can never get into the earth's shadow, to be eclipsed herself, except when she is in op- position to the sun, and it is full moon. 250. If the moon's orbit lay exactly in the plane of the ecliptic, she would eclipse the sun at every change, and be eclipsed herself at every full / but as her orbit departs from the ecliptic over 5 (216), she may pass either above or below the sun at the time of her change, or above or below the earth's shadow at the time of her full. NEW AND FULL MOONS WITHOUT ECLIPSES. Shadow abort the Earth. Above the Earth's shadow. Shadow below the Earth. Below the Earth's shadow t. Let the line joining the earth and the sun represent the plane of the ecliptic. Now as the orl)it of the moon departs from this plane about 5 0', she may appear either above or below the sun at new moon, as represented in the figure, and her shadow may fall above the north pole or below the south. At such times, then, there can be no solar eclipse. 2. On the right, the moon is shown at her full, both above and below the eart x ' shadow, in which case there can be no lunar eclipse. was largest? If brought nearer ? How if planets smallest ? How affected by distance ? (How, then, with planets nearest the sun ? More remote ' Does any primary throw its shadow out to the next exterior planet?) 249. At what time of the moon do solar eclipses always occur? Lunai * Why? 250 Why not two eclipses every lunar month? (IlluBtrate.) ECLIPSES OF THE SUN AND MOON. 119 251. Eclipses of the sun always come on from the west, and pass over eastward ; while eclipses of the moon come f the Gas t, and pass over BOLAB ECLIP8E> westward, ibis is a necessary result of the eastward motion of the moon in her orbit. 1. In the right hand cut, the moon is seen re- volving eastward, throwing her shadow upon the earth, and hiding the western limb of the sun. In some Instances, however, when the eclipse is very slight, it may first appear on the northern otfto-uifmm limb of the sun that is, the upper or lower side; but even then its direction must be from west to east. It will also be obvious from this figure, that the shad- ore of the moon upon the earth must also trav- erse her surface from west to east; conse- quently the eclipse will be visible earlier in the west than in the east. 2. On the left, the moon is seen striking into the earth's shadow from the west, and having her eastern limb first obscured. By holding the book up south of him, the student will see at once why the revolution of the moon east- ward must cause a solar eclipse to proceed from west to east, and a lunar eclipse from east to west. To locate objects and motions correctly, the student should generally imagine himself looking to the south, as we are situated north of the equinoctial. The student should bear in mind that nearly all the cuts in the book are drawn to represent a view from northern lati- tude upon the earth. Hence by holding the book up south, of him, the cuts will' generally afford an accurate illustration both of the posi- tions and motions of the bodies represented. 252. Eclipses can never take place, except when the moon is near the ecliptic ; or, in other words, at or near one of her nodes. At all other times, she passes above or below the sun, and also above or below the earth's shadow. It is not necessary that she should be exactly at hei node.,, irk jorder that an eclipse occur. If she is withinmtf? of lifer node at the time oJier change, she will eclipse the sun ; and if within 12 0< of her node at her full, she will strike into the earth's shadow, and be more or less eclipsed. These distances are called, respectively, the solar and lunar ecliptic limits. 251. What is the direction of a solar eclipse ? A lunar? Why this dif fercrce? 252. Where must the moon be, with respect to the ecliptic and her nodes, in order to an eclipse ? What meant by ecliptic limits ? Name the distance uf each, respectively, from the node. (Illustrate.) 120 ASTRONOMY . This subject may be understood by consulting the following figure: THE MOON CHANGING AT DIFFEKENT DISTANCES FROM HER NODI 1. Let the line EE represent the ecliptic, and the line OO the plane of the moon's orbit. The light globes are the sun, and the dark ones the moon, which may be imag- ined as much nearer the student; hence their apparent diameter is the same. 2. Let the point A represent the node of the moon's orbit. Now if the change occur when the moon is at B, she will pass belmo the sun. If when at C, she will just touch liis lower lirnb. At C, she will eclipse him a little, and so on to A ; at which point, if the change occurs, the eclipse would be central, and probably total. 8. If the moon was at G, H, I, or J, in her orbit, when the change occurred, she would eclipse the upper or northern limb of the sun, according to her distance from her nodo at the time ; but if she was at K, she would pass above the sun, and would not eclipse him at all The points C and J will represent the Solar Ecliptic Limits. 253. All parts of a planet's shadow are not alike dense. The darkest portion is called the umbra^ and the partial shadow the penumbra. TJMBEA AND PENTTMBBA OF THE EABTH AND MOON. Penumbra is from the Latin pene, almost, and vmlra, a shadow. In this cut, the earth's umbra and penumbra will be readily found by the lettering: while A is the um- bra, and B B the penumbra, of the moon. The latter is more broad than it should be, owing to the nearness of the sun in the cut, as it never extends to much over half the earth's diameter. The student will see at once that solar eclipses can be total only to persons within the umbra; while to all on which the penumbra falls, a portion of the Bun's disk will be obscured. 254. The average length of the earth's umbra is about 860,000 miles ; and its breadth, at the distance of the moon, is about 6,000 miles, or three times the moon's diameter. As both the earth and moon revolve in elliptical orbits, both the above estimates are subject to variations. The length of the earth's umbra varies from 842.217 to 8?i,26 miles ; and its diameter There the moon passes it, varies from 5,235 to 6,365 miles. 255. The average length of the moon's umbra is about 239,000 miles. It varies from 221,148 to 252,638 miles, U53. What is the umbra of the earth or moon ? The penumbra ? (Deriva tioti ? Within which are solar eclipses total {\ 254. The average length of the earth's shadow ? Breadth at the moonV distance ? (Do they vary ? W r hy I) ECLIPSES OF THE SUN AND MOOX. 121 according to the moon's distance from the nm. Its greatest diameter, at the distance of the earth, is 170 miles ; but the penumbra may cover a space on the earth's surface 4,393 miles in diameter. 256. When the moon but just touches the limb of the sun, or the umbra of the earth, it is called an appulse. (See D and G-, in the first cut on the opposite page.) A partial eclipse is one in which only part of the sun or moon is obscured. A solar eclipse is partial to all places outside the umbra ; but within the umbra, where the whole disk is obscured, the eclipse is said to be total. A central eclipse is one taking place when the moon is exactly at one of her nodes. If lunar, it js total, as the earth's umbra is always broad enough, at the moon's distance, if centrally passed, to obscure her whole disk. But a solar eclipse may be central and not total) as the moon is not always of sufficient apparent diameter to cover the whole disk of the sun. In that case, the eclipse would be annular (from annulus, a ring), because the moon only hides the center of the sun, and leaves a bright ring unobscured. PROGRESS OF A CEKTEAL -ECLIPSE. Going off Annular. Coming on. 257. It has already been shown (50) that the apparent magnitudes of bodies vary as their distances vary ; and as both the earth and moon revolve in elliptical orbits, it 255. Average length of the moon's umbra? Does it vary ? Why ? Great- est diameter at the earth's surface ? Of penumbra ? 256. What is an appnlw f A partial eclipse ? A total ? A central? Arc all central eclipses total ? Why not ? What called then ? Why ? 257. AVhy are some central eclipses total, and others par al and annular? (Diagram.) 6 122 ASTRONOMY". follows that the moon and sun must both vary in their respective apparent magnitudes. Hence some central eclipses of the sun are total, while others are partial and annular. TOTAL AND ANNTTLAB ECLIPSES OF THB SFW. 1. At A, the earth Is at her aphtli&n, and the sun being at his most distant point, will have his least apparent magnitude. At the same time, the moon is in perigee, and ap- pears larger than usual If, therefore, she pass centrally over the sun's disk, the eclipse will be total. 2. At B, this order is reversed. The earth is at her perihelion, and the moon in apogee; so that the sun appears larger, and the moon smaller than usual. If. then, a central eclipse occur under these circumstances, the moon will not be lanre enough tc eclipse the whole of the sun, but will leave a ring, apparently around hevself, unoh- scured. Such eclipse will be annular. 258. As the solar ecliptic's limits are further from the moon's nodes than the lunar, it results that we have more eclipses of the sun than of the moon. There may be seven in all in one year, viz., five solar and two lunar; but the most usual number is four. There can never be less than two in a year ; in which case, both must be of the sun. Eclipses both of the sun and moon recur in nearly the same order, and at the same intervals, at the expiration of a cycle of 223 lunations, or 18 years of 365 days and 15 hours. This cycle is called the Period of ike Eclipses. At the expiration of this time, the suri and the moon's nodes will sustain the same relation to each other as at the beginning, and a new cycle oi eclipses begins. 259. In a total eclipse of the sun, the heavens are shrouded in darkness, the planets and stars become visi- ble, the temperature declines, the animal tribes become agitated, and a general gloom overspreads the landscape. Such were the effects of the great eclipse of 1806. In a lunar eclipse, the moon begins to lose a portion of her 258. Which kind of eclipses is most frequent? Why? The greatest number in a year ? How many of each ? Least number, and which ? Usu.\I number? What said of the order of eclipses ? Time of cycle ? 259. Describe the effects of a total eclipse of the smi. The process of vnar eclipse ? ECLIPSES OF THE SUN AND MOON. 123 light and grows dim, as she enters the earth's penumbra, till at length she comes in contact with the umbra, and the real eclipse begins. 260. In order to measure and record the extent of eclipses, the apparent diameters of the sun and moon are divided into twelve equal parts, called digits / and in predicting eclipses, astronomers usually state which "limb" of the body is to be eclipsed the southern 01 northern the time of the first contact, of the nearest approach of centers, direction, and number of digits eclipsed. FIVE DIGITS ECLIPSED. TWELVE DIGITS. 261. The last annular eclipse visible in the United States occurred Oct. 19, 1865. The next total eclipse of the sun will be August 7, 1869-. Some of the ancients and all barbarous nations formerly regarded eclipses with amazement and fear, as supernatu- ral events, indicating the displeasure of the gods. Colum- bus is said to have made a very happy use of this supersti- tion. When the inhabitants of St. Domingo refused to allow him to anchor, in 1502, or to furnish him supplies, he told them the Great Spirit was offended at their conduct, and was about to punish them. In proof, he said the moon would be darkened that very night / for he knew an eclipse was to occur. The artifice led to a speedy and ample supply of his wants. 262. Eclipses can be calculated with the greatest pre- cision, not only for a few years to come, but for centuries 260. How are eclipses measured and recorded ? 2tU. When the next annular eclipse visible in *liis country? The next total ? How have the ignorant and superstitious regarded eclipses ? Aneo- dote of Columbus 2 121 ASTRONOMY and ages either past or to come. This met demonstrates the truth of the Copernican theory, and illustrates the order and stability that everywhere reign throughout the planetary regions. CHAPTER VII. TELESCOPIC VIEWS OF THE MOONS OP JUPITEK. SATELLITES OF THE EXTERIOR PLANETS. 263. JUPITER is attended by four satellites or moons. They are easily seen with* a common spy-glass, appear- ing like small stars near the primary. (See adjoining cut, and note at 178.) By watch- ing them for a few evenings, they will be seen to change their places, and to occupy dif- ferent positions. At times, only one or two may be seen, as the others are either betw y een the observer and the planet, or leyond the primary, or eclipsed by his shadow. 264. The size of these satel- lites is about the same as our moon, except the second, which is a trifle less. The first is about the distance of our moon ; and the others, respect- ively, about two, three, and five times as far off. COMPARATIVE DISTANCES OF JUPITEE'S MOONS. 4th. - 3d. 1st 262. What said of the calculation of eclipses ? srratc and illustrate? What does this demon- 263. Ilow many moons has Jupiter? How seen ? Why not all seen at once I 264. Their size? Distances? Per 'da? Why so rupi< ipid ? SATELLITES OF THE EXTERIOR PLANETS. 125 Their periods of revolution are from 1 day 18 hours to 17 days, according to their distances. This rapid mo- tion is necessary, in order to counterbalance the power- ful centripetal force of the planet, and to keep the satel- lites from falling to his surface. The magnitudes, distances, and periods of the moons of Jupiter are as follows : Diameter in mites. Distance. Periodic times. lt,t 2,500 259,000 1 day 18 hours. 2d 2,068 ... 414,000 3 " 12 " * 3d 3,377 647,000 7 " 14 " 4th 2,890 1,164,000 1? " " 265. The orbits of Jupiter's moons are all in or near the plane of his equator ; and as his orbit nearly coin- cides with the ecliptic, and his equator with his orbit, it follows that, like our own moon, his satellites revolve near the plane of the ecliptic. On this account, they are sometimes between us and the planet, and sometimes beyond him, and seem to oscillate, like a pendulum, from their greatest elongation on one side to their greatest elongation on the other. 266. Their direction is from west to east, or in the direction their primary revolves, both upon his axis and in his orbit. From the fact that their elongations east and west of Jupiter are nearly the same at every revolu- tion, it is concluded that their orbits are but slightly elliptical. They are supposed to revolve on their re- spective axes, like our own satellite, the moon, once during every periodic revolution. 267. As these orbits lie near the plane of the ecliptic, they have to pass through his broad shadow when in opposition to the sun, and be totally eclipsed at every revolution. To this there is but one exception. As the fourth satellite departs about 3 from the plane of Jupi- ter's orbit, and is quite distant, it sometimes passes above or below the shadow, and escapes eclipse. But such escapes are not frequent. 265. How are their orbits situated ? How satellites appear to move ? 266. Direction of secondaries ? Form of orbits ? How ascertained ? What motion on axes ? 2<>7. What said of eclipses ? Of fourth satellite? Of solar eclipses upon Jupiter ? Number of solar and lunar ? 126 ASTRONOMY. These moons are not only often eclipsed, but they often eclipse Jupiter, by throwing their ow r n dark shadows upon his disk. They may be seen like dark round spots traversing it from side to side, causing, wherever that shadow falls, an eclipse of the sun. Altogether, about forty of these eclipses occur in the system of Jupiter every month. 268. The immersions and emersions of Jupiter's moons have reference to the phenomena of their being eclipsed. Their entrance into the shadow is the immersion and their coming out of it the emersion. ECLIPSES OF JUPITKK'S MOONS, EMKRSIONS, ETC. ^F \ 4# 1. The above is a perpendicular view of the orbits of Jupiter's satellites. His broau Bliadow is projected in a direction opposite the sun. At C, the second satellite is suiter- ing an immersion, and will soon be totally eclipsed ; while at D, tlie first is in the act of emersion, and will soon appear with its wonted brightness. The other satellites are Been to cast their shadows off into space, and are ready in turn to eclipse the sun, or cut off a portion of his beams from the face of the primary. 2. If the earth were at A in the cut, the immersion, represented at C, would be in- visible ; and if at B, the emersion at D could not be seen. So, also, if the earth wero exactly at F, neither could be seen ; as Jupiter and all his attendants would be directly beyond the sun, and would be hid from our view. 269. The system of Jupiter may be regarded as a miniature representation of the solar system, and as fur- nishing triumphant evidence of the truth of the Coper nican theory. It may also be regarded as a great natu- ral clock, keeping absolute time for the whole world ; as the immersions and emersions of his satellites furnish a uniform standard, and, like a vast chronometer hung up in the heavens, enable the mariner to determine his lon- gitude upon the trackless deep. 268. What are the immersions and emersions of Jupiter's moons ? (Arc the immersions and emersions always visible from the earth? Why not ? Illustrate.) 269. How may the system of Jupiter be regarded ? What use mac's of in naviga ion ? (lll-'strate method. Much u^ed ?) SATELLITES OF THE EXTERIOR PLANETS. 127 By l*m and careful observations upon these satellites, astronomers have been al;'e to construct tables, showing the exact time when each immersion and emersion will take place, at Greenwich Observatory, near London. Now suppose the tables fixed the time for a certain satellite to be eclipsed at 12 o'clock at Greenwich, but we find it to occur at 9 o'clock, for instance, by our local time: this would show that our time was three- hours behind the time at Greenwich; or, in other words, that we were three hours, or 45, tcet of Greenwich. If our time was ahead of Greenwich time, it would show that we were efint of that meridian, to the amount of 15 for every hour of variation. But this method of finding the longitude is less used than the " lunar method" (Art. 245), on ac- count of the greater difficulty of making the necessary observations. 270. By observations upon the eclipses of Jupiter's moons, as compared with the tables fixing the time of their occurrence, it was discovered that light had a pro- gressive motion, at the rate of about 200,000 miles per second. 1. This discovery may be illustrated by again referring to the opposite cut In th* year 1675, it was observed by Roemer, a Danish astronomer, that when the earth was nearest to Jupiter, as at E, the eclipses of his satellites took place 8 minutes 13 seconds foont>r than the mean time of the tables ; but when the earth was farthest from Jupiter, as at F, the eclipses took place 8 minutes and 13 seconds later than the tables predicted tiie entire difference being 16 minutes and 26 seconds. This difference of time he ascribed to the progressive motion of light, which he concluded required 16 minutes and 26 seconds to cross the earth's orbit from E to F. 2. This progress may be demonstrated as follows: 16m. 26s. = 986s. If the radius of the earth's orbit be 95 millions of miles, the diameter must be twice that, or 190 mil- lions. Divide 190,000,000 miles by 986 seconds, and we have 192,697$ miles as the progress of light in each second. At this rate, light would pass nearly eight times around the glob? at every tick of the clock, or nearly 500 times every minute 1 SATURN. 271. The moons of Saturn are eight in number, and are seen only with telescopes of considerable power. The best time for observ- ing them is when the planet is at his equinoxes, and his rings are nearly invisible. In January, 1849, the author saw five of these satellites, as represented in the adjoining cnt The rings appeared only nfl a line of light, extending each way from the planet, and the satellites were in the direction of the line, at different distances, as here represented. 