PAT'l' OF B1ELA5S COMET. LETTERS ON ASTRONOMY, IN WHICH THE ELEMENTS DF THE SCIENCE ARE FAMUIIALRY EXPLAINED IN CONNECTION WITH BIOGRAPHICAL SKETCHES OF TIlE MOST EMINENT ASTRONOMERS. WITH NUMEROUh ENGRAVINGS. BY DENISON OLMSTED,'LL.D., PROFESSOR OF NATURAL PHILOSOPIIY AND ASTRONOMY IN YALE COLLEGI ebfseb ~btffon. INCLUDING THE LATEST DISCOVERIES NEW YORK: HARPER & BROTHERS, PUBLISHERS, 329 & 331 PEARL S TR EET, FR&NKLIN SIUARE Eutefed according to Act of Congress, in the year 1840, by MARSH, CAPEN, LYON, AND WEBB, in the Clerk's Office of the District Court of Massachusetto. A D VERTI SE ME N T TO THE REVISED EDITION. SINCE the first publication of these Letters, in 1840, the work has passed through numerous editions, and received many tokens of public favor, both as a classbook for schools and as a reading-book for the family circle. The valuable discoveries made in the science within a few years have suggested an additional Letter, which is accordingly annexed to the series in the present revised form, giving a brief but comprehensive notice of all the leading contributions with which Astronomy has of late been enriched. The form of Letters was chosen on account of the greater freedom it admits, both of matter and of style, than a dress more purely scientific. Thus the technical portion of the- work, it was hoped, might be relieved, and the whole rendered attractive to the youthful reader of either sex by interspersing sketches of the master-builders who, in successive ages, have reared the great temple of Astronomy, composing, as they do, some of the most remarkable and interesting specimens of the human race. iv ADVERTISEMENT. The work was addressed to a female friend (now; no more), who was a distinguished ornament of her sex, and whose superior intellect and refined taste required that the work should be free frbm every thing superficial in matter or negligent in style;- and it was deemed by the writer no ordinary privilege that, in the composition of the work, an image at once so exalted and so pure was continually present to his mental vision. YALE COLLEGE, January, 1853. CONTENTS. PREFACE,.... LETTER I. Introductory Observations,......... 9 LETTER II. Doctrine of the Sphere,...... 16 L~ETTER III. Astronomical Instruments.-Telescope,.9. 29 LETTER IV. Telescope continued,. 36 LETTER V. Observatories,...........42 LETTER VI. Time and the Calendar,. 69 LETTER VII. Figure of the Earth,.. 69 LETTER VIII. Diurnal Revolution,. 81 LETTER IX. Parallax and Refraction,... 89 1* i6 CONTENTS. LETTER X. The Sun,...10 LETTER XI. Annual Revolution.-Seasons,..111 LETTER XII. Laws of Motion,. 126 LETTER XIII. Terrestrial Gravity,......... 34 LETTER XIV. Sir Isaac Newton. —Universal Gravit ation.-Figure of the Earth's Orbit.-Precession of the Equinoxes., 143 LETTER XV. The Moon,......... 157 LETTER XVI. The Moon. —Phases.-Harvest Moon.-Librations,. 172 LETTER XVII. MIoon's Orbit. —Her Irregularities,. 180 LETTER XVIII. Eclipses,... 195 LETTER XIX. Longitude.-Tides,...........208 LETTER XX. Planets.-Mercury and Venus,...... 225 LETTER XXI. Superior Planets: Mars, Jupiter, Saturn, and Uranus, 243S CONTENTS. 7 LETTER XXII. Copernicus.-Galileo,...... 254 LETTER XXIII. Saturn. —Uranus. —-Asteroids,.. 274 LETTER XXIV. The Planetary Motions.-Kepler's Laws.-Kepler,. 291 LETTER XXV. Comets, 312 LETTER XXVI. Comets, 334 LETTER XXVII. Meteoric Showers,.. 346 LETTER XXVIII. Fixed Stars,........... 365 LETTER XXIX. Fixed Stars,.. 383 LETTER XXX. System of the World,. 392 LETTER XXXI. Natural Theology,.... e 406 LETTER XXXII. Recent Discoveries,... 414 IN1EX,........... 423 LETTERS ON ASTRONOMY. LETTER I. INTRODUCTORY OBSERVATIONS. "Ye sacred Muses, with whose beauty fired, hMy soul is ravished, and my brain inspired, Whose priest I am, whose holy fillets wear; Would you your poet's first petition hear, Give me the ways of wandering stars to know, The depths of heaven above, and earth below i Teach me the various labors of the moon, And whence proceed th' eclipses of the sun; Why flowing tides prevail upon the main, And in what dark recess they shrink again; What shakes the solid earth, what cause delays The Summer nights, and shortens Winter days." Dryden's Virgtl ro MRs. C- - M DEAR MADAM, —In the conversation we recently held )f the study of Astronomy, you expressed a strong desire to become better acquainted with this noble sci-.*nce, but said you had always been repelled by the air of severity which it exhibits, arrayed as it is in so many technical terms, and such abstruse mathematical processes: or, if you had taken up some smaller treatise, with the hope of avoiding these perplexities, you had always found it so meager and superficial, as to afford you very little satisfaction. You asked,. if a work might not be prepared, which would convey to the general reader some clear and adequate knowledge of the great discoveries in astronomy, and yet require for its perusal no greater preparation, than may be presumed of every well-educated English scholar of either sex. You were pleased to add the request, that I would 10 LETTERS ON ASTRONOMY. write such a work, —a work which should combine with a luminous exposition of the leading truths of the science, some account of the interesting historical facts with which it is said the records of astronomical discovery abound. Having, moreover, heard much of the grand discoveries which, within the last fifty years, have been made among the fixed stars, you expressed a strong desire to learn more respecting these sublime researches. Finally, you desired to see the argument for the existence and natural attributes of the Deity, as furnished by astronomy, more fully