272. These satellites all revolve eastward with the rings of the planet, in orbits nearly circular, and, with the exception of the eighth, in the plane of the rings. Their mean distances, respectively, from the planet's cen- 2TO. What discovery by observing these eclipses ? (Illustrate method. Diagram. Demonstration.) 271. Number of Saturn's moons ? How seen ? Best time ? 272. How revolve? Shape of orbits? How situated? Distances? Period* ? SATELLITES OF SATURN. 128 ASTRONOMY. tcr are from 123.000 to 2,366,000 miles; and their pe- riods from 22 Lours to 79 days, according to their dis- tances. The distances and periods of the satellites of Saturn are as follows: Distance in miles. Periodic tiins. Distance in miles. Periodic tmie. 1st 118,000 day 22| hours. 2<1 152,000 1 " 9 8d. 188,000 1 " 21 " 4th 240,000 2 " 17 " 5th 336.000 4 days 12 hours, 6th ..778,000 15 " 22 " 7th 940,000 22 * " Sth.... 2,268,000 70 u 1 " COMPAKATIVB DISTANCES OF THE MOONS OF SATURN. 2. - 273. The sixth of these satellites is the largest, sup- posed to be about the size of Mercury; and the remain- der grow smaller as they are nearer the primary. They are seldom eclipsed, on account of the great inclination of their orbits to the ecliptic, except twice in thirty years, when the rings are edgewise toward the sun. The eighth satellite, which has been studied more than all the rest, is known to revolve once upon its axis during every periodic revolution ; from which it is inferred that they all revolve on their respective axes in the same manner. 1. Let the line A B represent the plane of the planet's orbit, C D his u-xis, and F the plane of his rings. The satellites being in the plane of the rings, will revolve around the shadow of the primary, instead of passing through it, and being eclipsed. 2. At the time of his equinoxes, how- ever, when the rings are turned toward the sun (see A and E, cut, page 92), they must be in the center of the shad- ow on the opposite side ; and the moons, revolving in the plane of the rings, must pass through the shadow at every revolution. The eighth, however, may sometimes escape, on account of Ms departure from the plane of the rings, as shown in the cut URANUS. 274. Uranus is supposed to be attended by six secon- daries. Sir Win. Herschel recorded that he saw this number, and computed their periods and distances ; and on his authority the opinion is generally received, though 273. Size? Eclipses of ? When? Why not oftener ? (Illustrate.) 274. Satellites of Uranus ? Upon what authority ? Distances? Periods? Situation of orbits? Form? Direction in revolution 2 Remark of Dr. Kerschel \ NATURE AND CAUSE OF TIDES. 129 no other observer Las ever been able to discover mori than three. They are situated at various distances, and revolve in from 1 day and 21 hours to 117 days. Their orbits are nearly perpendicular to the ecliptic, and they revolve backward, or from east to west, contrary to all the other motions of our planetary system. Their or- bits are nearly circular, and they are described by Dr. Herschel as " the most difficult objects to obtain a sight of, of any in our system." The distances and periods of the system of Uranus, as laid down by Dr. Herschel, are as follows : Distance in miles. Periodic times. 1 st 224,000 Iday 21 hours. 2d 296,000 8 " 17 " 3d 340,000 10 " 23 " Distance in miles. Periodic times. 4th 890,000 13 days 11 hours. 5th.... '..777,000 38 " 2 " 6th.... 1,556,000 117 " 17 " NEPTUNE. 275. Neptune is known to be attended by one satel- lite, and suspected of having two. Professor Bond, of Cambridge, Mass., states that he has at times been quite confident of seeing a second. The mean distance of the known satellite from its primary is 236,000 miles, or near the distance of our moon. Its period is only 5 days and 21 hours. "We have here another illustration of the great.law of planetary motion explained nt 74 So great is the attractive power of Neptune, that to keep a satellite, at the distance of our moon, from falling to his surface, it must revolve some five times as swiftly as si; o revolves around the earth. The centripetal and centrifugal forces must be balanced in all cases, as the laws of gravitation and planetary motion, discovered by Newton aud Kepler, extend to and prevail among all the secondaries. CHAPTER VIII. NATURE AND CAUSE OP TIDES. 276. TIDES are the alternate rising and falling of the waters of the ocean, at regular intervals. Flood tide is when the waters are rising ; and eUb tide, when they are 275. What said of Neptune's secondaries? Eemark of Prof. Bond ? Dis- nce and period of th< 276. What are tides \ lo they ebb und flow ? tance and period of the Known satellite ? (Remark in note.) 276. What are tides ? Flood and ebb tides ? High and low? How cften. 130 ASTRONOMY. falling. The highest and lowest points to which they go are called, respectively, high and low tides. The tides ebb and flow twice every twenty-four hours i. ., we have two flood and two ebb tides in that time. 277. The tides are not uniform, either as to time or amount. They occur about 50 minutes later every day (as we shall explain hereafter), and sometimes rise much higher and sink much lower than the average. These extraordinary high and low tides are called, respectively, spring and neap tides. 278. The cause of the tides is the attraction of the sun and moon upon the waters of the ocean. But for this foreign influence, as we may call it, the waters having found their proper level, would cease to heave and swell, as they now do, from ocean to ocean, and would remain calm and undisturbed, save by its own inhabitants and the winds of heaven, from age to age. In this figure, the earth is represented as surrounded by water, in a stiite of rest or equilibrium, as it would be were it not acted upon by the sun and moon. 279. To most minds, it would seem that the natural effect of the moon's attraction would be to produce a single tide-wave on the side of the earth toward the moon. It is easy, therefore, for students to conceive how the moon can produce one flood and one ebb tide in twenty-four hours. 1. In this cut, the moon is shown at a distance above the earth, and attracting the waters of the ocean, so as to produce a high tide at A. But as the moon makes her apparent westward revolution around the earth but once a day, the simple raising of a flood tide on the side of the earth toward the moon, would give us but one flood and one ebb tide in twenty-four hours ; whereas it is known that we have two of each. 2. " The tides," says Dr. Herschel, " are a subject on which many persons find a strange difficulty of conception. That the moon, by her attraction, should heap up the waters of the ocean under her, seems to many persons very natural. That the same cause should, at the same time, heap them up on the opposite side of the earth (viz., at B in the figure), seems to many palpably absurd. Yet nothing is more true." 280. Instead of a single tide-wave upon the waters of 277. Are the tides uniform? What variation of time ? As to amount? What are these extraordinary high and low tides called ? 278. The cause of tides ? How but for this influence ? 279. What most obvious effect of the moon'a attraction ? (Substance ot note 1 ? Remark of Dr. Herschel ?) NATURE A.ND CAUSE OF TIDES. 131 the globe, directly under the moon, it is found that on the side of the earth directly opposite there is another high tide ; and that half way between these two high tides are two low tides. These four tides, Two TIDE . WAVE(i- viz., two high and two low, traverse the ocean from east to west every day, which accounts for both a flood and an ebb tide every twelve hours. In this cut, we have a representation of the tide-waves as they actually exist, except that their hight, as compared with the magni- tude of the earth, is vastly too great It is designedly exaggerated, the better to illustrate the principle under consideration. While the moon at A attracts the waters of the ocean, and produces a high tide at B, we see another high tide at C on the opposite side of the globe. At the same time it is low tide at D and E. 281. The principal cause of the tide-wave on the side of the earth opposite the moon is the difference of the moon's attraction on different sides of the earth. If the student well understands the subject of gravitation (65), he will easily perceive how a difference of attraction, as above described, would tend to produce an elongation of the huge drop of water called the earth. The diameter of the earth amounts to about 7j\,th of the moon's distance; so that, by the rule (69;, the difference in her attraction on the side of the earth toward her, and the opposite side, would be about -,-^th. The attraction being stronger at B {in the last cut) than at the earth's center, and stronger at her center than at C. would tend to separate these three portions of the globe, giving the waters an elongated form, and producing two opposite tide-waves, as shown in the Hit 282. A secondary cause of the tide-wave on the side of the earth opposite the moon, is the revolution of the earth around the common center of gravity between the earth and rnoon, thereby generating an increased centri- fugal force on that side of the earth. The center of gravity between the earth and moon is the point where they would exactly balance each other, if connected by a rod, and poised upon a fulcrum. CENTEB OF GRAVITY BETWEEN THE EARTH AND MOON. Moon This point, which, according to Ferguson, is about 6,000 miles from the earth's center Is represented at A in the above, and also in the next cut 280. How many tide-waves are there on the globe, and how situated ? 281. State the principal cause of the wave opposite the moon 1 (Demon- btrate by diagram.) 282. What other cause operates with the one just stated to produce tho tide-wave opposite the moon ? (What is the center of gravity between the earth and the moon ? Where is it situated ? Illustrate the operation of thia secondary cause. Diagram.) 132 ASTRONOMY. /SECONDARY CAUSE OF HIGH TIDE OPPOSITE THE MOON. 1. The point A represents the center of gravity between the ear h and moon ; and aa is this point which traces the regular curve of the earth's orbit, it is represented in the arc of that orbit, while the earth's center is 6,000 miles one side of it. Now the law of gravitation requires that while both the moon and earth revolve around the sun, they should also revolve around the common center of gravity between them, or around the point A. This would give the earth a third involution, in addition to that around the sun and on her axis. The small circles show her path around the center ot gravity, and the arrows her direction. 2. This motion of the earth would slightly increase the centrifugal tendency at B, and thus help to raise the tide-wave opposite the moon. But as this motion is slow, corresponding with the revolution of the moon around the earth, the centrifugal fored could not be greatly augmented by such a cause. 283. As the moon, which is the principal cause of the tides, is revolving eastward, and comes to the meridian later and later every night, so the tides are about 50 minutes later each successive day. This makes the in- terval between two successive high tides 12 hours and 25 minutes. Besides this daily lagging with the moon, the highest point E-WAVES BEHIND TnE MOON. of the tide-wave is found to be about 45 behind or east of the moon, so that high tide does not /' occur till about three hours after the moon has crossed the merid- ian. The waters do not at once yield to the impulse of the moon's attraction, but continue to rise after she has passed over. In the cut, the moon is on the meridian, but the highest point of the wave is at A, 01 46 east of the meridian ; and the corresponding wave on the opposite side at B is equally behind. 284. The time and character of the tides are also affected by winds, and by the situation of different places. Strong winds may either retard or hasten the tides, or may increase or diminish their hight ; and if a place is situated on a large bay, with but a narrow opening into the sea, the tide will be longer in rising, as the bay has 283. What daily lagging of the tides ? Interval between two successive hig \\ tides I What other lagging ? Cause of this last $ 264. What modification of the time and character of the tides ? NATURE AND CAUSE OF TIDES. 133 to fill up through a narrow gate. Hence it is not usually high tide at New York till eight or nine hours after the moon has passed the meridian. . 285. As botH the sun and moon ;are concerned in the production of tides, and yet arer constantly changing their positions with respect to the earth and to each other, it follows that I they /sometimes act against each others and measurably"i*eutralize each other's influence ; while at other times iiiQjicotnbine their forces, and mutu- ally assist each other, in the latter case, an unusually high tide occurs, called the Spring Tide. This happens both at new and full moo3| CAUSE OF 8PBING TIDES. 1. Here tlie sun and moon, being in conjunction, unite their forces to produce an ex- traordinary tide. The same effect follows when they are in opposition ; so that \ve have two spring tides every month namely, at new and full moon. 2. If the tide-waves at A and B are one-third higher at the moon's quadrature than usual, those of C and D will be one-third lower than usual. 286. Although the sun attracts the earth much more powerfully, as a whole, than the moon does, still the moon contributes more than the sun to the production of tides. Their relative influence is as one to three. The nearness of the moon makes the difference of her attraction on different sides of the earth much greater than the difference of the sun's attraction on different sides. It must not be forgotten that the tides are the result not so much of the attraction of the sun and moon, as a whole, as of the difference in their attraction on different sides 235. Do the sun and moon always act together in attracting 1 the watere I "Why not? How affect each other's' influence ? Effect on the tides ? What ure Xpi-ing Tiles ? When do they occur ? (Illustrate by diagram the cause of spring tide, when the sun and moon are in conjunction.) 286. Comparative influence of sun and moon in the production of tides ? Why inoon' influence the greatest ? (Substance of note ? Demonstration.? 12 134: ASTRONOMY. SPKLNG AND NEAP TIDES. of the earth, caused by a difference in the dintnnces of the several parts. The attrr.'v tion being inversely, as the square of the distance (69 , the influence of the sun and moon, respectively, must be in the ratio of the earth's diameter to their distances. Now the difference in the distance of two sides of the earth from the moon is jA th of tho moon's distance ; as 240,000 -J- 8,000 = 30 ; while the difference, as compared with tho distance of the sun, is only -pf^^th, as 95,000,000 -f- 8,000 = 11,875. /^ 287. \When the moon is in quadrature^ and her influ- / ence is partly neutralized by the sun, which now acts against her, the result is a very low tide, called Neap \ Tide. \ The whole philosophy of spring hnd neap tides may be illustrated by the annexed diagram. 1. On the right side of the cut, the Bun and moon are in conJimction, and unite to produce a spring tide. 2. At the first quarter, their at- traction acts at right angles, and the sun. instead of contributing to the lunar tide-waves, detracts from it to the amount of his own attractive force. The tendency to form a tide of his own, as represented in the figure, reduces the moon's wave to the amount of one-third. 3. At the full moon, she is in oppo- sition to the sun, and their joint at- traction acting again in the same line, tends to elongate the fluid por- tion of the earth, and a second spring tide is produced. 4. Finally, at the third quarter, the sun and moon act against each other again, and the second neap tide is the result. Thus we have two spring and two neap tides during every lunation the former at the moon's syzygies, and the latter at '.ier quadratures. 288. The tides are subject to another periodic varia- tion, caused by the declination of the sun and moon north and SOUth of the equator. As TIDES AFFECTED BY DECLINA- the tendency of the tide-wave is to rise directly under the sun and moon, when they are in the south, as in winter, or in the north, as in summer, every alternate tide is higher than the intermediate one. At the time of the equinoxes, the sun being over the equator, and the moon within 5 of it, the crest of the great tide-ware will be on the equator ; but as the sun and moon decline south to A, one tide-wave forms in the south, as at B, and the opposite one in the north, as at C. If the declination wan nort/i, as shown at D, the order of the tides would be reversed. The following diagram, -287. What are Neap Tides f Their cause? (Illustrate entire philosophy by diagram.) '.88. "What other periodic variations mentioned? (Explain cause, and illustrate,) NATURE AND CAUSE OF TIDES. 135 if carefully studied, will more fully illustrate the subject of the alternate high and low tides, in high latitudes, in winter and summer: ALTERNATE HIGH AND LOW TIDES. 1. Let the line A A represent the plane of the ecliptic, and B B the equinoctial. On the 21st of June, the day tide-wave is north, and the evening wave south, so that tho tide following about three hours after the sun and moon will be higher than the inter- mediate one at 3 o'clock in the morning. 2. On the 23d of December, the sun and moon being over the southern tropic, the highest wave in the southern hemisphere will be about 3 o'clock P. M., and the lowest about 3 o'clock A. M. ; while at the north, this order will be reversed. It is on this ac- count that in high latitudes every alternate tide is higher than the intermediate ones; the evening tides in summer exceeding the morning tides, and the morning tides in win- ter exceeding those of evening. 289. All spring and neap tides are not alike as to their elevation and depression. As the distances of the sun and moon are varied, so are the tides varied, especially by the variations of the moon. VARIATIONS IN THE SPRING TIDES. 1. At A, the earth is in aphelion, and the moon in apogee. As both the sun and moon are at their greatest distances, the earth is least affected by their attraction, and the spring tides are proportionately low. 2. At B, the earth is in perihelion, and the moon in perigee ; so that both the sun and moon exert their greatest influence upon our globe, and the spring tides are highest, as shown in the figure. In both cases, the sun and moon are in conjunction, but the varia- tion in the distances of the sun and moon causes variations in the spring tides. 290. In the open ocean, especially the Pacific, the tide rises and falls but a few feet ; but when pressed into nar- row bays or channels, it rises much higher than under ordinary circumstances. 289 Are all spring and neap tides alike ? By what are they modified ? (Illustrate by diagram.) 290. Hight of tides in open seas ? How in narrow bays and channels ? fllight at different points on our coast, ?) 136 ASTRONOMY. The average ele ition of the tide at seveiai points on on '.r^t is .is follows: Cumberland, head of the Bay of Fundy 71 feet. Boston 114 " New Haven 8 " Now York 5 " Charleston, 8. 0. . 6 " 291. As the great tide-waves proceed from east to west, they are arrested by the continents, so that the waters are permanently higher on their east than on their west sides. The Gulf of Mexico is 20 feet higher than the Pacific Ocean, on the other side of the Isthmus ; and the Red Sea is 30 feet higher than the Mediterranean. Inland seas and lakes have no perceptible tides, because they are too small, compared with the whole surface of the globe, to be sensibly affected by the attraction of the sun and moon. We have thus stated the principal facts connected with this complicated phenomenon, and the causes to which they are generally attributed. And yet it is not certain that the philosophy of tides is to this day fully under- stood. La Place, the great French mathematician and astronomer, pronounced it one of the most difficult prob- lems in the whole range of celestial mechanics. It is probable that the atmosphere of our globe has its tides, as well as the waters ; but we have no means, as yet, for definitely ascertaining the fact. CHAPTER IX. OF COMETS. 292. COMETS are a singular class of bodies, belonging to the solar system, distinguished for their long trains of light, their various shapes, and the great eccentricity ot their orbits. Their name is from the Greek coma, which 291. Direction of tide-waves ? What result ? Instances cited ? Have in- land seas and lakes anv tides ? Why not ? Remarks respecting philosophy of tides ? Of La Place ? Atmospheric tides ? 292. What are comets? Derivation of name ? Are they opake or sclf- luHiirfbus? OF COMETS. GREAT COMET OP 871 BEFORE CHRIST. signifies heard or Jiair, on account of their bearded or hairy appearance. They are known to be opake, from the iact that they sometimes exhibit phases, which show that they shine only by reflection. 293. Cornets usually consist of three parts the nn- cleus, the envelope, and the tail. The nucleus is what may be called the ~body or head of the comet. Thy envelope, is the nebulous or hairy covering that sur- rounds the nucleus ; and the tail is the expan- sion or elongation of the envelope. But all comets have not these parts. Some have no percept- ible nucleus ; their entire structure being like that of a thin vapory cloud passing through the dis- tant heavens. Others have but a slight envelope around a strongly marked nucleus. The great comet that appeared 371 years before Christ exhibited the different parts of a comet with great distinctness; on which account, as well as for its striking magnificence, we give a view of it in the above cut 294. The tails of comets usually lie in a direction opposite to the sun, so that from perihelion to aphelion they precede their nuclei or heads ; or, in other words, comets seem, after having passed their perihelion, to back out of the solar system. Their tails are usually curved more or less, being concave toward the region from whence they come. This is well shown in the comets of 1811, 1843, and in the following cut. That of 1689 is said to have been curved like a Turkish sabre. The cause of this curvature of the tails of comets is supposed to be a very rare ethereal substance which pervades 293. Parts of a comet? Describe each. Hove all cornets these three parte ? (What comet shown as a sample in the cut ?) 294. Direction of the tails of comets 2 How curved ' Cause of this cur- vature ? 138 ASTRONOMY. space, and offers a slight resistance to their progress. Of course it munt be almost infinitely attenuated, as the comets themselves are a mere vapor, which could make no progress through the spaces of the heavens, were they not very nearly a vacuum. They could no more pass u medium as dense as our atmosphere, than an ordinary cloud could pass through the waters of the sea. 295. The form of the comets' orbits is generally that of an ellipse greatly flattened or elongated. The sun being near one end of the ellipse, and the planets com- paratively in his immediate neighborhood, the comets are in the vicinity of the sun and planets but a short time, and then hasten outward again beyond the limits of human vision, with the aid of the best telescopes, to be sjone again for centuries. ORBIT OP A GOMKT. Here it will be seen that the orbit is very eccentric, that the perihelion point is very near the sun, and the aphelion point very remote. 296. The tails of comets do not continue of the same uniform length. They increase both in length and breadth as they approach the sun, and contract as they recede from him, until they often nearly disappear before the comet gets out of sight. Instances have occurred in which tails of comets have been suddenly expanded or elongated to a great distance. This is said to have been the case with the great comet of 1811. 295. Form of the orbits of comets ? What near the earth but little of 'Jie tame? 296. What said of the contraction and expansion of the tails of cort?ots What specimen shown in the cut I OF COMETS. 139 GREAT COMET OF 1S11. 297. Comets have been known to exhibit several tails at the same time. That of 1744, represented in the cut, had no less than six tails spread out in the heavens, like an enormous fan. The comet of 1823 is said to have had two tails, one of which extended toward the sun. The comet of 1744, represented in this cut, excited great attention and interest. It ex- hibited no train till within the distance of the orbit of Mars from the sun ; but early in March it appeared with a tail divided into six branches, all diverging, but curved in the same direction. Each of these tails was about 4 wide, and from 30 to 44 in length. The edges were bright and decided, the middle faint, and the intervening spaces as dark as the rest of the firmament, the stars shining in them. When circumstances were favor- able to the display of this remarkable body, the scene was striking and magnificent, al- most beyond description. 298. The heads or nuclei of comets are comparatively small. The following table shows the estimated diam- eter in five different instances : The comet of 1778 . diameter of head 33 miles. a 1805 . a (C 36 u (C 1799 . (C U 462 U U 1807 . u u 666 u u 1811 . u u 428 u 9.97. Have they ever more than one tail? What peculiarity of the comet of "323 ? (What specimen ^ comet with several tails ? Describe.) 110 ASTRONOMY. COMET OF 1585. Many comets have simply the envelope, without any tail or elongation. Such were those that appeared in 1585 and 1763, the former of which is represented in the adjoining cut, Cassini describes the comet of 1682 as being as round and as bright as Jupiter, without even an envelope. But these are very rare exceptions to the general charac- ter of cometary bodies. 299. The tails of comets are often of enormous length and magnitude. That of 371 before Christ was 60 long, covering one-third of the visible heavens. In 1618, a comet appeared, which was 101 in length. Its tail had not all risen when its head reached the middle of the heavens. That of 1680 had a tail 70 long ; so that though its head set soon after Rundown, its tail continued visible all night. GREAT COMET OF 1843. The following table will show the lens^h of the tails of some o f the most remarkable comets, both in degrees and in miles. They will be characterized only by the year when they appeared : Tieg. Miles. 80 85.000,006 48,000.000 Desr. Miles. B.C. 371 60 140,000,000 A. D. 1456 60 70,000,000 " 1618 104 65,000,000 " 1680 70 123.000,000 " 1689 68 100,000,000 A. D.I 744 " 1769 " 1811 23 132.000,000 " 1843 60 130,000,000 298. What of the size^ of the nuclei of comets ? Give a few examples. What comets without tails ? What specimen hi the cut? What said ot the comet of 1682 ? Are such comets numerous ? 299. What of the size of the tails of comets ? That of 871 B. C. ? Of A. D. 1618 ? Of 1680? (What specimen in cut, and its length? State tho length of some others in miles.) OF COMETS. 141 300. The velocity with which comets often move is truly wonderful. Their motions are accelerated as they approach, and retarded as they recede from the sun ; so that their velocity is greatest while passing their peri- helioris. The comet of 1472 described an arc of the heavens of 120 in extent in a single day ! That of 1680 moved, when near its perihelion, at the rate of 1,000,000 miles per hour. 301. The temperature of some comets, when nearest the sun, must be very great. That of 1680 came within 130,000 miles of the sun's surface, and must have re- ceived 28,000 times the light and heat which the earth receives from the sun a heat more than 2,000 times as great as that of red-hot iron ! What substance can a comet be composed of to endure the extremes of heat and cold to which it is subject ? Some have supposed that their tails were caused by the sun's light and heat rarefying and driving back the vapory substance com- posing the envelope. 302. The periods of but few comets are known. That of 1818, called Enckds Co- met, lias a period of only 3 J yeaw. JBiclds Comet lias a period of 6f years. That of 1862 (then first noticed with care, and identified as the same that had appeared in 1456, 1531, and 1607) has a period of about 76 years. It is called Ilal- Uy^s Comet, after Dr. Halley, who determined its periodic time. The great comet of 1680 has a periodic time of 570 years, so that its next return to our system will be in the year 300. Velocity of comets ? Uniform or not ? Comet of 1472 ? Of 1680 ? 801. Temperature? Comet of 1680? Supposed cause of their tails ? 302. Periods? Encke's ? Biela's ? Halley's ? That of 1680? Supposed periods of ot!/ ers ? Opinions of Prof. Nichol and Dr. Herschel ? 14:2 2250. Many are supposed to have periods of thousands of years ; and some have their orbits so modified by the attraction of the planets, as to pass off in parabolic curves, to return to our system no more. Prof. Nichol is of opinion that the greater number visit our system but once, and then fly off in nearly straight lines till they pass the center of attraction between the solar system and the fixed stars, and go to revolve around other suns in the far distant heav- ens. Sir John Herschel expresses the same opinion. 303. The distances to which those comets that return must go, to be so long absent, must be very great. Still their bounds are set by the great law of gravitation, for were they to pass the point " where gravitation turns the other way," they would never return. But some, at least, do return, after their " long travel of a thousand years." "What a sublime conception this affords us of the almost infinite space between the solar system and the fixed stars. ORBIT or HALLEY'S COMET. 304:. The perihelion distances of the various comets that have appeared, and whose elements have been esti- mated by astronomers, are also exceedingly variable. While some pass very near the sun, others are at an im- mense distance from him, even at their perihelion. Of 137 that have been particularly noticed, 30 passed between the sun and the orbit of Mercury. 44 between the orbits of Mercury and Yenus. 34: " " Yenus and the earth. 23 the earth and Mars. 6 " " Mars and Jupiter. 803. Distances to which they go ? Remark respecting the law of gravi- uition ? What specimen of orbit given ? 304. What said of perihelion distances ? How many noticed ? Where dij OF COMETS. 143 '" fhc orbit of Encke's comet is wholly within the orbit of Jupiter, while that of Biel.Vs extends but a short distance beyond it The aphelion distance of Haliey's comet Is 3,400 millions of miles, or 550 millions of miles beyond the orbit of Neptune. ORBITS OF SEVERAL COMETS. But these are all comets of short periods. 305. The number of comets belonging to, or that visit the solar system, is very great. Some have estimated them at several millions. When we con- sider that most comets are seen only through tele- scopes an instrument of comparatively modern date and that, notwith- standing this, some 440 are mentioned in ancient annals and chronicles, as having been seen with the naked eye, it is probable that the above opinion is by no means extravagant. It is supposed that not less than 650 have been seen at different times since the birth of Christ. The paths of only about 140 have been deter- mined. The extreme difficulty of ooserving comets wnose nearest point is beyond the orbit of Mars, is supposed to account for the comparatively small number that have been seen without that limit; and the proximate uniformity of the distribution of their orbits over the space included within the orbit of Mars, seems to justify the conclusion, that though seldom detected beyond his path, they are nevertheless equally distributed through all the spaces of the solar heavens. Eeasoning upon this hypothesis, Prolessoi Arago concludes that there are probably seven millions of comets that belong to 01 visit the solar system. 306. The directions of comets are as variable as their forms or magnitudes. They enter the solar system from all points of the heavens. Some seem to come up from the immeasurable depths below the ecliptic, and, having doubled u heaven's mighty cape," again plunge down- ward with their fiery trains, and are lost for ages in the ethereal void. Others appear to come down from the zenith of the universe, and, having passed their peri- they pass ? (What samples given in cut ? W T here does the orbit of Encke's comet lie ? Of Biela's ? Of 'Haliey's ?) 305. The number of comets? What estimate? Why probably correct? Row many supposed to have been seen since the birth of Christ ? (Why so few seen ? How supposed to be distributed ? What conclusion of Arago?) 306. Direction of comets ? (Remark of late writer?) 144 ASTEONOMY. hclion, reascend far above all human vision. Others again are dashing through the solar system, in all possible directions, apparently without any prescribed path, or any guide to direct them in their eccentric wanderings. In- stead of revolving uniformly from east to west, like the planets, their motions are direct, retrograde, and in every conceivable direction. It is remarked by a late writer, that the average inclinations of all the planes in which the comets now on record have been found to move, is about 90. This lie re- gards as a wonderful instance of the goodness of Providence, in causing their motions to be performed in a manner least likely to come in contact with the earth and the other planets. 307. Of t\\Q physical nature of comets, little is known. That they are, in general, very light and vapory bodies, is evident from the fact that stars have sometimes been seen even through their densest portions, and are gene- rally visible through their tails, and from the little attrac- tive influence they exert upon the planets in causing perturbations. While Jupiter and Saturn often retard and delay comets for months in their periodic revolutions, cornets have not power, in turn, to hasten the time of the planets for a single hour ; showing conclusively that the relative masses of the comets and planets are almost in- finitely disproportionate. Such is the extreme lightness or tenuity of cometary bodies, that in all probability the entire mass of the largest of them, if condensed to a solid substance, would not amount to more than a few hundred pounds. Sir Isaac Newton was of opinion, that if the tail of the largest comet was compressed within the space of a cubic inch, it would not then be as dense as atmospheric air! The comet of 1770 got entangled, by attrac- tion, among the moons of Jupiter, on its way to the sun, and remained near them for four mtHUM ; yet it did not sensibly affect Jupiter or his moons. In this way the orMte of comets are often entirely changed. 308. Comets were formerly regarded as harbingers of famine, pestilence, war, and other dire calamities. In one or two instances, they have excited serious appre- hension that the day of judgment was at hand, and that they were the appointed messengers of Divine wrath, hasting apace to burn up the world. A little reflection, however, will show that all such fears are groundless. The same unerring hand that guides the ponderous planet 807. Physical nature of comets ? What proofs of their light and vapory character? (What said of their probable mass? Opinion of Newton? What said of the comet of 1770 ? What effect on orbits ?) 308. How comets formerly regarded ? Why no fears of collision ("Wh.it> estimate of 'chances 2 "} OF COMETS. .115 m its way, directs also the majestic comet , and where infinite wisdom and almighty power direct, it is almost profane to talk of collision or accident. Even those who have calculated the " chances" of collision as if chance had any thing to do among the solar bodies have concluded the chances of collision are about as one to 281,000,000 i. e., like the chance one would have in a lottery, where there were 281,000,000 black balls, and but one white one ; and where the white ball must be produced at the first drawing to secure a prize. 309. Were a collision actually to take place between a comet and the earth, it is not probable that the former would even penetrate our atmosphere, much less dash the world to pieces. Prof. Olinsted is of opinion that in such an event, not a particle of the comet would reach the earth that the portions encountered by her would be arrested by the atmosphere, and probably inflamed ; and that they would perhaps exhibit, on a more magnifi- cent scale than was ever before observed, the phenomena of shooting stars or meteoric showers. The idea, there- fore, that comets are dangerous visitants to our system, has more support from superstition than from reason or science. The air is to us what the waters are to fish. Some fish swim around in the deep, while others, like lobsters and oysters, keep on the bottom. So birds wing the air, while men and beasts are the " lobsters" that crawl around on the bottom. Now there is no more probability that a comet would pass through the atmosphere, and injure us upon the earth, than there is that a handful offoff or vapor thrown down upon the sur face of the ocean, would pass through and kill the shell-fish at the bottom. 310. After all that is supposed to be known respecting comets, it must be admitted that they are less under- stood than any other bodies belonging to our system. " What regions these bodies visit, when they pass beyond the limits of our view ; upon what errands they come, when they again revisit the central parts of our system ; what is the difference between their physical constitution and that of the sun and planets ; and what important ends they are destined to accomplish in the economy of the universe, are inquiries which naturally arise in the mind, but which surpass the limited powers of the human understanding at present to determine." 809. What probable effect in case of collision? Prof. Olmsted's opinion ? (Remark respecting the air, fish, lobsters, &c. ?) 310. Are we as well acquainted with comets as with other bodies of our eystem 2 What inquiries suggested ? How answered ? 7 110 ASTRONOMY. CHAPTER X. OF THE SUN. 311. OF all the celestial objects with which we an; acquainted, none make so strong and universal an im- pression upon our globe as does the sun. He is the great center of the solar system a vast and fiery orb, kindled by the Almighty on the morn of creation, to cheer tlu dark abyss, and to pour his radiance upon surrounding worlds. Compared with him, all the solar bodies are of inconsiderable dimensions ; and without him, they would be wrapped in the gloom of interminable night. 312. The form of the sun is that of an oblate sphe- roid, his equatorial being somewhat greater than his polar diameter. The mean of the two is 886,000 miles. He is 1,400,000 times as large as the mighty globe we inhabit, and 500 times" as large as all the planets put together. Were he placed where, the earth is, he would fill all the orbit of the moon, and extend 200,000 miles be- yond it in every direction. It would take 112 such worlds as ours, if laid side by side, to reach across his vast diameter. 1. The vast magnitude of the sun may be inferred from the fact, that when rising or set- ting, he often appears larger than the largest building, or the tops of the largest trees. Now if the angle filled by him at the distance of two miles is over 100 feet across, what must it be at the distance of 95 millions of miles? 2. "Were a railroad passed through the sun's center, and should a train of cars start from one side, and proceed on at the rate of 30 miles an hour, it would require 34 years 811. Describe the sun. How compare with the rest of the system ? 