and clearly exhibited, than is done in any work which you have hitherto perused. In the preparation of the proposed treatise, you urged me to supply, either in the text or in notes, every elementary principle which would be essential to a perfect understanding of the work; for although, while at school, you had paid some attention to geometry and natural philosophy, yet so much time had since elapsed, that your memory required to be refreshed on the most simple principles of these elementary studies, and you preferred that I should consider you as altogether unacquainted with them. Although, to satisfy a mind, so cultivated and inquisitive as yours, may require a greater variety of powers and attainments than I possess, yet, as you were pleased to urge me to the trial, I have resolved to make the attempt, and will see how far I may be able to lead you into the interior of this beautiful temple, without obliging you to force your way through the "jargon of the schools." Astronomy, however, is a very difficult or a comparatively easy study, according to the view we take of it. The investigation of the great laws which govern the motions of the heavenly bodies has /commanded the highest efforts of the human mind; but profound truths, which it required the mightiest efforts of the intellect to disclose, are often, when once discovered, simple in their complexion, and may be expressed in very simple terms Thus, the creation of that element, on whose mysteri INTRODUCTORY OBSERVATIONS. I ous agency depend all the forms of beauty and loveliness, is enunciated in these few monosyllables, "And God said, let there be light, and there was light;" and the doctrine of universal gravitation, which is the key that unlocks the mysteries of the universe, is simply this, —that every portion of matter in the universe tends towards every othel The three great laws of motion, also, are, w-hen sty_, so plain, that they seem hardly to assert any thing but what we knew before. That all bodies, if at rest, will continue so, as is declared by the first law of motion, until some force moves them; or, if in motion, will continue so, until some force stops them, appears so much a matter of course, that we can at first hardly see any. good reason why it should be dignified with the title of the first great law of motion; and yet it contains a truth which it required profound sagacity to discover and expound. It is, therefore, a pleasing consideration to those who have not either the leisure or the ability to follow the astronomer through the intricate and laborious processes, which conducted him to his great discoveries, that they may fully avail themselves of the results of this vast toil, and easily understand truths which it required ages of the severest labor to unfold. The descriptive parts of astronomy, or what may be called the natural history of the heavens, is still more easily understood than the laws of the celestial motions. The revelations of the telescope, and tlw wonders it has disclosed in the sun, in the moon, in the planets, and especially in the fixed stars, are facts not difficult to be understood, al.. though they may affect the mind with astonishment. Thb great practical purpose of astronomy to the world is, enabling us safely to navigate the ocean. There are indeed many other benefits which it confers on man; but this is the most important. If, however, you ask, what advantages the study of' astronomy promises, as a branch of education, I answer, that few subjects promise to the mind so much profit and entertainment. It is agreed by writers on the human LETTERS ON ASTRONOMY. mind, that the intellectual powers are enlarged and strengthened by the habitual contemplation of great objects, while they are contracted and weakened by being constantly employed upon little or trifling subjects. The former elevate, the latter depress, the mind, to their own level. Now, every thing in astronomy is great. The magnitudes, distances, and motions, of the heavenly bodies; the amplitude of the firmament itself; and the magnificence of the orbs with which it is lighted, supply exhaustless materials for contemplation, and stimulate the mind to its noblest efforts. The emotion felt by the astronomer is not that sudden excitement or ecstasy, which wears out life, but it is a continued glow of exalted feeling, which gives the sensation of breathing in a purer atmosphere than others enjoy. We should at first imagine, that a study which calls upon its votaries for the severest efforts of the human intellect, which demands the undivided toil of years, and which robs the night of its accustomed hours of repose, would abridge the period of life; but it is a singular fact, that distinguished astronomers, as a class, have been remarkable for longevity. I know not how to account for this fact, unless we suppose that the study of astronomy itself has something inherent in it, which sustains its votaries by a peculiar aliment. It is the privilege of the student of this department of Nature, that his cabinet is already collected, and is ever before him; and he is exempted from the toil of collecting his materials of study and illustration, by traversing land and sea, or by penetrating into the depths of the earth. Nor are they in their nature frail and perishable. No sooner is the veil of clouds remov ed, that occasionally conceals the firmament by night, than his specimens are displayed to view, bright and changeless. The renewed pleasure which he feels, at every new survey of the constellations, grows into an affection for objects which have so often ministered to his happiness. His imagination aids him in giving them a ersonification, like that which the ancients gave to the INTRODUCTORY OBSERVATIONS. 