312. What is his form? Diameter? Mass, jus compared with our globe? With all other bodies of the system ? With moon's orbit ? (What sensible evidence of *he vast magnitude of the. sun? Illustration from railroad 8 Pemor^tration as to its comparison with moon's orbit THK SUN AND THE MOON'b ORBIT. OF THE SUN. BPOTS ON THE SUN. t.o cross over ms diameter. To traverse his vast circumference, at the same rate ol speed, would require nearly 11 years. 8. The mean distance of the moon from the earth's center is 240.000 miles : conse- quently the diameter of her orbit, which is twice the radius, is 480,000. Subtract this from 886,000, the sun's diameter, and we have 406,000 miles left, or 203,000 miles on each side, beyond the moon's orbit 313. By the aid of telescopes, a variety of spots have been discovered upon the sun's disk. Their number is exceedingly variable at different times. From 1611 to 1629, a period of 18 years, the sun was never found clear of spots, except for a few days in December, 1624. At other times, twenty or thirty were frequently seen at once ; and at one period, in 1825, upwards of fifty were to be seen. Prof. Olmsted states that over 100 are sometimes visible. From 1650 to 1670, a pe- riod of 20 years, scarcely any spots were visible ; and for eight years, from 1676 to 1684, no spots whatever were to be seen. For the last 46 years, a greater or less number of spots have been visible every year. For several days, during the latter part of September, 1846, we could count sixteen of these spots, which were dis- tinctly visible, and most of them well defined ; but on the 7th of October following, only six small spots were visible, though the same telescope was used, and circum- stances were equally favorable. The sun is a difficult object to view through a telescope, even when the eye is pro- tected in the best manner by colored glasses. In some cases (as in one related to the author by Professor Caswell, of Brown University), the heat becomes so great as to spoil the eye-pieces of the instrument, and sometimes the eye of the observer is irrepa- rably injured. 314. The solar spots are all found within a zone 60 wide i. e., 30 each side of the sun's equator. They are generally permanent, though they have been known to 313. View of sun's surface through telescopes ? Number of spots seen ? Are they always to be seen? How from Ittll to 1629? In 1825? Prof. OlmstedV statement ? How from 1050 to 1670? From 1676 to 1684? Ii. 1846 ? (What said of difficulties of observing ?) 314. Where are these spots situated ? Are they permanent ? What mo- 148 ASTRONOMY. un revolves in the direction of the arrows, and in 25 days 10 hours the spot comes break in pieces, and disappear in a very short time. They sometimes break out again in the same places, or where none were perceptible before. They pass from left to right over the sun's disk in 13 days, 15 hours, and 45 minutes ; from which it has been ascertained that he performs a sidereal revolution on his axis, from west to east, or in the di- ' - -, ' BIDEEKAL AND SYNODIC INVOLUTIONS OF THE BUN. rection of all the planets, every 25 E / days, 7 hours, and * -^ / ' a/Dm- 48 minutes. 1. His apparent or synodic revolution requires 27 days 7 hours; but this is as much more than a complete revolution upon his axis, as the earth has ad- vanced in her orbit in 25 days 8 hours. Let S represent the sun, and A the earth in her orbit. When she is at A, a spot is seen upon the disk of the sun at B. The sun revolves in the direction round to B again, or opposite the star E. This is a ftidereal revolution. 2. During these 25 days 8 hours, the earth has passed on in her orbit some 25, or m-arly to 0, which will require nearly two days for the spot at B to get directly toward the earth, as shown at D. This last is a synodic revolution. It consists of one com- plete revolution of the sun upon his axis, and about 27 over. 315. Of the nature of these wonderful spots, a variety of opinions have prevailed, and many curious theories have been constructed. Lalande, as cited by Herschel, suggests that they are the tops of mountains on the sun's surface, laid bare by fluctuations in his luminous atmos- phere ; 'and that the penumbrse are the shoaling declivi- ties of the mountains, where the luminous fluid is less deep. Another gentleman, of some astronomical knowl- edge, supposes that the tops of the solar mountains are exposed by tides in the sun's atmosphere, produced by planetary attraction. To the theory of Lalande, Dr. Herschel objects that it is contradicted by the sharp termination of both the in- ternal and external edges of the penumbraa ; and ad vances as a more probable theory, that " they are the tion have they? What conclusion from it? (What revolution is this? What time required for a synodic revolution ? Illustrate.) 315. What are these spots supposed to be ? Lalande ? &c. Dr. Herschel'a 'e-oark? Prof Olmsted? Prof. Wilson? Experiments of Prof. Henry ? OF THE SUN. 149 divrk, or, at least, comparatively dark, solid body of the Bim itself, laid bare to our view by those immense fluc- tuations in the luminous regions of the atinospnere, to which it appears to be subject." Prof. Olmsted supports this theory by demonstrating that the spots must bo " nearly or quite in contact with the body of the sun." In 1773, Prof. Wilson, of the University of Glasgow, ascertained, by a series of observations, that the spot were probably u vast excavations in the luminous matter of the sun ;" the nuclei being their bottom, and the urn* brae their shelving sides. This conclusion varies but little from that of Dr. Herschel, subsequently arrived at. In a series of experiments conducted by Prof. Henry, of the Smithsonian Institute, at Washington, by means of a thermo-electrical apparatus, applied to an image of the sun thrown on a screen in a dark room, it was found that the spots were perceptibly colder than the surrounding light surface. 316. The magnitude of the solar spots is as ariable as their number. Upon this point, the second cat pre- ceding gives a correct idea, as it is a pretty accurate rep- resentation of the sun's disk, as seen by the writer on the 22d of September, 1846. In 1779, Dr. Herschel ob- served a spot nearly 30,000 miles in breadth ; and he further states, that others have been observed, whose diameter was upward of 45,000 miles. Dr. Dick ob- serves that he has several times seen spots which were not less than ^ of the sun's diameter, or 22,192 miles across. It is stated, upon good authority, that solar spots have been seen by the naked eye a fact from which Dr. Dick concludes that such spots could not be less than 50,000 miles in diameter. The observations of the writer, as above referred to, and represented in the cut, would go to confirm this deduction, and to assign a still greater magnitude to some of these curious and interest- ing phenomena. 317. The axis of the sun is inclined to the ecliptic 7J, 316. What said of the size of the solar spots? Dr. Herschel'a observa tions ? Dr. Disk's 8 The writer's ? 150 ASTRONOMY. or, more accurately, 7 20'. This is but a slight deviation from what we may call a perpendicular ; so that, in rela- tion to the earth, he may be considered as standing up and revolving with one of his poles resting upon a point, just half his diameter below the ecliptic. As the result of the sun's motion upon his axis, his spots always appear first on his eastern limb, and pass off or disappear on the west. But though the direction of the spots, as viewed from the earth, is from east to west, it only proves his motion to coincide with that of the earth, w r hich we call from west to east; as when two spheres revolve in the same direction, the sides toward each other will appear to move in opposite directions. During one-half of the passage of the spots across the sun's disk, their apparent motion is accelerated / and during the remainder, it is retarded. This apparent irregularity in the motion of the spots upon the sun's surface, is the necessary result of an equable motion upon the surface of a globe or sphere. "When near the eastern limb, the spots are coming partly toward us, and their angular motion is but slight ; but w r hen near the center, their angular and real motions are equal. So, also, as the spots pass on to the west, it is their angular motion only that is diminished, while the motion of the sun upon his axis is perfectly uniform. 318. The figure of the sun affects not only the appa- rent velocity of the spots, but also their forms. When first seen on the east, they appear narrow and slender, as represented in the cut, page 147. As they advance westward, they continue to widen or enlarge till they reach the center, where they appear largest; when they again begin to contract, and are constantly diminished, till they disappear. 319. Another result of the revolution of the sun upon an axis inclined to the ecliptic, and the revolution of the 317. How is the sun's axis situated ? What said of the direction of the c Em , ov THB at a time, gives it the ap- pearance of two pyramids with their bases joined at the sun. It is an interest- ing fact, stated by Prof. Nichol, that this light or nebulous body lies in the plane of the sun's equator. A line drawn through its transverse diameter, or from one apex of the pyra- mids to the other, would cross the axis of the sun at right angles. This fact would seem to indicate a revolution of this curious sub- stance with the sun upon his axis. Let A, in the above cut, represent the sun, B B his axis ; then C C will represent the extent, and D D the thickness of this curious appendage. 325. At the meeting of the American Association for the Advancement of Science, held in Providence, R. I., August 18, 1855. The Rev. George Jones, of the U. S. navy, read an elaborate paper upon the Zodiacal Light, founded upon his own observations during a cruise in the United States' steam frigate Mississippi, from 41 K lat. to 52 S. lat. From a record of 331 observations, each accompanied by a drawing, showing the exact form and position of the Light among the stars, Mr. Jones was decided in the conviction that the Zodiacal Light is a luminous ring around the earth, like that which surrounds the planet Saturn. Prof. Pierce, of Harvard College, is said to have concurred with him in this opinion. 326. After all the observations that have been made, 325. Mr. Jones' observations ? Extent ? Where made, and when ? Prof. Pierce' s reported opinion ? 155 and the theories that have been advanced, it must be ad- mitted that the subject of the zodiacal light is but imper' iectly understood. Prof. Olmsted supposes it to be a nebulous body, or a thin vapory mass revolving around the sun ; and that the meteoric showers which have oc- curred for several years in the month of November, may be derived from this body. This is the opinion of Arago, Biot, and others. The best time for observing the zodiacal light is on clear evenings, in the months of March and April. It may be seen, however, in October, November, and De- cember, before sunrise ; and also in the evening sky. 327. Although, in general terms, we speak of the sun as the fixed center of the system, it must not be under- stood that the sun is absolutely without motion. On the contrary, he has a periodical motion, in nearly a circular direction, around the common center of all the planetary bodies; never deviating from his position by more than twice his diameter. 1 rom the known laws of gravita- tion, it is certain that the sun is affected in some measure by the attraction of the planets, especially when many of them are found on the same side of the ecliptic at the same time ; but this would by no means account for so great a periodical motion. 328. In addition to the motion above described, the sun is found to be moving, with all his retinue of planets and comets, in a vast orbit, around some distant and hitherto unknown center. This opinion was first ad vanced, we think, by Sir William Herschel ; but the honor of actually determining this interesting fact be- longs to Struve, who ascertained not only the direction of the sun and solar system, but also their velocity. 326. Is this subject well understood as yet? Prof. Olmsted's theory? When the best time for observing the zodiacal light ? 327. Is the sun really stationary ? What motion \ How affected by plan- et^ 828. What other motion ? Who first advanced the opinion that he had 3-iyh a motion ? Who demonstrated it? Toward what point is the sun and 156 ASTRONOMY. The point of tendency is toward the constellation Her- cules, right ascension 259, declination 35. The ve- locity of the sun in space is estimated at 8 miles per second, or 28,000 miles per hour. Its period is about 18,200,000 years ; and the arc of its orbit, over which the sun has traveled since the creation of the world, amounts to only about ^oVo tn P art f m ' s orbit, or about 7 minutes an arc so small, compared with the whole, as to be hardly distinguishable from a straight line. 329. With this wonderful fact in view, we may no longer consider the sun as fixed and stationaiy, but rather as a vast and luminous planet, sustaining the same rela- tion to some central orb that the primary planets sustain to him, or that the secondaries sustain to tLeir primaries. Nor is it necessary that the stupendous mechanism of nature should be restricted even to these sublime propor- tions. The sun's central body may also have its orbit, and its center of attraction and motion, and so on, till, as Dr. Dick observes, we come to the great center of ail to the THRONE OF GOD ! Professor Madler, of Dorpat, in Kussia, has recently announced as a discovery that the star Alcyone, one of the seven stars, is the center around which the sun and sour Bystem are revolving. CHAPTER XI. MISCELLANEOUS REMARKS UPON THE SOLAR SYSTEM. NEBFLAR THEORY OF THE ORIGIN OF THE SOLAR SYSTEM. 330. IT was the opinion of La Place, a celebrated French astronomer, that the entire matter of the solar system, which is now mostly found in a consolidated solar system tending ? Its velocity ? Period of revolution ? Amount of its progress since the creation of the world ? 829. How, thm, should the sun be considered ? How extend the analogy ? What further recont discovery, and by whom ? 830. State the " nebular theory" of the origin of the solar system ? Who ^rst started this theory ? ORIGIN OF THE SOLAR SYSTEM NEBULAR THEORY. 157 state, in the sun and planets, was once a vast nebula or gaseous vapor, extending beyond the orbits of the most distant planets that in the process of gradual conden- sation, by attraction, a rotary motion was engendered and imparted to the whole mass that this motion caused the consolidating matter to assume the form of various concentric rings, like those of Saturn ; and, finally, that these rings collapsing, at their respective distances, and still retaining their motion, were gathered up into plan- ets, as they are now found to exist. This opinion is sup- posed to be favored, not only by the fact of Saturn's revolving rings, but by the existence of the zodiacal light, or a resisting medium about the sun ; and also by the character of irresolvable or planetary nebulae, hereafter to be described. 331. To this theory, however, there are many plau- sible, if not insurmountable, objections. (a.) It. seems to be directly at variance with the Mosaic account of the creation of the sun, moon, and stars. The idea that the sun and all the planets were made up, so to speak, out of the same general mass, not only throws the creation of this matter back indefinitely into eternity, but it substitutes the general law of attraction for the more direct agency of the Almighty. The crea- tion spoken of in the Bible thus becomes not the origi- nating of things that did not previously exist, but the mere organization or arrangement of matter already existing. (5.) The supposed consolidation of the nebulous mass, in obedience to the general law of attraction, does not of itself account for the rotary motion which is an essen- tial part of the theory. Under the influence of mere at- traction, the particles must tend directly toward the cen- ter of the mass, and consequently could have no tendency to produce a rotary motion during the process of conden- sation. (c.) The variation of the planetary orbits from the 331. What said of it? State the first objection named? The second? Third? Fourth ? Fifth ? What remark added by the author? 14 158 ASTRONOMY. plane of the sun's equator contradicts the nebular theory. If the several primary planets were successively thrown off from the general mass, of which the sun is a part, they could not have been separated from the parent body till they were near the plane of its equator. Now, as the sun is assumed to be a part of the same mass, re- volving still, the theory would require that the portions now separated from him, and called planets, should still revolve in the plane of his equator. But instead of this, it is found that some of them vary from this plane to the amount of nearly 42. (d.) This theory assumes not only that the primary planets were thrown off from the parent mass by its rapid revolution, but that the primaries, in turn, threw off their respective satellites. These, then, should all revolve in the plane of the planetary equators respect- ively, and in the direction in which their primaries re- volve. But their orbits not only depart from the plane of the equators of their primaries (Jupiter's satellites excepted), but the moons of Uranus actually have a retrograde or backward revolution. (lanets. Substance ? Forms ? Gravitation ? Magnitude ? Day* and aights? Seasons? Atmospheres? Moons? Mountains? &e. ARE THE PLANETS INHABITED? 163 tected by its intercepting or refracting the light, it may be of a nature too clear and rare to produce such phe- nomena. (h.) The principal primary planets are provided with moojis or satellites, to afford them light in the absence of the sun. It is not improbable that both Mars and Yenus have each, at least, one moon. The earth has one ; and as the distances of the planets are increased, the number of moons seems to increase. The discovery of six around Uranus, and only one around Neptune, is no evidence that others do not exist which have not yet been dis- covered. (i.) The surfaces of all the planets, primaries as well as secondaries, seem to be variegated with hill and dale, mountain and plain. These are the spots revealed by the telescope. (j.) Every part of the globe we inhabit is adapted to the support of animal life. It would, therefore, be contrary to the analogy of nature, as displayed to us, to suppose that the other planets are empty and barren wastes, utterly devoid of animated being. And if ani- mals of any kind exist there, why not intelligent beings 2 338. If other worlds are not the abodes of intellectual life, for what were they created ? What influence do they exert upon our globe, especially those most remote ? There are doubtless myriads of worlds beyond our system that will never even be seen by mortal eye, and that have no perceptible connection with our globe. If, then, they are barren and uninhabited islands in the great ocean of immensity, we repeat, for what were they created ? The inquiry presses itself upon the mind with irresistible force, Why should this one small world be inhabited, and all the rest unoccupied ? For what purpose were all these splendid and magnificent worlds fitted up, if not to be inhabited ? Why these days and years this light and shade these atmospheres, and seasons, and satellites, and hill and dale? 338. What difficulty on the supposition that the planets are not inha' - ited 1 164: ASTRONOMY. 339. To suppose all these worlds to be fitted up upon one general plan, provided with similar conveniences as abodes for intellectual beings, and yet only one of them to be inhabited, is like supposing a rich capitalist would build some thirty fine dwellings, all after one model, though of different materials, sizes, and colors, and pro- vide in all for light, warmth, air, &c. ; and yet, having placed the family of a son in one of them, allow the remaining twenty-nine to remain unoccupied forever ! And as God is wiser than man, in the same proportion does it appear absurd, that of nearly ninety planetary temples now known to exist, only one has ever been occu pied; while the remainder are mere specimens of Divine architecture, wheeling through the solitudes of immen- sity ! The legitimate and almost inevitable conclusion, therefore, is, that our globe is only one of the many worlds which God has created to be inhabited, and which are now the abodes of his intelligent offspring. It seems irrational to suppose that we of earth are the only intel ligent subjects of the " Great King," whose dominions border upon infinity. It is much more in keeping with sound reason, and with all the analogies of our globe, to suppose that " Each revolving sphere, a seeming point, Which through night's curtain sparkles on the eye, Sustains, like this our earth, its busy millions." 340. The fact that we neither see, nor hear, nor hear from the inhabitants of other worlds, is no evidence that such inhabitants do not exist. It would have been Eremature in Columbus had he concluded, when he saw md in the distance, that it was uninhabited, simply be- cause he could not hear the shout of its savages, or see them gathered in groups upon the beach. So in regard to the distant planets. Our circumstances forbid our knowing positively that they are inhabited ; so that the absence of that knowledge is no argument against the inhabitedness of other worlds. 339. What illustration ? Conclusion ? Poetry ? 840. What said of the objection that we neither see, hear, nor hear from the inhabitants of the other worlds ? AKE THE PLANETS INHABITED? 165 341. It may be thought that the extremes of heat and cold on some of the planets must be fatal to the idea of animal life, at least. But even this does not follow. Upon our globe, some animals live and flourish where others would soon die from heat or cold. And some ani- mals, having cold blood, may be frozen, and yet live. So in other worlds. He who made the three Hebrews to live in the fiery furnace, can easily adapt the inhabit- ants of Mercury to their warm abode. And of the exte rior planets we have only to say : " Who there inhabit must have other powers, Juices, and veins, and sense, and life, than ours ; One moment's cold, like theirs, would pierce the bone, Freeze the heart's blood, and turn us all to stone 1" Adaptation is a law of the universe; and this at once obviates every difficulty in regard to the temperature of the planets, which might otherwise be urged as a reason why they were not inhabited. 841. Objection drawn from extremes of temperature? Poetry? What grev. law answers every such objection? PART II. THE SIDEREAL HEAVENS. CHAPTER I. THE FIXED STARS CLASSIFICATION, NUMBER, DISTANCE, ETC. 34:2. THE sidereal heavens embrace all those celestial bodies that lie around and beyond the solar system, in the region of the fixed stars. The fixed stars are distinguished from the planetary bodies by the following characteristics : (a.) They shine by their own light, like the sun, and not by reflection. (&.) To the naked eye, they seem to twinkle or scintil- late while the planets appear tranquil and serene. (c.) They maintain the same general positions, with respect to each other, from age to age. On this account, they are called fixed stars. (d.) They are inconceivably distant/ so that, whel viewed through a telescope, they present no sensible disk, but appear only as shining points on the dark concave of the sky. To these might be added several other peculi- arities, which will be noticed hereafter. 343. For purposes of convenience, in finding or refer- ring to particular stars, recourse is had to a variety ol artificial methods of classification. 842. What parts of the book have we now gone over ? Upon what do we now enter? What is meant by the sidereal heaveus ? How are the fixed stars distinguished from planetary bodies ? 843. What are constellations ? Their origin ? THE FIXED STAKS CLASSIFICATION. 107 First, The whole concave of the heavens is divided into sections or groups of stars of greater or less extent. The ancients imagined that the stars were thrown toge- ther in clusters, resembling different objects, and they consequently named the different groups after the objects which they supposed them to resemble. These clusters, when thus marked out by the figure of some animal, person, or thing, and named accordingly, were called constellations. 344. Secondly, The stars are all classed according to their magnitudes. There are usually reckoned twelve different magnitudes, of which the first six only are visible to the naked eye, the rest being telescopic stars. These magnitudes, of course, relate only to their apparent brightness ; as the faintest star may appear dim solely on account of its immeasurable distance. The method by which stars of different magnitudes are distinguished in astronomical charts is as follows : 8TAB8 OF DIFFERENT MAGNITUDES. 3 4 5 6 7 8 9 10 11 12 "It must be observed," says Dr. Herschel, "thatttits classification in to magnitudes Is entirely arbitrary. Of a multitude of bright objects, differing, probably, intrinsically botli in size and in splendor, and arranged at unequal distances from us, one must oi necessity appear the brightest ; the one next below it brighter still, and so on." 345. The next step is to classify the stars of each con stellation according to their magnitude in relation to each other, and without reference to other constellations. In this classification, the Greek alphabet is first used. For instance, the largest star in Taurus would be marked (a) Alpha ; the next largest (/3) Beta ; the next (7) Gamma, &c. "When the Greek alphabet is exhausted, the Roman or English is tuken up ; and when these are all absorbed, recourse is finally had to figures. As Greek letters so frequently occur in catalogues and maps of the stars, and on the celestial globes, the Greek alphabet is here inserted, for the benefit of those who are not 344. How classified by magnitudes ? (Remark of Dr. Herschel ?) 345. Next step iri classifying ? How conducted? Greek letters \ (Eepou Iho ulphubet.) 168 ASTRONOMY. ccqnainted with it; but as the capitals are seldom used for designating tho stars, the small characters only are given : THE GREEK ALPHABET. a Alpha a Beta b r Gamma g Delta d e Epsilou e short $ Zeta z rj Eta e long e Theta th 1 Iota K Kappa k X Lambda 1 M Mu m Nu n Xi x Omicron o short Pi p Rho T Sigma s Tau t Upsilon n Phi ph Chi ch Psi ps Omega o long 346. To aid in finding particular stars, and especially in determining their numbers, and detecting changes, should any occur, astronomers have constructed cata- logues of the stars, one of which is nearly 2,000 years old. Several of the principal stars have a specific name as Sirius, Aldebaran, Megulus, &c. ; and clusters of stars in a constellation sometimes receive a specific name, as the Pleiades and Hyades in Taurus. 347. The stars are still further divided into double, triple, and quadruple stars, binary systems, variable stars, periodic stars, nebulous stars, &c., all of which will be noticed hereafter. NUMBER OF THE FIXED STABS. 348. The actual number of the stars is known only to Him who " telleth the number of the stars, and calleth them all by their names." The powers of the human mind are barely sufficient to form a vague estimate of the number near enough to be seen by our best tele- scopes, and here our inquiries must end. The number of stars, down to the twelfth magnitude, has been estimated as follows : 846. What farther methods for finding particular stars ? 847. How are the stars still further distinguished ? 348. Number of the stars? Of each magnitude? Number visible to naked eye ? Additional seen through telescopes ? Total ? Kemarks of hcl and Olmstcd ? NUMBER OF THE FIXED STABS. 169 Visible to the naked eye. Visible only through telescopes. 1st ?iiao-nitude 18 Yth magnitude 26,000 2d " 52 8th " 170,000 3d " 177 9th 1,100,000 4th " 376 10th " 7,000,000 5th " 1,000 llth " 46,000,000 6th 4,000 12th " 300,000,000 Total 5,623 Grand total, 354,301,623 NUMBER OF 8TAK8 OF EACH MAGNITUDE. Of these stars, Dr. Herschel remarks that from 15,000 to 20,000 of the first seven magnitudes are already regis- tered^ or noted down in catalogues ; and Prof. Olmsted observes that Lalande has registered the positions of no less than 50,000. 349. The reason why there are so many more of the binall stars than of the large ones is, that we are in the midst of a great cluster, with but few stars near us, the number increasing as the circumference of our view is enlarged. (See second cut, page 28, and also the adjoining.) Let the ;entral star represent the sun (a star only among the rest), with the solar system revolving between him nd the first circle. The 18 stars in space 1st will apperr to be of the first magnitude, on account of their nearness, and they are thus few be- cause they emDrace but a small part of the entire cluster. The stars of space 2d will app*wr smaller, being more distant; but as it embraces more space, they will be more numerous. Thus as we advance from one circle to another, the apparent magnitude constantly diminishes, but the num- ber constantly increases. The large white circle marks the limit of our natural vision. Even this cut foils to present fully to the eye the cause of the rapid in- crease in numbers, for we can only show the surface of a cut section of our firmament of stars, which exhibits the increase in a plane only, whereas our sun seems to be im- bedded in the midst of a magnificent cluster (like a single apple in the midst of a large tree richly laden with fruit), the stars of which we vievv around us in every direction. 349. Why so many more of small stars than of the larsrer ? (Illustrate by diagram. Does this convey a complete idea of the position of the sun, with reference to the fixed stars? Whj not? What docs his position more nearly 8 ITO ASTKONOMY. 350. If we suppose that each of these suns is accom- panied only by as many planets as are embraced in onr solar system, we have nine thousand millions of worlds in our firmament. ]STo human mind can form a concep- tion of this number ; but even these, as will hereafter be shown, form but a minute and comparatively insignifi- cant portion of the boundless empire which the Creator has reared, and over which he reigns. " Lo, these are parts of his ways ; but how little a portion is heard of fern? but the thunder of his power who can under- stand." (Job xxvi. 14.) DISTANCES AND MAGNITUDES OF THE STAKS. 351. It has been demonstrated that the nearest of tne fixed stars cannot be less than 20,000,000,000 (twenty billions) of miles distant ! For light to travel over this space, at the rate of 200,000 miles per second, would re- quire 100,000,000 seconds, or upwards of three years. What, then, must be the distances of the telescopic stars, of the 10th and 12th magnitudes ? " If we admit," says Dr. Herschel, " that the light of a star of each mag nitude is half that of the magnitude next above it, it will follow that a star of the first magnitude will require to be removed to 362 times its distance, to appear no larger than one of the twelfth magnitude. It follows, therefore, that among the countless multitude of such stars, visible in telescopes, there must be many whose light has taken at least a thousand years to reach us ; and that when we observe their places, and note their changes, we are, in fact, reading only their history of a thousand years' date, thus wonderfully recorded." Should such a star be struck out of existence now, its light would continue to stream upon us for a thousand years to come ; and should a new star be created in those distant regions, a thousand years must pass away before its light could reach the solar system, to apprise us of its existence. 350. What supposition and conclusion ? Scripture qnotation ? S51. Distances of the nearest stars? Time for light to travel over tiu space ? Suppositions and conclusions of Dr. Ilersch^i 3 MAGNITUDE OF THE STARS. 171 352. From what we have already said respecting the almost inconceivable distances of the fixed stars, it will readily be inferred that they must be bodies of great magnitude, in order to be visible to us upon the earth. It is probable, however, that " one star differeth from another" in its intrinsic splendor or " glory," although we are not to infer that a star is comparatively small be cause it appears small to us. 353. The prevailing opinion among astronomers is. that what we call the fixed stars are so many suns and' centers of other systems. From a series of experiments upon the light received by us from Sirius, the nearest oi the fixed stars, it is concluded that if the sun were re- moved 141,400 times his present distance from us, or to a point thirteen billions of miles distant, his light would be no stronger than that of Sirius ; and as Sirius is more than twenty billions of miles distant, he must, in intrinsic magnitude and splendor, be equal to two suns like ours. Dr. Wollaston, as cited by Dr. Herschel, con- cludes that this star must be equal in intrinsic light to nearly fourteen suns. According to the measurements of Sir Wm. Herschel, the diameter of the star Vega in the Lyre is 38 times that of the sun, and its solid con- tents 54,872 times greater! The star numbered 61 in the Swan is estimated to be 200,000,000 miles in di- ameter. 354. Sir John Herschel states, that while making ob- servations with his forty-feet reflector, a star of the first magnitude was unintentionally brought into the field of view. " Sirius," says he, " announced his approach like the dawn of day ;" and so great was his splendor when thus viewed, and so strong was his light, that the great astronomer was actually driven from the eye-piece of his telescope by it, as if the sun himself had suddenly burst upon his view. 352. Whal inference from the great distance of the stars ? What proba- bility as to the real magnitude c.f the stars ? 353. The prevailing opinion among astronomers ? Conclusions from ex- periments with Sirius? Magnitude of Vega? Of No. 61 in the Swan? 354. Incident stated by Dr. ilerscliel? (Relative light of tne stars ol the first six mugnitadea ?) 172 ASTRONOMY. According to Sir Wm. Herschel, the relative light of the stars of tho first sis magni- tudes is as follows: Light of a star of the average 1st magnitude 100 8 * 2d " 25 " 3d " 12 4th " 6 " " 5th " 2 " 6th " .... .1 CHAPTER II. DESCRIPTION OF THE CONSTELLATIONS. 355. ALTHOUGH this work is designed particularly to illustrate the mechanism of the heavens, as displayed in the solar system, we are desirous of furnishing the learner with a sufficient guide to enable him to extend his inquiries and investigations not only to the different classes of bodies lying beyond the limits of the solar system in the far off heavens, but also to the constella- tions, as such. For this purpose, we shall here furnish a brief description of the principal constellations visible in the United States, or in north latitude ; by the aid of which, the student will be able to trace them, with very little difficulty, upon that glorious celestial atlas which the Almighty has spread out before us. If the student will be at the trouble to identify the constellations by the aid of these descriptions, and without the aid of chart?, it will give him a practical familiarity with the heavens which can be acquired in no other way. Indeed, this exercise is indispen- sable to a competent knowledge of sidereal astronomy, even where maps of the constel- lations are used. Let all students, therefore, embrace every favorable opportunity for looking up the constellations. Those who wish to study their mythological history will consult the author's edition of the " Geography of the Heavens,' 1 ' 1 by E. II. Burritt the most reliable and popular work upon this subject in the English language. 356. Of the nature and origin of the constellations we have already spoken, at 343. Their formation has been the work of ages. Some of them were known at least 3,000 years ago. In the 9th chapter of Job, we 355. Principal design of this text-book ? What further object? What clone for this purpose'? (Substance of note?) 356. What said of the formation of the constellations ? Antiquity ? Scripture .-illusions? DESCRIPTION OF THE CONSTELLATIONS. 173 re/id of " Arcturus, Orion, and Pleiades, and the cham- bers of the south ;" and in the 38th chapter of the same book, it is asked, " Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion ? Canst thou bring forth Mazzaroth in his season ? or canst thou guide Arcturus with his sons ?" 357. The constellations are divided into ancient and modern. According to Ptolemy's catalogue, the ancients had only 48 constellations ; but being found convenient in the study of the heavens, new ones were added to the list, composed of stars not yet made up into hydras and dragons, till there a^e now scarcely stars or room enough left to construct the smallest new constellation, in all the spacious heavens. The present number, according to the catalogue of the Observatory Eoyal of Paris, is 93. 358. The constellations are further divided into the Zodiacal, Northern, and Southern. The zodiacal con- stellations are those which lie in the sun's apparent path, or along the line of the zodiac. The northern are those which are situated between the zodiacal and the north pole of the heavens ; and the southern, those which lie between the zodiacal and the south pole of the heavens. They are distributed as follows viz., 12 zodiacal, 35 northern, and 46 southern. This division is convenient for reference ; but in tracing the constellations in the heavens, or upon a map, it is better to begin with those that are on or near the meridian, mid proceed eastward, taking northern and southern together, so far as they are in view. And where classes in astronomy are organized during "the fall months, it will be found advantageous to begin with the constellations that are in view at seasonable hours during those months. 359. In consequence of the eastward motion of the earth in its annual revolution, the constellations rise ear- lier and earlier every night ; so that if an observer were to watch the stars from the same position for a whole year, he would see each constellation, in turn, coming to the meridian at midnight (or at any other hour fixed 357. How are the constellations classified ? How many of eacn ? In ull ? 358. How further classified ? Describe each. How many of each ? (Whut Baid in note ?) 359. What said of the rising of the constellations? How proceed in de- scribing aud tracing ? 174: ASTRONOMY. upon), till he had seen the whole panorama of the heav- ens. Beginning, therefore, with the constellations that are on or near the meridian at 9 o'clock, on the 15th ot November, and going eastward, we shall now proceed with our description of the constellations. OCTOBER, NOVEMBER, AND DECEMBER. 360. ANDROMEDA. Almost directly over head, at 9 o'clock, on the 15th of November, may be seen the con- stellation Andromeda. The figure is that of a woman in a sitting posture, with her head to the southv/est. Andromeda may be known by three stars of the second magnitude, situated about 12 apart, nearly in a straight line, and extending from east to west. The middle star of the three is situated in her girdle, and is called Mirach. The one west of Mirach is Alplieratz, in the head of Andromeda; and the eastern one, called Al- maak, is in her left foot. The star in her head is in the equinoctial colure. The three largest stars in this con- stellation are of the second magnitude. Near Mirach, are two stars of the third and fourth magnitudes, and the three in a row constitute the girdle. This constellation embraces 66 stars, of which three are of the 2d magnitude, two of the 3d, and the rest small. About 2 from v, at the northwestern extremity of tho girdle, is a remarkable cluster or nebula of very minute stars, and the only one of the kind which is ever visible to the naked eye. It resembles two cones of light, joined ot their base, about (j in length, and in breadth. 