13 constellations; (as is evident from the names which they have transmitted to us;) and he walks abroad, beneath the evening canopy, with the conscious satisfaction and delight of being in the presence of old friends. This emotion becomes stronger when he wanders far from home. Other objects of his attachment desert him; the face of society changes; the earth presents new features; but the same sun illumines the day, the salme moon adorns the nioht, and the same bright stars still attend him.'When, moreover, the student of the heavens can command the aid of telescopes, of higher and higher powers, new acquaintances are made every evening The sight of each new member of the starry train, that the telescope successively reveals to him, inspires a peculiar emotion of pleasure; and he at length finds himself, whenever he sweeps his telescope over the firmameent, greeted by smiles, unperceived and unknown to his fellow-mortals. The same personification is given to these objects as to the constellations, and he seems to himself, at times, when he has penetrated into the remotest depths of ether, to enjoy the high prerogative of holding converse with the celestials. It is no small encouragement, to one who wishes to acquire a knowledge of the heavens, that the subject is embarrassed with far less that is technical than most other branches of natural history. Having first learned a few definitions, and the principal circles into which, for convenience, the sphere is divided, and receiving the great laws of astronomy oil the authority of the eminent persons who have investigated them, you wil. find few hard terms, or technical distinctions, to repel or perplex you; and you will, I hope, find that nothingl but an intelligent mind and fixed attention are requisite for perusing the Letters which I propose to address to you. I shall indeed be greatly disappointed, if the perusal does not inspire you with some portion of that pleasure, which I have descr'bed as enjoyed by the as tronomer himself. I,. A. 14 LETTERS ON ASTRONOMY. The dignity of the study of the heavenly bodies, and its suitableness to the most refined and cultivated mind, has been recognised in all ages. Viirgil celebrates it in the beautiful strains with which I have headed this Letter, and similar sentiments have ever been cherished by the greatest minds. As, in the course of these Letters, I propose to trace an outline of the history of astronomy, from the earliest ages to the present timhe, you may think this the most suitable place for introducing it; but the successive discoveries in thie science cannot be fully understood and appreciated, until after an acquaintance has beenforned with the science itself. We must therefore reserve the details of this subject for a future opporlunity; but it may be stated, here, that astronomy was cultivated the earliest of all the sciences; that great attention was paid to it by several very ancient nations as the Egyptians and Chaldeans, and tIhe people of india and China, before it took its rise in Greece. More than six hundred years before the Christian era, however, it began to be studied in this laitter country. Thales and Pythagoras were particularly disting'uished for their devotion to this science; and the celebrated school of Alexandria, in Egypt, which took its rise about three hundred years before the Christian era, and flourishe d for several hundred years, numbered am-ong its disciples a succession of eminent- astronomers, among whom wrel e Hipparchus, Eratosthenes, and Ptolemy. The last of these composed a great work on astronomy, called the' Almagest,' in which is transmitted to us an account of all that was known of the science by the Alexandmrian school. The'Almagest' was the principal text-bool in astronomy, for many centuries afterwards, and conmparatively few improvements were made until the age of Copernicus. Copernicus was born at Thorn, in Prussia, in 1473. Previous to his tim'ie, the doctrine was held, that the earth is at rest in the centre ot the universe, and that the sun5, moon, and stars, revolve,.bout it, every day, from east o wVest; in short, that INTRODUCTORY OBSERVATIONS. 15 the apparent motions of the heavenly bodies are the same with their r'eal motions. But Copernicus expounded what is now known to be the true theory of the celestial motions, in which the sun is placed in the centre of the solar system, and the earth and all the planets are made to revolve around him, from west to east, while the apparent diurnal motion of the heavenly bodies,-from east to west, is explained by the revolution of the earth on its axis, in the same time, from west to east; a motion of which we are unconscious, and which we erroneously ascribe to external objects, as we imag nme the shore is receding from us, when we are uncon scious of the motion of the ship that carries us from Although many of the appearances, presented by tna motions of the heavenly bodies, may be explained or the former erroneous hypothesis, yet, like other hypoth eses founded in error, it was continually leading its votaries into diffiLRlties, and blinding their minds tI the perception of truth. They had advanced nearly afar as it was practicable to go in the wrong road; an:, the great and sublime discoveries of modern. times arc owing, in no small degree, to the fact, that, since the days of Copernicus, astronomers have been pursuing the plain and simple path of truth, instead of threading their way through the mazes of error. Near the close of the sixteenth century, Tycho Brahe, a native of Sweden, but a resident of Denmark, carried astronomical observations (which constitute the basis of all that is valuable in astronomy) to a far greater degree of perfection than had ever been done before. Kepler, a native of Germany, one of the greatest geniuses the world has ever seen, was contemporary with Tycho Brahe, and was associated with him in a part of his labors. Galileo, an Italian astronomer of great eminence, flourished only a little later than Tycho Brahe. He invented the telescope, and, both by his discoveries and reasonings, contributed greatly to establish the true system of the.world. Soon after the com mencement of the seventeenth century, (1620,) Lord 1 6 LETTERS ON ASTRONOMY. Bacon, a celebrated English philosopher, pointed out the true method of conducting all inquiries into the phenomena of Nature, and introduced the inductive method of philosophizing. According to the inductive method, we are to begin our inquiries into the causes of any events by first examining and classifying all the facts that relate to it, and, from the comparison of these, to deduce our conc.usions. But the greatest single discovery, that has ever been made in astronomy, was the law of universal gravitation, a discovery made by Sir Isaac Newton, in the latter part of the seventeenth century. The discovery of this law made us acquainted with the hidden forces that move the great machinery of the universe. It furnished the key which unlocks the inner temple of Nature; and from this time we may regard astronomy as fixed on a sure and immovable basis. I shall Iereafter endeavor to explain to you the leading principles of universal gravitation, when we come to the proper place for inquiring into the causes of the celestial motions, as exemplified in the motion of the earth around the sun LETTER II. DOCTRINE OF THE SPHERE. "All are but parts of one stupendous whole, Whose body Nature is, and God the soul."-Pope. LET us now consider what astronomy is, and into what great divisions it is distributed; and then we will take a cursory view of the doctrine of the sphere. This subject will probably be less interesting to you than many that are to follow; but still, permit me to urge upon you the necessity of studying it with attention, and reflecting upon each definition, until you fully understand it; for, unless you.fully and clearly comprehend the circles of the sphere, and the use that is made DOCTRINE OF TlHE SPHERE, 17 of them in astronomy, a mist will hang over every subsequent portion of the science. I beg you, therefore, to pause upon every paragraph of this Letter; and if there is any point in the whole which you cannot clearly understand. I would advise you to mark it, and to recur to it repeatedly; and, if you finally cannot obtain a clear idea of it yourself, I would recommend to you to apply for aid to some of your friends, who may be able to assist you. Astronomy is that science which treats of the heavcely bodies. More particularly, its object is to teach what is known respecting the sun, moon, planets, coinets, and fixed stars; and also to explain the methods by which this knowledge is acquired. Astronomy is sometimes divided into descriptive, physical, and practical. Descriptive astronomy respects facts; physical astronomy, causes; practical astronomy, the mea~ns of investigating the facts, whether by instruments or by calcula.tion. It is the province of descriptive astronomy to observe, classify, and record, all the phenomena of tlhe heavenly bodies, whether pertaining to those bodies individually, or resulting from their motions and mutual relations. It is the part of physical astronomy to explain the causes of these phenomena, by investigating the general laws on which they depend; especially, by tracing out all the consequences of the law of universal gravitation. Practical astronomy lends its aid to both the other departments. The definitions of the different lines, points, and circles, which are used in astronomy, and the propositions founded upon them, compose the doctrine of the sphere. Before these definitions are given, I must recall to your recollection a few particulars respecting the method of measuring angles. (See Fig. 1, page 18.) A line drawn from the centre to the circumference of a circle is called a radius, as C. D, C B, or C K. Any part of the circumference of a circle is called an art, as A B, or B D. An angle is measured by an arc included between.-qt 318 LETTERS ON ASTRONOMY. Fig. 1. two radii. Thus, in Fig 1, the angle contained be tween the two radii, C A and C B, that is, the angle A C B, is measured by the are A B. Every circle, it Iw~~r will be recollected, is divided into three hundred and sixty equal parts, called degrees; and any are, as A B, contains a certain number of degrees, according to Ats length. Thus, if the are A B contains iorty degrees, then the opposite angle A C B is said to be an angie of forty degrees, and to be nmeasured by A B. BuS this are is the same part of the smaller circle that E F is of the greater. The arc A1 B, therefore, contains the same number of degrees as the are E F, and either may be taken as the measure of the angle A C B. As the whole circle contains three hundred and sixty degrees, it is evident, that the quarter of a circle, or quad-?ant, contains ninety degrees, and that the semicircle A B D G contains one hundred and eighty degrees. The conmplement of an are, or angle, is what it wants of ninety degrees. Thus, since A D is an are of ninety degrees, ]B D is the complement of A B, and A B is the complement of B D. If A B denotes a certain number of degrees of latitude, B D will be the compienent of the latitude, or the colatitude, as it is commonly