361. PEGASUS (the Flying Horse}. The figure is the head and fore parts of a horse, with wings. The three principal stars are of the 2d magnitude viz., Algenib, about 15 south of Alpheratz, in Andromeda ; Markab, about 18 west of Algenib; and 8Jceat,\5 north of Markab. These three, with Alpherat in Andromeda, form what is called the Square of Pegasus. The head of the figure is to the southwest, almost in a line with Algenib and Markab, and about 20 from the latter. 360. Constellations on the meridian, in what months taken up ? An, dromeda where situated ? Figure ? Position ? How known ? F-mie principal stars. (How many stars in constellation?- What cluster, and where ?) 861. Figure of Pegasus? Principal stars? How situated? Fonr'ig what ? How the horse situated ? His head where DESCRIPTION OF THE CONSTELLATIONS. 175 362. PISCES (the Fishes) consists of two fishes, distin- guished as the northern and western, connected by an irregular line of stars. The Western Fish is situated directly south of the square of Pegasus is about 20 long, with its head to the west. It includes a number of small stars, just south of Pegasus. The Northern Fish is about the same size, with its head near Mirach in Andromeda, and its body extending to the south. This, also, includes small stars only, and is by no means conspicuous. The flexure or ribbon, uniting the tails of the northern and western fishes, extends eastward from the latter, from star to star, till it comes opposite the former, when it turns to the north, taking several small stars in its way, till it joins the northern fish. 363. AQUARIUS (the Water-bearer) is represented by the figure of a man in a reclining posture, with his head to the northwest. Its four largest stars are of the third magnitude. It is situated directly south of the head of Pegasus, and from 5 to 30 north of a star of the first magnitude, in the southern fish. Three of the principal stars of Aquarius are near each other in the water-pot which he holds in his right hand. 364. PISCIS AUSTRALIS (the Southern Fish) is situated directly south of Aquarius. Its largest star is Fomal- haut, of the 1st magnitude, which constitutes the eye of the fish. The body extends westward about 20. 365. GRUS (the Crane) is situated directly south of the southern fish, with its head to the north. It is composed of a few stars only, of the fourth magnitude. As it is 45 south of the equinoctial, it appears low down in the south to persons situated in the Middle or Eastern States. 366. THE PHCENIX is about 25 east of the Crane. It 862. Describe Pisces. The Western Fish ? The Northern ? Flexure ? 363. Figure of Aquarius f Largest stars 1 Situation and extent I Fur- ther description. Stj-L Pisces Aastralis largest star? Situation of figure ? 865. Gru& 'how situated ? Where ? Composition ( 3(J6. Situation of the Phcenix f Principal stun* ? 17G ASTRONOMY. has two stars of the 2d magnitude, about 12 apart east and west. The most western of these, in the neck of the bird, is about 25 southeast of Fomalhaut, in the South- ern Fish. The other stars of the figure are of the 3d and 4th magnitudes. 367. CASSIOPEIA (the Queen). About 30 northeast of Andromeda is Cassiopeia. The figure is that of a woman sitting in a chair, with her head from the pole, and her body in the Milky Way. Its four largest stars are of the 3d magnitude. 368. PERSEUS (the King). Directly north of the " seven stars," and east of Andromeda, is Perseus. The figure is that of a man with a sword in his right hand, arid the head of Medusa in his left. Algol, a star of the 2d magnitude, is about 18 from the Pleiades (or seven stars), in the head of Medusa ; and 9 northeast of Al- gol is Algenib, of the same magnitude, in the back of Perseus. It embraces four other stars of the third mag- nitude, besides many smaller. 369. MUSCA (the Fly) is about 12 south of Medusa's head. It is a very small constellation, embracing one star of the 2d magnitude, two of the 3d, and a few smaller. 370. THE TRIANGLES include a few small stars, about half-way between Musca in the southeast, and Mirach in Andromeda in the northwest. Its two principal stars are of the 3d magnitude. 371. ARIES (the Ram). The head of Aries is about 10 south of the Triangles. It may be known by two stars about 4 apart, of the 3d and 4th magnitudes. The most northeasterly of the two is the brightest, and is called a Arietis. The back of the figure is to the north, and the body extends eastward almost to the Pleiades. ' 367. Where is Cassiopeia f Fisrure ? Situation? Largest stars ? 368. Perseus figure ? Two principal stars ? Names ? Situation ? Mag- nitude ? 369. Where is Musca ? Size ? Composition ? 370. The Triangles where ? Principal stars ? 871. Where is Aries f How known? Which of two principal brightest ? Name ? How figure situated ? Extent 3 DESCRIPTION OF THE CONSTELLATIONS. 177 372. CETUS (the Whale). Directly southeast of Ari- etis, and about 25 distant, is Menkar, a star of the 2d magnitude, in the mouth of Cetus. This is the largest constellation in the heavens. It is situated below 01 south of Aries. It is represented with its head to the east, and extends 50 east and west, with an average breadth of 20. The head of Cetus may be known by five remarkable stars, 4 and 5 apart, and so situated as to form a regular pentagon, or five-sided figure. About 40 southwest of Menkar, is another star in the body ot the figure, near which are four small stars nearly in a row, and close together, running east and west. Passing eastward, we next take the constellations that are on the meridian in JANUARY, FEBRUARY, AND MARCH. 373. TAURUS (the Bull) will be readily found by the seven stars or Pleiades, which lie in his neck. The largest star in Taurus is Aldebaran, in the Bull's eye, a star of the first magnitude, of a reddish color, somewhat resembling the planet Mars. Aldebaran, and four other stars in the face of Taurus, compose the Hyades. They are so placed as to form the letter Y. 374. ORION lies southeast of Taurus, and is one of the most conspicuous and beautiful of the constellations. The figure is that of a man in the act of assaulting the bull, with a sword in his belt, and a club in his right hand. It contains two stars of the first magnitude, four of the second, three of the third, and fifteen of the fourth. Be- telguese forms the right, and Bellatrix the left shoulder. A loose cluster of small stars forms the head. Three small stars, forming a straight line about 3 in length, constitute the lelt, called by Job " the lands of Orion" They are sometimes called the Three Kings, because they point out the Hyades and Pleiades on the one hand. 372. Cetus what star pointed out ? Size of constellation ? Situation Extent? How know its head? "What other star pointed out ? "Whutcvm- Btellutions next described in order ? 37 ; 3. Taurus how found ? Largest star ? Hyades? 374. O'-ion, situation I (Jhui-uotur I i'Ujuro I Composition ? 178 ASTRONOMY. and Sinus on the other. A row of very small stars runs down from the belt, forming the sword. These, with the stars of the belt, are sometimes called the Ell and Yard. Mintaka, the northernmost star in the belt, is less than J south of the equinoctial. Rigel, a bright star of the first magnitude, is in the left foot, 15 south of Bellatrix ; and Saiph, of the third magnitude, is situ- ated in the right knee, 8J east of Rigel. 375. LEPUS (the Hare) is directly south of and near Orion. It may be known by four stars of the third mag- nitude, in the form of an irregular square. Zeta, of the fourth magnitude, is the first star, situated in the back, and about 5 south of Saiph in Orion. About the same distance below Zeta are the four principal stars, in the legs and feet. 376. COLUMBA NOACHI (Noah's Dove) lies about 16 south of Lepus. It contains but- four stars, of which Phaet is the brightest. It lies on the right, a little higher than Beta, the next brightest. This last may be known by a small star just east of it. 377. EKIDANUS (the River Po) is a large and irregular constellation, very difficult to trace. It is 130 in length, and is divided into the northern and southern streams. The former lies between Orion and Cetus, commencing near Kigel in the foot of Orion, and flowing out westerly in a serpentine course, near 40, to the Whale. 378. CANIS MAJOR (the Greater Dog) lies southeast of Orion, and may be readily found by the brilliancy of its principal star, Sirius. This is the largest of the fixed stars, and was once supposed to be the nearest to the solar system. Several others are now supposed to be nearer. 879. ARGO NAVIS (the Ship Argo) is a large and splendid constellation southeast of Sirius, but so low down in the south that but little of it can be seen in the United 875. Where is Lepus? How known? Describe. 876. Cbktmba Noachi situation ? Composition ? 877. Describe Eridanus. Length? Division? Situation? 873. Where is Canis Major situated ? How found ? What of Sit 879. Describe Argo Xusis. Where situated? Principal stum and where? DESCRIPTION OF THE CONSTELLATIONS. 179 States. It lies southeast of Canis Major, and may be known bj the stars in the prow of the ship. Harkeb, of the fourth magnitude, is 16 southeast of Sirius. Naas and 7, still further south, are of the second magnitude, and Canopus and Miaplacidus of the first. 380. CANIS MINOR (the Lesser Dog) is situated about 25 northeast of Sirius, and between Canis Major and Cancer. It is a small constellation, having one star, Procyon, of the 1st magnitude, and Gomelza, of the 2d. 381. MONOCEKOS (the Unicorn). A little more than half way from Procyon to Betelguese in Orion, are three stars in a row, about 4 apart, and of the 4th magnitude. They extend from northeast to southwest, and constitute the face of Monoceros. His head is to the west, with Canis Minor on his back, and his hind feet about 25 southeast of Procyon. It is a large constellation, with but few stars, and those mostly small. 382. HYDRA (the Water Serpent). About 20 east of Procyon are four stars of the fourth magnitude, situated about 4 apart, and so as to form a diamond; the longer axis running east and west. These constitute the head of Hydra,, which points to the west. The figure extends to the south and east more than 100, taking in an ir- regular line of stars of the 3d and 4th magnitudes. The largest star is about 15 southeast of the head. It is of the 2d magnitude, and is called AlpJiard. 383. CANCER (the Crab) is the least remarkable of the zodiacal constellations. It is situated about 15 north of the diamond in Hydra. It has no stars larger than the 3d magnitude, and is distinguished for a group of small stars called the Nebula of Cancer, which is often mis- taken, for a comet. A common telescope resolves this nebula into a beautiful assemblage of bright stars. 384. GEMINI (the Twins) may be known by two bright 380. Cam* Minor where ? Describe. 881. Where is Monoceros? How situated? Composed? Character? 882. Where is the head of Hydra? How formed \ Extent and position ? Largest, star ? 883. Describe Cancer, Situation? Composition? For what distiu- 1 SO ASTRONOMY. stars of the 2d magnitude one in the head of each figure. They are about 5 apart ; the northeasterly one, and the brightest of the two, being about 25 due north ofProcyon. This ia PottuM j and the other one is called Castor. The bodies of the Twins extend from Castor and Pollux about 15 to the southwest, or toward Betel- guese, in the right shoulder of Orion. " This constellation," says Dr. Adam Clark, " was deemed propitious to mariners ;" and on this account, the ship in which St Paul sailed from Alexandria (Acts xxviii. 11) had the sign of Castor and Poll'ix. 385. HERSCHEL'S TELESCOPE covers two stars of the 5th magnitude, near each other, and about 10 north of Cas- tor ; and one other star of the same magnitude, about 10 northwest of the two first named. It is a small aftair to immortalize Herschel's grand telescope. 386. THE LYNX is situated between Gemini and Can- cer on the south, and the Pole in the north, the head being to the northwest. It has no stars larger than the 4th magnitude, and these are in two pairs the first 15 northeast of Cancer, and the other 30 north of it. It is a loose and tame constellation, with nothing striking or peculiar by which it may be identified. 387. CAMELOPAKDALUS (the Camelopard) extends from Perseus to the Pole. This, too, is a tame and uninterest- ing constellation, with but few stars in it, and those of the 4th magnitude, or less. The hind feet of the figure touch the llilky Way, and the head is composed of two stars of the 5th magnitude, 5 and 10 from the Pole star, toward the " dipper" in the Great Bear. We now pass eastward to constellations that are on the meridian in APRIL, MAY, AND JUNE. 388. URSA MAJOR (the Great Bear) is one of the most conspicuous in the northern heavens. It may be known 384. Gemini how known ? Names and situation of principal stars ? Of figures? (Note.) 385. fferschel's Telescope where? Character? 886. Situation of the Lynx ? Position ? Character ? 887. Position of CameUpardalis t Extent? Character? Where the feet ? Tlie head, uud how composed ? What range of constellations next do- &cribei cut ? Describe it. DOITBLS NZMTIuS. NEBUL.E. 203 It ANNULAR NEBULA. the mid ale star in the tail of the Great Bear. consists of a large and bright globular nebula, surrounded by a double ring, at a considerable distance from the globe; Dr rather a single ring divided through about two-fifths of its circum- ference, and having one portion turned up, as it were, out of the plane of the rest. A faint .nebu- lous atmosphere, and a small round nebula near it, like a satellite, com- pletes the figure. 459. Another very conspicuous nebula of this class may be found half-way between (3 and 7, in the Lyre, and may be seen with a telescope of moderate power. It is small, and particularly well defined, so as, in fact, to have much more the appearance of a flat oval solid ring, than of a nebula. The space within the ring is filled with a faint hazy light, uniformly spread over it, like a fine gauze stretched over a hoop. 460. " Planetary Nelulce" says Dr. Herschel, " are very extraordinary objects. They have, as their name imports, exactly the appearance of planets round or Slightly oval discs in some instances quite sharply ter- minated, in others a little hazy at the borders, and of a light exactly equable, or only a very little mottled, which, in some of them, approaches in vividness to that of the actual planets. Whatever be their nature, they must be of enormous magnitude." 461. Stellar Nebulce, or Nebulous Stars, are such as present the appearance of a thin cloud, with a bright star in or near the center. They are round or oval- 459. What other annular nebulae 1 Describe. 4t50. Planetary nebulae i Describe. 4(51. /Stellar nebulae ? Remarks of Professor Mitchcl? 204 ASTRONOMY. STELLAR NBBWUB. shaped, and look like a star with a burr around it, or a candle shining through horn. "It was an object of this kind," says Prof. Mitchel, " which first suggested to Sir "W. Ilerschel his great theory of the formation of suns out of a nebulous fluid. He thought it impossible to ac- count for the central location of stars, surrounded by nebulous matter, in any way except by sup- posing this to be a sort of atmos- phere attracted to, and sustained in its spherical form by, the power of the central body. I have examined specimens of these objects, and always with increasing wonder. Their magnitude must be enor- mous, as the stars are certainly not nearer than other stars ; and yet the circular halo around them is of a, diameter easily measured, and proves them to have a circumference perhaps greater than the entire orbit of Neptune." 462. One of the most remark- able nebula in all the heavens may be found around the mid- dle star in the sword of Orion. It is easily seen with a common telescope. It is shaped like the head of some animal a fish, for instance with its mouth open. Near the inner surface of this mouth are four stars, ranged in the form of a trapezium. It requires a good telescope to see four stars ; but, with powerful instruments, six are visible, instead of four. GREAT NEBULA. IN OHIO 462. "Describe the nebula of Orion? stars iu 5U Where situated ? Shape ? What NEBULA. 205 463. The sun is considered by astronomers as belong- ing to this class of nebulous stars ; and the Zodiacal Light (322 and 325) has been regarded as of the nature of the gaseous matter with which the nebulous stars are surrounded. It is supposed that if we were as far from the sun as from the stellar nebulas, he would appear to us only as a small and nebulous star ! 464. Until recently, the most powerful instruments have failed to reveal any thing like distinct stars, as com- posing the body of the remarkable nebula in Orion. Both the Herschels regarded it as positively irresolvable ; or, in other words, as composed of nebulous fluid or un- organized matter. But it has recently been seen to be composed of distinct stars, both by the monster telescope of Lord Rosse, and the great refractor of Cambridge, near Boston. 465. The magnitude of this nebula must be beyond cill human conception. " If," says Mr. Smyth, " the parallax of this nebula be no greater than that of the stars, its breadth cannot be less than a hundred times that of the diameter of the earth's orbit ; but if, as is more probable, it is a vast distance beyond them, its magnitude must be utterly inconceivable." 466. Prof. Mitcliel observes, that in cio light be not absorbed in its journey through the celestial spaces, the light of the nebula of Orion cannot reach the eye in less than 60,000 years, with a velocity of twelve millions of miles in every minute of time! And yet this object may be seen from this stupendous distance, even by the naked eye ! What, then, must be its dimensions ? Here, indeed, we behold a universe of itself too vast for the imagination to grasp, and yet so remote as to appear y taint spot upon the sky." 467. The number of such nebulous bodies is unknown 463. Kemarks respecting the sun ? 404. How the nebula in Orion regarded ? What recent discovery ? 465. Its probable magnitude ? Kemark of Smyth ? 406. Prof. Mitchel's observations respecting its distance and dimensions? 467. What said of the number of nebulous bodies in the heavens ? Where n ^st abundant I II orschel's catalogue t Various forms I 206 ASTRONOMY. perhaps we should say innumerable. The} 7 are especially abundant in the Galaxy or Milky Way. Sir W. II er- schel arranged a catalogue, showing the places of two thousand of these objects. They are of all shapes and sizes, and of all degrees of brightness, from the faintest milky appearance to the light of a fixed star. 468. Star Dust is a name given to those exceedingly faint nebulous patches that appear to be scattered about at random in the far-distant heavens. It is barely visible through the best telescopes, and seems to form a sort of back-ground, far beyond all stars, clusters, and nebulae, resolvable or irresolvable. 469. " The. nebulae," says Sir John Ilerschel, " fur nish, in every point of view, an inexhaustible field of speculation and conjecture. That by far a larger share of them consist of stars, there can be little doubt ; and in the interminable range of system upon system, and firmament upon firmament, which we thus catch a glimpse of, the imagination is bewildered and lost." 470. It is a general belief among astronomers that the material universe consists of distinct clusters, separated from each other by innumerable chasms : that the fixed stars by which we are surrounded constitute one great cluster the sun being a star with the rest, arid appearing as he does to us, solely on account of our nearness to him ; that the nebulae are far beyond our cluster, like so many distinct continents in the boundless ocean of immensity. 