written. The supplement of an arc, or angle, is what it wants of one hundred and eighty degrees. Thus, B A is the supplement of G D B, and G D B is the supplement of B A. If B A were twenty degrees of longitude, G D B, its suplplement, would be one hundred and sixty degrees. An angle is said to be subtended by the side which is opposite to it. Thus, in the triangle A C K, the angle at C is subtended by the side A K, the angle at A by C K, and the angle at K by C A. In like man DOCTRINE OF THIE.I'HERE, 19 ner, a side is said to be subtended by an angle, as A K by tile angle at C. Let us now proceed with the doctrine of the sphere. A section of a sphere, by a plane cutting it in any manner, is a circle. Great circles are those which pass through the centre of the sphere, and divide it into two equal hemispheres. Smnall circes are such as do not pass through the centre, but divide the sphere into two unequal parts. The axis of a circle is a straight line passing through its centre at right angles to its- plane. The pole of a great circle is the point on the sphere where its axis cuts through the sphere. Every great circle has two poles, each of which is every where ninety degrees fromrn the great circle. All great circles of the sphere cut each other in two points diametrically opposite, and consequently their points of section are one hundred and eighty degrees apart. A great circle, which passes through the pole of another great circle, cuts the latter at right angles. The great circle which passes through the pole of another great circle, and is at right angles to it, is called a seconda'ry to that circle. The angle made by two great circles on the surface of the sphere is measured by an are of another great circle, of which the angular point is the pole, being the are of that great circle intercepted between those two circles. In order to fix the position of any place, either on the surface of the earth or in the heavens, both the earth and the heavexs are conceived to be divided into separate portions, by circles, which are imagined to cut through them, in various ways. The earth thus intersected is called the ferrestrial, and the heavens the celestial, sphere. We rnust bear in mind, that these circles have no existence in Nature, but are mere landmarks, artificially contrived for convenience of reference. On account of the immense distances of the heavenly bodies, they appear to us, wherever we are placed, to be fixed in the same concave surface, or celestial vault. The great circles of the globe, eu.aded O LET TERS ON ASTRONOMY. every way to meet the concave sphere of tne heavens, become circles of the celestial sphere. The horizon is the great circle which divides the earth into upper and lower hemispheres, and separates the visible heavens from the invisible. This is the rational horizon. The sensible horizon is a circle touching the earth at the place of the spectator, and is bound ed by the line in which the earth and skies seem to meet. The sensible horizon is parallel to the rational, but is distant from it by the semidiameter of the earth, or nearly four thousand miles. Still, so vast is the distance of the starry sphere, that both these planes appear to cut the sphere in the same line; so that we see the same hemisphere of stars that we should see, if the upper half of the earth were removed, and we stood on the rational horizon. The poles of the horizon are the zenith and nadir. The zenith is the point directly over our heads; and the nadir, that directly under our feet. The plumbline (such as is formed by suspending a bullet by a string) is in the axis of the horizon, and consequently directed towards its poles. Every place on the surface of the earth has its own horizon; and the traveller has a new horizon at every step, always extending ninety degrees from him, in all directions. Vertical circles are those which pass through the poles of the horizon, (the zenith and nadir,) perpendicular to it. The mneridian is that vertical circle which passes through the north and south points. The prime vertical is that vertical circle which passes through the east and west points. The altitude of a body is its elevation above the horizon, measured on a vertical circle. The azimuth of a body is its distance, measured on the horizon, from the meridian to a vertical circle passing through that body. The amplitiude of a body is its distance, on the horizon, from the prime vertical tc a vertical circle pass. ing through the body. )DOCTRINE OF THE SIPHElE.R 2[1 Azimuth is reckoned ninety degrees from either the north or south point; and amplitude ninety degrees from either the east or west point. Azimuth and amplitude are mutually complements of each other, for one makes up what the other wants of ninety degrees. Wfhen a point is on the horizon, it is only necessary to count the number of degrees of the horizon between that point and the meridian, in order to find its azimuth; but if the point is above the horizon, then its azimuth is estimated by passing a vertical circle through it, and reckoning the azimuth from the point where this circle cuts the horizon. The zenith distance of a body is measured on a vertical circle passing through that body. it is the complement of the altitude. The axis of the earthl is the diameter on which the earth is conceived to turn in its diurnal revolution, The same line, continued until it meets the starry concave, constitutes the axis of the celestial sphere. The poles of the earth are the extremities of the earth's axis: the poles of the heavens, the extremities of the celestial axis. The equator is a great circle cutting the axis of the earth at