471. Could we leave our system, and pass outward toward the fixed stars, they would doubtless expand to the dimensions of suns as we approached them, while our own central luminary would dwindle to a glimmering star. Reaching the frontier of the cluster, and plunging off into the awful solitudes of space, toward the distant nebulas beyond, we should see them also expand as we drew near, while our vast firmament of stars seemed to 468. What is meant by star dust? Where supposed to be situated ? 4t>9. Herschel's remark respecting the nebulae i 470. What the prevailing opinion among astronomers, as to the structure of the universe ? 471. What imaginary journey and scenery described by the author ? NEBULA. 207 be gathering into a compact group ; till at length, enter- ing the bosom of the distant nebulas, we should find our- selves surrounded by new and strange constellations; and if we saw our own firmament at all, should see it only as a faint annular nebula, far beyond the reach ot all unassisted vision. 472. The great stellar cluster in which the sun and solar system are imbedded is supposed, in its form, to resemble a double convex lens, with the sun and solar system near its center; and by being viewed edgewise from our central position, to give us the phenomenon of the Milky Way. OHUAT NKEULA OF Tlffl 8OLAE SYSTEM. The above is an edgewise view of the great stellar cluster, in the midst of which tho solar system is placed, as drawn by Sir William Herschel. Its figure was ascertained by ganging the space-penetrating power of his telescope, and then " sounding the heavens," to ascertain the distance through the cluster, in all directions, to the open void. Tho nebula? lie in distinct and independent islands, far beyond the limits of our cluster. Let the student imagine the sun to be one of the stars near the middle of the lens- shaped cluster, of which the above is an edge view, with the planets revolving close around it If, then, he look out upon the surrounding stars, the number visible, and their distinctness, will depend upon the direction in which he looks. If toward the thin part of the cluster (either up or down in the cut), fewer stars will be seen, while they will be comparatively distinct But if the view be toward the edge of the cluster, instead of the sides (or horizontally, in the cut), there will be seen beyond the large stars, and fading away to an indistinct and mingled light, a numberless host of stars; and this zone of distant stars will extend quite around the heavens. Such is the Galaxy or Milky Way. The zone of milky light is the light of the stars in the remote edge of the great cluster. The opening in the left end of the figure is a split in the cluster, and constitutes the division seen in the milky way, extending part way around the heavens. See cut page 203. The vast apparent extent of the Galaxy, as compared with other nebulje, is supposed to be justly attributable to its comparative nearness. Were we as far from the solar system as from the nebulae in the Lyre, the Milky Way would doubtless appear as an annular nebula no larger than that. It may therefore with propriety be called "tho great nebula of the solar system." 473. Sir W. Herschel estimated that 50,000 stars passed the field of his telescope, in the Milky Way v in a 472. Supposed form of our own stellar cluster f Philosophy of Galaxy (Why apparently PO large ? How appear at a great distance ?) 473. Slurs in Milky Way ? Mutual distances ? Character of each sfcvr? 208 ASTRONOMY. single hour! And yet the space thus examined was hardly a point in the mighty concave of our own <; sun strown firmament." What an idea is here conveyed to the mind, of the almost boundless extent of the uni- verse ! The mutual distances of these innumerable orbs are probably not less than the distance from our sun to the nearest fixed stars, while they are each the center of a distinct system of worlds, to which they dispense light and heat. 474. "Were the universe limited to the Great Solar Cluster, in the midst of which we are placed, it would be impossible to conceive of its almost infinite dimen- sions ; but when we reflect that this vast and glowing zone of suns is but one of thousands of such assem- blages, which, from their remoteness, appear only as fleecy clouds hovering over the frontiers of spa*ce, we are absolutely overwhelmed and lost in the mighty abyss of being ! 475. And here we close our rapid and necessarily im- perfect survey of the Sidereal Heavens. And while the mind of the student is filled with awe, in contemplating the vastness and majesty of creation, let him not forget that over all these Jehovah reigns that " these are but parts of his ways ;" and yet so perfect is his knowledge and providence in every world, that the very hairs of our heads are numbered, and not a sparrow falls without his notice. And while we behold the wisdom, power, and goodness of God so gloriously inscribed in the heav- ens, let us learn to be humble and obedient to love and serve our Maker here that we may be prepared for the still more extended scenes of another life, and for the society of the wise and good in a world to come. 474. Magnitude of our own cluster ? What in comparison with all others? 475. Remarks in closing paragraph ? Moral reflections ? PART III, PRACTICAL ASTRONOMY CHAPTER I. PROPERTIES OF LIGHT. 476. Practical Astronomy has respect to the means employed for the acquisition of astronomical knowledge. It includes the properties of light, the structure and use of instruments, and the processes of mathematical calcu- lation. In the present treatise, nothing farther will be attempted than a mere introduction to practical astronomy. In a work designed for popular use, mathematical demonstrations would be out of place. Still, every student in astronomy should know how telescopes are made upon what laws they depend for their power, and how they are used. It ia for this purpose mainly that we add the following chapters on Practical Astronomy. 477. Light is that invisible ethereal substance by whi'cli we are apprised of the existence, forms, and colors of material objects, through the medium of the visual organs. To this subtile fluid we are especially indebted for our knowledge of those distant worlds that are the principal subjects of astronomical inquiry. 478. The term light is used in two different senses. It may signify either light itself, or the degree of light by which we are enabled to see objects distinctly. In this last sense, we put light in opposition to darkness. But 476. Parts of the book gone over ? Subject of Part III. ? Of Chapter ]. I What is practical astronomy? (How far discussed in this treatise ?) 477. Define light. For what indebted to it ? 478. Different senses in which the term is used ? "What is darkness ? Can it be uark and light at the same time ? Is there any place without light ? (Quotation from Dick ?) 210 ASTRONOMY. it should be borne in mind that darkness is merely the absence of that degree of light which is necessary to human vision ; and when it is dark to us, it maybe light to many of the lower animals. Indeed, there is more or less light even in the darkest night, and in the deepest dungeon. " Those unfortunate individuals," says Dr. Dick, " who have been confined in the dark- est dungeons, have declared, that though, on their first entrance, no object could be per- ceived, perhaps for a day or two, yet, in the course of time, as the pupils of their eyes expanded, they could readily perceive mice, rats, and other animals that infested their cells, and likewise the walls of their apartments; which shows that, even in such situa- tions, light is present, and produces a certain degree of influence," 479. Of the nature of the substance we call light two theories have been advanced. The first is, that the whole sphere of the universe is filled with a subtile fluid, which receives from luminous bodies an agitation j so that, by its continued vibratory motion, we are enabled to per ceive luminous bodies. This was the opinion of Des- cartes, Euler, Huygens, and Franklin. The second theory is, that light consists of particles thrown off from luminous bodies, and actually proceeding through space. This is the doctrine of Newton, and of the British philosophers generally. "Without attempting to decide, in this place, upon the relative merits of these t*v> hy- potheses, we shall use those terms, for convenience sake, that indicate the actual passage of light from one body to another. 480. Light proceeds from luminous bodies in straight lines, and in all directions. It will not wind its way through a crooked passage, like sound ; neither is it con- lined to a part of the circumference around it. As the sun may be seen from every point in the solar system, and far hence into space in every direction, even till he appears but a faint and glimmering star, it is evident that he fills every part of this vast space with his beams. And the same might be said of every star in the firmament 481. As vision depends not upon the existence of light merely, but requires a certain degree of light to emanate from the object, and to enter the pupil of the eye, it is obvious that if we can, by any means, concentrate the 479. What theories of the nature of light, and by whom supported respect- vely ? (Remark of author ?) 480. How light proceeds from luminous bodies ? (Eadiations from sun and stars ?) 481. How improve vision, and why ? (Animals ?) REFRACTION OF LIGHT. 211 light, so that more may enter the eye, it will improve our perception of visible objects, and even enable us to see objects otherwise wholly invisible. Some animals have the power of adapting their eyes to the existing degrw of light. The cat, horse, &c., can see day or night; while the owl, that sees well in the night, sees poorly in the day-time. 482. Light may be turned out of its course either by reflection or refraction. It is reflected when it falls upon the highly polished surface of metals and other intrans- parent substances ; and refracted when it passes through transparent substances of different densities. REFRACTION OF LIGHT. 483. Whenever light passes from a rare medium to one more dense, and enters the latter obliquely, it inva- riably leaves its first direction, and assumes a new one. This change or bending of the rays of light is what is called Refraction. The term refract is from the Latin re, and frango, to break ; and signifies the break Ing of the natural course of the rays. 484. As air and water are both transparent, but of different densities, it follows that, when light passes obliquely from one to the other, it will be refracted. If it pass from the air into the water, it will be re- fracted toward a per- pendicular. Here the ray A C strikes the water perpendicularly, and passes directly through to B without oeing refracted. But the ray I) C strikes the water at C obliquely ; and instead of passing straight through to E, is refracted at C. and reaches the bottom of the water at F. If, therefore, a person were to receive the ray into the eye at F. and to judge of the place of the object from which the lijiht emanates from the direction of the ray "C F, he would conclude that he saw the object at G, unless he made allowance for the refraction of the light at C. LIGHT EEFKACTED BY WATEB. G A E 4S2. How \\fr\\t turned out of course ? 483. \V hat is refraction ? How produced ? (Derivation of term refract 'A 184. How refracted by air and water? (Illustrate by diagram.') 212 ASTRONOMY. LIGHT PBOCEEDINO FEOU WA1EC. B 485. When light passes obliquely from a denser to a rarer medium, as from water into air, it is refracted from a perpendicular to- ward a horizontal. Here the lamp A shines up through water into air. The ray that strikes the surface per- pendicularly passes on to B without being refracted; but - - the other rays that leave the water obliquely are refracted toward a horizontal direction, in proportion to their distance from the perpend icfular; or, in other words, in propor- tion to the obliquity of their contact with the surface of the water. 486. In consequence of the refraction of light toward a horizontal direction, in passing from water into air, a pole, half of which is in the water, seems bent at the surface, and the lower end seems nearer the surface than it really is. For the same reason, the bottom of a river seems higher, if seen obliquely, than it really is ; and the water is always deeper than we judge it to be. In this cut, the oar, the blade of which is in the water, seems bent at the surface of the water. The rays of light passing from the part under water to the surface at D, are refract- ed toward a horizontal direction at that point, and received into the eye of the observer at B, who, judging of the position of the immersed portion of the oar from the direction of the rays D B, locates the blado of the oar at C; thus reversing the effect illustrated at 484 487. The refracting power of different transparent substances depends mainly upon their density. Water refracts more than air, glass more than water, and dia- mond most of all. But the angle of incidence, or the obliquity of the contact of the rays with the denser sub- EFFECT OF REFRACTION. 485. How when light passes from denser to rarer mediums ? (Diagram.) 486. Effect of refraction upon objects seen under water ? (Diagram.) 487. Upon what does the refracting power of different transparent media openOTTBLE-CONVKX FOCAL DISTANCE. 500. The distance of the focus of a double-con- vex glass lens is the ra- dius of the sphere of its convexity. In this cut, it will be seen that the parallel rays A are refracted to a focus nt C, by the double convex lens B, the convexity of whose surfaces is juBt equal to the curve of the circle D. 501. The focal distance of a plano-convex lens is equal to the diameter of the sphere formed by the convex surface produced. PLANO-CONCAVE FOCAL DIOTANOB. It must be borne in mind that light is refracted boUi when it enters and when it leaves a double-convex lens, end in both instances in the same direction ; and, so far as the distance of the focus is con- cerned, to the same extent. But when the lens is convex only on one side, half its re- fracting power is gone, so that the rays are not so soon re- fracted to a focus. In this case, the focal distance is equal to ttie diameter of the sphere formed by extending the convex surface of the lens ; while with the double-convex lens, the focal distance is only equal to the radius of such -sphere. In the cut, the parallel rays A are refracted to a focus at B, by the plano-concave lens C ; and the distance C B is the diameter of the circle D, formed by the convex surface of the lens produced. 502. A double- concave lens dis- perses parallel rays, as if they diverged from the center of a circle formed by the con- vex surface pro- duced. In this cut, the parallel rays A are dispersed by the double-concave lens B, as shown at C; and their direction, as thus refracted, is the same as if they proceeded from tno point D, which is the center of a circle formed by the concave surface of the lens pi in- duced. 500. How focal distance governed ? (Diagram.) f01. What is the focal distance of & plano-convex lens ? (Diagram.) 502. Effect of douUct-wnwjc lens ? Amount of divergency of rayu ? (Dia- gram.) 10 KAYS DISPERSED BY EEFEACTIOM. 218 ASTRONOMY. BUENINQ-GLAbS, 503. Common spectacles, opera-glasses, burning-glasses, and refracting telescopes are made by converging light to a focus, by the use of double-convex lenses. The ordinary burning-glass, which may be bought for a few shillings, is a double-convex disk of glass two or three inches in diameter, inclosed in a slight metallic frame with a handle on one side. Old tobacco-smokers some- times carry them in their pockets, to light their pipes with when the sun shines. In other instances, they have been so placed as to fire a cannon in clear weather, by igniting the priming at 12 o'clock. The adjoining cut represents a large bnrn- Ing-glass converging the rays of the sun to a focus, and setting combustible substances on fire. Such glasses have been made power- ful enough to melt the most refractory sub- stances, as platinum, agate, &c. " A lens three feet in diameter," says Professor Grny, u has been known, to melt carnelian in 75 seconds, and a piece of white agate in SO seconds." REFLECTION OF LIGHT. 504. We have now shown how light may be turned out of its course, and analyzed, dispersed, or converged to a point by refraction. Let us now consider how it may be converged to a focus by reflection. 505. When light falls upon a highly polished surface, especially of metals, it is reflected or thrown off in a new direction, and the angles of con- tact and departure are always equal. Let A B represent the polished metallic sur- face. C the source of light, and the arrows the direction of the ray. Then D would represent lue angle of incidence or contact and E the angle of reflection or departure which angles are seen to be equal. MFLKCTIolf BY A 503. What articles made with double-ec nvex lenses? Ues ? (Power 01 burning-glasses ?) 504. What now shown in this chapter ? What next ? 505. What is rwCfetiMfc and when does it take piaee ? What law govern* it? (Diagram.) REFLECTION OF LIGHT. 506. A concave mirror reflects parallel rays back to a focus, the distance of which is equal to half the radius of the sphere formed by the concave surface produced. BEFLECTION BT A CONCAVE MIKKOR. In this cut, the parallel rays A fall upon the concave mirror B B, and are reflected to the focus C, which is half the radius of the sphere formed by the surface of the mirror produced. If, therefore, it was desirable to construct a concave mirror, having its focus 10 feet distant, it would only be necessary to grind it on the circle of a sphere having a nidi us of 20 feet. ~. 507. In reflection, a. portion of the light is absorbed or otherwise lost, so that a reflector of a given diameter will not converge as much light to a focus as a double-convex lens of the same size. In the latter case, all the light is transmitted. Still, reflectors have been formed of such power as to melt iron, and other more difficult sub- stances. We have now considered so much of optics as is necessary to an understanding of the principles upon which telescopes are constructed; and, for further particulars, shall refer tl'.e student to books of Natural Philosophy. 506. How does a concave mirror reflect parallel rays ? Distance of focus ? (Diagram. How would you construct a concave mirror with a 10 feet focus ?) 507. Is all the light falling upon a polished surface reflected ? What then f (Closing note ?) 20 ASTRONOMY. CHAPTER II. TELESCOPES. 508. A Telescope is an optical instrument employed in viewing distant objects, especially the heavenly bodies. The term telescope is derived from two Greek words, viz., tele, at a distance, and skopeo, to see. 509. So far as is now known, the ancients had no knowledge of the telescope. Its invention, which oc- curred in 1609, is usually attributed to Galileo, a phi- losopher of Florence, in Italy. The discovery of the principle upon which the refracting telescope is constructed was purely accidental. The children of one Jansen, a spectacle-maker of Middleburgh, in Holland, being at play in their father's shop, happened to place two glasses in such a manner, that in looking through them, at the \veather-cock of the church, it appeared to be nearer and much larger than usual. This led their father to fix the glasses upon a board, that they might be ready for observation ; and the news of the discovery was soon conveyed to the learned throughout Europe. Galileo hearing of the phenomenon, soon discovered the secret, and put the glasses in a tube, instead of on a board ; and thus the first telescope was constructed. 