right angles. Hence, tihe axis of the earth is the axis of the equator, and its poles are the poles of the equator. The intersection of the plane of the equa tor with the surface of the earth constitutes the terrestrial, and its intersection with the concave sphere of the heavens, the celestial, equator. The latter, by way of distinction, is sometimes denominated the equinoctial. The secondaries to the equator,-that is, the great circles passing through the poles of the equator,-are called meridians, because that secondary which passes through the zenith of any place is the meridian of that place, and is at right angles both to the equator and thie horizon, passing, as it does, through the poles of both. These secondaries are also called houtr circles because the arcs of the equator intercepted betweep then are used as mleasures of time. I ET,:T'ERS ON ASTRO:';:i,'." o The latitude of a place on the earthl is its distance fi'om the equator north or south. The polar dit ance, ol- angular distance from,the nearest pole, is thle complemrent of the latitude. The lota'itude of a place is its distance from some standard meridian, eithc er east or west, mieasured on thle equator. The meridian, usually taken as the standard, is that of the Observatory of Greeinwich, in London. If a place is directly on the equator, we h ave only to niquire, how many degrees of the equator there are h)e tween that place and the point whelre -the meridia n of Greenwich cuts the equator. If the place is north or south of the equator, then its longiftude is tle arc of t0he equator intercepted between the meridian t which passes thlrough the place and the neridlian of Greesnwiiclh The ecliptie is a great circle, in whih-ica the eam-i rth pe1rrif s its annual revolutions around the su-n. at passes through the centre of the'earn and the elmtre of the su-n~ It is foumnd by observtaion, t-enatt t. e taL' does not lie wih11 its axis at ri'ht.a arnles to the plane of.he ecliptic, so as to make tle eq.utoor coincide with it but that it is turne'd aboue twe ntdy-three ad lalf det^rees out of a Tpcrpendicular'CIeeno i making' aDn angle iwith the plane itself of sixty-six nid a'half dfetrcees. e equcator, therefore, mnus be turne d le saiae dis an.nce out of a c oincidene i le t t liptic, tlhe tiwo() ci r cles m ai king an an le with each ot of tw ent1tlhare a nd a half degrees It Is pal'rticularlay impo,:tn i that we sho'uld foIn corr.ect ildeas of the ecli;? ic!l en, d of its relations to the equator, since to these twvo citr c!es a great number of astronomical measurenents a-n phenomena are referred. The e qunroction pozints, or eqvin oxe8, ai'e the in - sections of the ecliptic and equatoer. he,iu e when the sun crosses the eiuator, an oing ~ north0rd, is Td c,. ed the ve',nal, and in returnienm southvwad, the oait,hnla[, equinox. The vernal equirox occurs abou0ot the (1i, t wenty-first of _Iarch, an Id - a tumnali abou the?tver~ l 0's' i t. i,i,?w i3 +; o *v-' i. DOCTRINE OF TH'E SPHIER. E. The solstitiat points are the two pohnts of the eclip tic most distaint fromn the equator. The times when the sun comes to them are called solstices. The Sum-'rser solstice occurs about the twenty-second of June, and the Ainter solstice about the twenty-second of December. Thie ecliptic is divided into twelve equal parts, of t;irty do'rees each, called sig s i, which. bc ginninga at the vernal equinox, succeed each other, in the following order: 1. Aries, 7.:Linbra,. TauruS) 8. S corpio, r 3. Gemini, i 9.O Sagittarius, t 4. Cancer, s 10. Capricornus, i 5. Leo, St JII. Aquarius,, 6. Virgo, ir IS9. Pisces. %S Thfe mode of c.ckon.ng oin tihe ecliptic is by signs deg'rees, minutes, aid seconds, The sign is denoted either -by its name or its number. Thus, one hundred degrees nmay be expressed eithle as the tenth degree of Cancer, or as 3s 10~. it wi 11 be fotund an advantage to riepeat the s s..n theil proper order, until they are well fixed in the Smemory, and to be able to recognise each sign by its a.propriate chC;ar.cc. Of the various mneridians, two are disuntiguishied by the nnare of colncores. The eui,6'. octira colure is l thet ineridian wlhch passes thlrough th:e cqucinsoctial points. From. this mRleridan, rl''at ascension avd celestial ondl - rude are rec-hooned, as longitude on top. heertdi is reckoned from the meridian of Greenwich. The solsbtitia-:, coture is the meridian which passes thlrough the solstitial points. The position of a celestial body is referred to the equator by its righti ascension and declination. Right ascension is the angular distance from the vernal equinox measured on the equato. If a star is situated on the equator, then its right ascension is the number of degrees of thle equator betweena the star arnd the verna equinox. Bat if the star is nOLrt-h or soulth of the equa it"m. Ii,-"t bei:!,'......;si od ti" th-' I, tl t le nlun)er of d,le:rees of t4 kLETTERS ON AST'1RONOMY. the equator, intercepted between the vernal- equinox and that secondary to the equator which passes through the star. Declination is the distance of a body from the equator measured on a secondary to the latter. Therefore, right ascension and declination correspond to terrestrial longitude and latitude,-right ascension being reckoned from the equinoctial colure, in the sanme manner as longitude is reclioned from the meridian of Greenwich.. On the other hand, celestial longitude and latitude are referred, not to the equator, but to the ecliptic. Celestial longitude is thle distance of a body from the vernal equinox measured on the ecliptic. Ce. lestial latitude is the distance from the ecliptic measured on a secondary to the latter. Or, more