510. The telescope of Galileo was but one inch in di ameter, and magnified objects but 30 times. Yet with this simple instrument he discovered the face of the moon to be full of inequalities, like mountains and val- leys; the spots on the sun ; the phases of Venus ; the satel- lites of Jupiter ; and thousands of new stars in all parts of the heavens. Notwithstanding this propitious commencement, so slow was the progress of the telescope toward its present state, that in 1S16, Bonnycastle speaks of the 80-fold mag- nifying power of the telescope of Galileo as "nearly the greatest perfection that this kind of telescope is capable of I" 511. If he be the real author of an invention who, from a knowledge of the cause upon which it depends, deduces it from one principle to another, till he arrives 508. Subject of Chap. II. ? Telescope ? Derivation ? 509. Ancient or modern ? Inventor ? (Incidents of discovery ?) 510. Galileo's telescope? Discoveries with it? (Progress in telescoro making ?) 511. IB Galileo entitled to the lionor of i- venting the telescope? (AA tsro 'a his ?) DIFFERENT KINDS OF TELESCOPES. 221 at the end propobed, then the whole merit of the inven- tion of the telescope belongs to Galileo. The telescope of Jansen was a rude instrument of mere curiosity, acci- dentally arranged ; but Galileo was the first who con- structed it upon principles of science, and showed the practical uses to which it might be applied. It is said that the original telescope constructed by Galileo Is still preserved in the Britihh Museum. A pigmy, indeed, in its way, but the honored progenitor of a race of giants! 512. The discovery of the telescope tended greatly to sustain the Copernican theory, which had just been pro- mulgated (10), and of which Galileo was an ardent dis- ciple. Like Copernicus, however, his doctrines subjected him to severe persecutions, and he was obliged to re- nounce them. The following is his renunciation, made June 28, 1633 : " I, Galileo, in the seventieth year of my age, on bended knees before your eminences, having before my eyes and touching with my hands the Holy Gospels, I curse and detest the error of the earth's movement" As he left the court, however, after this forced renunciation, he is said to have stamped upon the earth, and exclaimed, " It does move, after all !" Ten years nt'ter this he was sent to prison for the same supposed error; and soon, his age advan- cing, the grave received him from the malice of his persecutors. DIFFERENT KINDS OF TELESCOPES. 513. Telescopes are of two kinds Reflectors and Re- fractors. Refracting telescopes are made by refracting the light to a focus with a glass lens (499) ; and reflect- ing telescopes, by reflecting it to a focus with a concave mirror (506). Besides this general division, there are various kinds, both of reflectors and refractors. 514. Telescopes assist vision in various ways first, by enlarging the visual angle under which a distant ob- ject is seen, and thus magnifying that object ; and, secondly, by converging to a point more light than could otherwise enter the eye thus rendering objects distinct or visible that would otherwise be indistinct or invisible. All the light falling upon a six or a twelve inch lens may be converged to a focus, so as to be taken into the human eye through the pupil, which is but one-fourth of an inch in diameter. Our vision is thus made as perfect by art as if nature had given us ability to enlarge the eye till the pupil was a foot in diameter. 512. Eelation of discovery to Copernican theory ? Effects upon Galileo ? (His renunciation ? Death ?.) 513. Kinds of telescopes ? Describe. 514. How os.-ust vision ? (Illustrative note ?) 22 ASTEONCXMY. 515. Refracting telescopes may consist of a double- couvex lens placed upon a stand, without tube or eye- Eiece. Indeed, a pair of ordinary spectacles is nothing jss than a pair of small telescopes, for aiding impaired vision. REFRACTING TELESCOPE WITH A SINGLE LEXS. Here the parallel rays ar seen to pass through the lens at A, and to be so converged to a point as to enter the eye of the beholder at B. His eye is thus virtually enlarged to the size of the lens at A. But it would be very difficult to direct such a telescope toward celestial objects, or to get the eye in the focus after it was thus directed. 516. The Galilean telescope consists of two glasses a double-convex next the object, and a double-concave near the eye. The former converges the light till it can be received by a small double-concave, by which the con- vergency is corrected (502), and the rays rendered paral- lel again, though in so small a beam as to be capable of entering the eye. GALILEAN TELESCOPE. Here the light is converged by the lens A, till it can be received by the double-con- cave lens B, by which the rays are made to become a small parallel beam, that can enter the eye at C. This was the form of the telescope constructed by Jansen, and improved by Galileo ; on which account it is called the Galilean telescope. In the cut, the two lenses are represented as fastened to a board, as first exhibited by Jansen. 517. The common astronomical telescope consists of two glasses viz., a large double-convex lens next tho 15. Simplest form of refracting telescope ? (Diagram ?) 16. Galilean telescope I (Diagram and < 515. 516. lean?) 617. How common astronomical telescopes made ? Why in tube t explanation I Wiry called Gall- DIFFERENT KINDS OF TELESCOPES. 223 object, called the object-glass / and a small double-convex lens or microscope next the eye, called the eye-piece. For the greater convenience in using, they are both placed in a tube of wood or metal, and mounted in various ways, according to their size, and the purposes to which they are devoted. LKXSES PLACKD IN A TUBE. REFRACTING TELESCOPE MOUNTED ON A STAND. A is the object-glass, B the eye-piece, and C the place where the tube in which the eye-piece is set slides in and out of the large tube, to adjust the eye-piece to the focal ilistance. By placing the lenses in a tube, the eye is easily placed in the focus, and th* object-glass directed toward any desired object. 518. The object-glass of a telescope is usually pro- tected, when not in use, by a brass cap that shuts over the end of the instrument ; and the eye-pieces, of which there are several, of different magnifying powers, are so fixed as to screw into a small movable tube in the lower end of the instrument, so as to adjust them re- s -Actively to the fo- cus, and to the eyes of different observ- ers. Such telescopes usually represent ob- jects in an inverted position. The adjoining cut represents the simplest form of a mounted refractor. The object-glass is at A, where the brass cap may be seen covering it. B is the small tube into which the eye-piece is screwed, and which is moved in and out by the small screw C. Two eye-pieces muy be seen at D one short one, fur astro- nomical observations; and a long one, for hind objects. For - iewing the sun, it is necessary to add a screen, made of colored glass. At E, a bolt goes i ito a socket in the top of the stand, in which it turns, allowing the telescope to sweep 518. How object-glass protected? What suid of eye-pieces? (Cut and explanation ?) 224 ASTRONOMY. erpund the horizon; while the joint, connecting the saddle in which the telescope rests with the top of the bolt, allows it to be directed to any point between the horizon and the zenith. But such stands answer only for comparatively small instruments. 510. Refracting telescopes are mounted in various ways. So important is it that they should not shake 01 vibrate, that, in most observatories, the stand rests upon heavy mason-work in no way connected with the build- ing, so that neither the wind nor the tread of the ob- server can shake it. They are then furnished with a double axis, which allows of motion up and down, or east and west ; and two graduated circles show the pre- cise amount of declination and right ascension. They are then furnished with clockwork, by which the tele- scope is made to move westward just as fast as the earth turns eastward ; so that the celestial object being once found, by setting the instrument for its right ascension and declination, or by the aid of the Finder a small telescope attached to the lower end of the large one it may be kept in view by the clockwork for any desirable length of time. A telescope thus furnished with right ascension and declination circles is called an Equatorial, or is said to be eqnatwially 'mounted, because it sweeps east and west in the heavens parallel to the equator. 520. The object-glasses of telescopes are not always made of a single piece of glass. They may be made of two concavo-convex glasses, like two watch crystals, with their concave sides toward each other, or with a thin double concave glass between them. They are thus double, or triple ; but when thus constructed, the whole is called a lens, as if composed of a single piece. Lenses have also been formed by putting two concavo-convex glasses together, and filling the space between them with some transparent fluid. These are called Barlow lenses, from Prof. Barlow, their inventor. 521. As a prism analyzes the light, and exhibits dif- ferent colors, so a double-convex lens may analyze the 519. How refractors mounted, and why ? When equatorial, and why ? 520. How object-glasses made ? What a lens ? A Barlow lens ( 521. What is an Achromatic telescope ? ( Derivation of term ?) DIFFERENT KINDS OF TELESCOPES. light that falls near its circumference, and thus represent the outside of the heavenly bodies as colored. But this defect is remedied by using disks made of different kinds of glass, so as to correct one refraction by another. Re- fracting telescopes thus corrected are called Achromatic telescopes. Achromatic is from the Greek a chroma, which signifies destitute of color. Most refracting telescopes are now so constructed as to be achromatic. 522. It is but recently that any good refracting tele- scopes have been made in this country. The best have been made in Germany and France. Several very good instruments have been made by Alvan Clark, Esq., of Boston, Charles A. Spencer, Esq., of Troy, N. Y., and the late Henry Eitz, Jun., of New York City. The author was personally well acquainted with Mr. Fitz, and during his life gave favorable descriptions of his instruments in tbase pages, and did all that he could to make his capabilities known to the American public. He made his first telescope in Io85. In the winter of 1841 he invented a method of perfecting object-glasses for refracting telescopes, making the first one of the bottom of an ordinary tumbler, la the fall of 1845 he exhibited at the Fair of the American Institute, an instrument of 6 inches aperture, which, although made of common American material, in the way of flint glass, was a very excellent instrument. It secured him the friendship of noted astronomers, and from that time forward he devoted himself to the business of telescope-making with a good degree of success. Continually progressing in size, he finally succeeded in making instruments of 16 inches aperture, one of which is now in the possession of Mr. Van Dusee, of Buffalo, N. \. lie made two of 18 inches, ono for the Dudley Observatory, at Albany, and the other for an association of gentlemen at Allegheny City, Pa. Of 12 inches aperture he' produced one for the Observatory at Ann Arbor, Michigan, and another for the Vassar Female College. He made for Mr. L. M. Rutherford, of New York, at various times, telescopes of 4, 5J, 6, 9, and 11J inches aperture; the last, an Instrument of remarkable defining power, is now mounted in Mr. Rutherford's Observatory in Eleventh Street, New York city. Mr. Vickers of Baltimore has a 10-inch. Several of the size of 8 and 9 inches are scattered over the country. The British Charge^ 33. Where located? How mounted? By whom mar unequal reduc- tion of the temperature. This speculum has a reflecting surface of 4,071 square inches. The tube is made of deal wood, one inch thick, land hooped with iron. Its diameter is seven feet, and its length 56. The entire weight of this telescope is twelve tons. It is mounted between two north and south walls, 21 feet apart, 72 feet long, and 48 feet high. The lower end rests upon a universal hinge. It can be lowered to the horizon, and raised to the zenith, and lowered northward till it takes in the Pole star. Its motion from east to west is limited to 15 degrees. This magnificent instru- ment is situated at Burr Castle, Ireland. It was con- structed by the Earl of Rosse, at an expense of $60,000 544. Lord Rosse's telescope ? "Weight of speculum? Diameter? Tliick- iH&a ? Cooling 9 %Tube 1 Entire weight ? How mounted! What motion 3 VVhcre located ? Co^, ' OBSERVATORIES AND TELESCOPES. OBSERVATORIES 'AND TELESCOPES IN THE TTNITKD STATES. CBSEKVAIOEIES. THEIK TELESCOPES. Wh-n procured. Name of utaker. Focr.l lengih. ot$t*kM. Cost. Ya'e College 1880 1836 (1836 1 1852 1S37 1S40 1S41 1844 1846 1848 1849 1850 1851 1852 1846 1854 1S53 1S57 ? 1S57 1846 1S47 J 1S50 1 1851 1S52 Dollond. Lerebours. Holcomb. A. Clark. Si nuns. Merz. Lerebours. Merz. Siinms. Fitz. Merz.,: Fitz. Clark. Fitz. Spencer. Fitz. ft. in. 10 rr 10 9 5 6 8 4 ^ 15 3 17 22 6 9 7 6 7 10 4 8 4 5 7 8 6 17 15 2 16 8 4 7 5 7 8 4 11 6 5 10 9 6 inches. 5 6 reflector. 7 4 6* 6 9-6 12 15 0-4 4-8 5-6 7-5 ? 5 7i 12| 18 18* 63-10 5 4 5 6| 8* 4* 4 8 9 $1.000 1,000 1,900 6,000 9.^37 19,S-i2 l.COO 1.030 8.500 1,200 2*5 i.soo 6.000 14.500 10.000? 1.833 900 4-25 750 l.OdO 2,220 300 2'25 1,150 2.200 *Yesleyan University "Williams Collpge Hudson Ohio Philadelphia West Point Washington Dartmouth College Georgetown fchelby Columbia (S. C.) College Columbia (Mo) Friends Philadelphia Amherst College Dudley, Albany, N. Y Hamilton College J. Jackson, near Philadelphia. .. Mr. Longstreet, Philadelphia S. G. Gummere, Burlington, N. J. II. Vanarsdale, Newark, N. J W. S. Van Duzee, Buffalo, N. Y. . "W S Dickie Elkton Ky D. Mosnian, Bangor, Me J. Campbell, Now York L. M. Rutherford, New York .... FOREIGN OBSERVATORIES TIIEIB LATITUDE AND LONGITUDE. OBSERVATORIES. Latitude. Lonpritud e iii Timo. Altona 53 54 52 50 52 83 55 58 58 55 51 51 54 48 38 48 50 41 45 48 82 21 80 51 12 56 40 22 23 57 31 28 42 8 6 50 56 53 4 12 45 12.7 16.7 10.7 51.8 8 53 47.1 13 23.2 47.9 38.2 50.4 45 44 13 29.7 54 6 85.5 N. N. N. N. N. S. N. N. N. N. N. N. N. N. N. N. N. N. N. N. 1) 1 1 1 g 1 m. 39 26 53 17 13 50 46 25 12 39 22 46 53 9 1 49 30 5 46'. 2 35.5 34.9 27.2 23.5 56.0 19.3 54.6 22 43.0 46.8 0.0 0.4 25.4 25.5 21.5 13.5 54.7 48.4 32.6 E. W. E. E. E. E. E. E. w! E. E. E. E. E. E. E. E. E. Berlin Brussels Cape of Good Hope Copenhagen Dorpat Dublin Edinburgh. Gottiniren Konigsberg Munich Palermo Paris Turin Vienna Public observatories in this country? Largest telescope ? Table? Private observatories names ? Telescopes by whom mostly made ? What other table ? PARALLAX OF TEE HEAVENLY BODIES. 545. Parallax is the difference between the altitude of any celestial object seen from the earth's surface, and tho altitude of the same object seen at the same time from the earth's center; or it is the angle under which the semi-diameter of the earth would appeal-, as seen from the object. The' true place of a celestial body is that point of the heavens in which it would be seen by an eye placed at the center of the earth. The apparent place is that point of the heavens where the body is seen from the surface of the earth. The parallax of a heavenly body is great- est when in the horizon, and is thence called the hori- zontal parallax. Parallax decreases as the body ascends toward the zenith, at which place it is nothing:. PARALLAX OF THE PLAXKT8. F The adjoining cut will afford a sufficient illustration. When the observer, standing upon the earth at A, vieivs the object at B, it appears to be at C, when, at the same time, if viewed from the center of the earth, it would appear to be atD. The parallax is the angle B G D or A B E, which is the difference between the altitude of the object B, when seen from the earth's surface, and when seen from her center. It is also the angle under which the semi-diameter of the earth, AE, is seen from the object B. As the object advances from the horizon to the ze- nith, the parallax is seen gradually to diminish, till at F it has no parallax, or its apparent and true place are the same. This diagram will also show why objects nearest the earth have the greatest parallax, and those most distant the least ; why the moon, the nearest of all the heavenly bodies, has the greatest parallax; while the "fixed stars, from their immense distance, have no appreciable horizontal parallax the semi-diaineter of the earth, at such a distance, being no more than a point 546. As the effect of parallax on a heavenly body is to depress it "below its true place, it must necessarily affect its right ascension and declination, its latitude and longi- tude. On this account, the parallax of the sun and moon must be added to their apparent altitude, in order to obtain their true altitude. The true altitude of the sun and moon, except when in the zenith, is always affected, more or less, both by parallax and refraction, but always in a contrary manner. Hence the mariner, in finding the latitude at sea, h ways adds the parallax, and ubtracta the refraction, to and from the sun's observed altitude, in order to obtain the true altitude, find thence the latitude. 545. Parallax? True place of a celestial body ? Apparent? W lien par- allax greatest ? Least? Called what, and why 2 (Diagram ? What objects greatest parallax?) 546. Effect of parallax? How obtain true altitude ? (IIo\v differ from ro- fhictiou ? How theu obtain true altitude I) 244 547. The principles of parallax are of great import- ance to astronomy, as they enable us to determine the distances of the heavenly bodies from the earth, the mag- nitudes of the planets, and the dimensions of their or- bits. The sun's horizontal parallax being accurately known, the earth's distance from the sun becomes known ; and the earth's distance from the sun being known, that of all the planets may be known also, because w r e know the exact periods of their sidereal revolutions, and, according to the third law of Kepler, the squares of the times of their revolutions are proportional to the cubes of their mean distances. Hence, the first great desideratum in astronomy, where measure and magnitude are concerned, is the determination of the true parallax. At a council of astronomers assembled in London some yenrs since, from the most learned nations in Europe, the sun's mean horixontal parallax was settled, as tlie result of their united observations, at 0' 8".57T6. Now the value of radius, expressed like- wise in seconds, is 2oG264".8; and tins divided by 8".577(>, gives 24047 for the distance of the sun from the earth, in semi-diameters of the latter. If we take the iuTO TABLES. 25 i V. TO FIND THE HOURLY MOTION OF A PLANET IN ITS ORBIT. RULE. Multiply the planet's mean distance from the Sun by 6.2831853, and divide the product by the time of the planet's sidereal revolution, expressed in- hours, and the deci mals of an hour. By Logarithms. Add 0.7981799 to the logarithm of the planet's mean distance from the Sun, and from the sum subtract the logarithm of the planet's revolution, expressed in hours. EXAMPLE. Required the Earth's hourly motion in its orbit Log. of Earth's distance=Y.97S073S + 0.79S1799= 8.7771587 Subtract loff. of Earth's revolution 8.9428090 Gives Earth's horary motion, 63,288 miles, 4.8343447 VI. TO FIND THE HOURLY MOTION OF A PLANET ON ITS AXIS. RULE. Multiply the diameter of the given planet by 3.141 59, and divide the product by the period of its diurnal rotation. By Logarithms. Add 4.0534524 to the logarithm of the planet's diameter, and from the sum subtract the logarithm of its diurnal rotation, expressed in seconds. Earth's diameter, 7924 lo