briefly, longitude is distance on, the ecliptic: latitude, distance from the ecliptic. The north polar distance of a star is the complement of its declination. Parallels of latitude are small circles parallel to the.quator. They constantly diminish in size, as we go from the equator to the pole. The tropics are the parallels of latitude which pass throug'h the solstices. The northern tropic is called the tropic of Cancer; the southern, the tropic of Capricorn. The polar cir;cles are the parallels of latitude that pass through the poles of the ecliptic, at the distance of twenty-three and a half deg'rees from the poles of the earth. The elevation of the pole of the heavens above the horizon of any place is always equal to th;e latitude of the place. Thus, in forty degrees of north latitude we see the north star forty degrees above the northern horizon; whereas, if we should travel southward, its eievation would grow less and less, until we reached the equator, where it would appear in the horizon. Or, if we should travel northxwards, the north star would rise continually higher and higher, until, if we could reach the pole of the earth, th-at star would appear directly over head. The elevation of the eqtauctor above the horizon of any place is' equal to the cornplen-ment of the latitude. ri11s, at the latitude of forlty (degrete DOCTRINE OF TIHE SPHERE. 25 north, the equator is elevated fifty degrees above the southern horizon. The earth is divided into five zones. That portion of the earth which lies between the tropics is called the torrid zone; that between the tropics and the polar circles, the temperate zones; and that between the polar circles and the poles, the frigid zones. The zodiac is the part of the celestial sphere which lies about eight degrees on each side of the ecliptic. This portion of the heavens is thus marked off by itself, because all the planets move within it. After endeavoring to form, from the definitions, as clear an idea as we can of the various circles of the sphere, we may next resort to an artificial globe, and see how they are severally represented there. I do not advise to begin learning the definitions from the globe; the mind is more improved, and a power of conceiving clearly how things are in Nature is more effectually acquired, by referring every thing, at first, to the grand sphere of Nature itself, and afterwards resorting to ar4ficial representations to aid our conceptions. We can get but a very imperfect idea of a man fiom a profile cut in paper, unless we know the original. If we are acquainted with the individual, the profile will assist us to recall his appearance more distinctly than we can do without it. In like manner, orreries, globes, and other artificial aids, will be found very useful, in assisting us to form distinct conceptions of the relations existing between the different circles of the sphere, and of the arrangements of the heavenly bodies; but, unless we have already acquired some correct ideas of these things, by contemplating them as they are in Nature, artificial globes, and especially orreries, will be apt to mnislead us. I trust you will be able to obtain the use of a globe,@ A small pair of globes, that will answer every purpose requiret by the readers of these Letters, may be had of the publishers of this Work, at a price not exceeding tell dollars; or half that sum for a celestial globe, which will serve alone for studying astronomy, 0 A1 2V6 LETTERS ON AST RONOMY. to aid you in the study of the foregoing definitions, ox doctrine of the sphere; but if not, I would recommend the following easy device. To represent the earth, select a large apple, (a melon, when in season, will be found still better.) The eye and the stem of the apple will indicate the position of the two poles of the earth. Applying the thumb and finger of the left hand to the poles, and holding the apple so that the poles may be in a north and south line, turn this globe friom west to east, and its motion will correspond to the diurnal movement of the earth. Pass a wire or a knitting needle through the poles, and it will represent the axis of the sphere. A circle cut around the apple, half way between the poles, will be the equator; and several other circles cut between the equator and the poles, parallel to the equator, will represent parallels of latituCde; of which, two, drawn twenty-three and a half degrees from the equator, will be the tropics, and two others, at the same distance from the poles, will be the polar circles. A great circle cut through the poles, in a north and south direction, will form the meridian, and several other great circles drawn through the poles, and of course perpendicularly to the equator, will be secondaries to the equator, constituting meridians, oi hour circles, A great circle cut through the centre of the earth, from one tropic to the other, would represent the plane of the ecliptic; and consequently a line cut round the apple where such a section meets the surface, will be the terrestrial ecli tic. The points xwhere this circle meets the tropics indicate the position of the solstices; and its intersection with the equator, that of the equinoctial points. The horizon is best represented by a circular piece of pasteboard, cut so as to fit closely to the fplul1e, be ing movable upon it. When this horizon;,' passed through the poles, it becomes thle horizon of tIhe equa tor; when it is so placed as to coincide witi: he earth's equator, it becomes the horizon of the po'es; and in every other situation it represents thle i )rizon of a DOCTRINE OF THEE SPIIERE 7 place on the globe ninety degrees every way from it. Suppose we are in latitude forty degrees; then let us place our movable paper parallel to our own horizon, and elevate the pole forty degrees above it, as near as we can judge by the eye. If we cut a circle around the apple, passing through its highest part, and through the east and west points, it will represent the prime vertical. Simple as tle foregoing device is, if you will take the trouble to construct one for yourself, it will lead you to more correct views of the doctrine of the sphere, than you would be apt to obtain from the most expensive attificial globes, although there are lmany other use.ful purposes whichl such globes serve, for which the apple would be inadequate. When you have thus made a sphere for yourself, or, wvith an artificial globe before you, if you have access to one, proceed to point out on it the various arcs of azinuth and altitude, riglt ascension and Ideclination, terrestrial and celestial latitude and longitude,-these last being referred to the equator on the earth, and to the ecliptic in the heavens. When the circles of the sphere are well learned, we may advantageously employ projections of them in various illustrations. By the projection of the sphere is meant a representation of all its parts on a plane. The plane itself is called the plane of projection. Let us take any circular rino, as a wire bent into a circle, and hold it in different positions before the eye. If we hold it parallel to thle face, with the whole breadth opposite to the eye, we see it as an entire circle. If we turn it a little sideways, it appears oval, or as an ellipse; and, as we continue to turn it more and more round, the ellipse grows narrower and narrower, until, when the edge is presented to the eye, we see nothing but a line. Now imagcine the ring to be near a perpendicular wall, and the eye to be removed at such a distance from.it, as not to distinguish any interval between the ring and the wall; then the several figures under which the ring is seen will appear to be inscribecd on thle wall, and we LETTERS ON AST'RnONOSr. shall see the ring as a circle, when perpendicular to a straight line joining the centre of the ring and the eye, or as an ellipse, when oblique to this line, or as a straight line, when its edge is towards us. It is in this manner that the circles of the sphere are projected, as represented in the following diagram, Fig. 20 Fig.2. Here, various circles zA a, r;e represented as -projected on the me. ~,.~ \, ridian. which is supposed to be situated directly before the eye, at some distance from it. The horizon /1 0, leing perpendicular to thle meridian, is seen edgewise, and consequently is pro-.ected into a straight line. The same is the case with the prime vertical Z N, with the equator E Q, and the several small circles parallel to the equator, which represent the two tropics and the txwo polar circles. In fact, all circles whatsoever, which are perpendicular to the plane of projection, will be represented by straight lines. But every circle which is perpendicular to the horizon, except the prine vertical, being seen obliquely, as Z Al N, will be projected into an ellipse, one half only of which is seen,-the other half being on the other side of the plane of projection. In the same manner, P R P, an hour circle, is represented by an ellipse on the plane of projection. ASTRONOMICAL INSTRUMENTS LETTEIR XI3I ASTRONOM3ICAL INSTRUME NTS.-TELESCOPE, "H ere truths sublime, and sacred science charm, Creative arts new faculties supply, Mechanic powers give more than giant's arm, And piercing optics more than eagle's eye; Eyes that explore creation's wondrous laws, And teach us to adore the great Designing Cause."-Beattze. iF, as I trust, you have gained a clear and familiar Kmowledge of the circles and divisions of the sphere. and of the mode of estimating the position of a heavenly body by its azimuth and altitude, or by its right ascension and declination, or by its longitude and latitude, you will now enter with advantage upon an account of those instruments, by means of which our knowledge of astronomy has been greatly promoted and per fected, The most ancient astronomers employed no instruments of observation, but acquired ttheir knowledge of thle heavenly bodies by long-continued and most attentive inspection with the naked eye. Instruments for measuring angles were first used in the Alexandrian school, about three hundred years before the Christian era. Wherever we are situated on the earth, we appear to be in the centre of a vast sphere, on the concave surface of which all celestial objects are inscribed. If we take any two points on the surface of the sphere, as two stars, for example, and imagine straight lines to be drawn to them from the eye, the ancgle included between these lines will be measured by the are of the sky contained between the two points. Thus, if D B I, Fig. 3, page 30, represents the concave surface of the sphere, A, B, two points on it, as two stars, and C A, C B, straight lines drawn fron- the spectator to those points, then the angular distance betwveen them is meas ured by the arc A B, or the angle A C B. But this an 3:x 30 LETTERS ON ASTRONOMY. Fig. S. gle may be measured on a much smaller circle, having the same centre, as G F K, since the are E F will have the same number of degrees as the are A B. The simplest mode of taking an angle between two stars is by means of an arm opening at a joint like the blade of a penknife, the end of the arm moving like C E upon the graduated circle K F G. In fact, an instrument constructed on this principle, resembling a carpenter's rule with a folding joint, with a semicircle attached, constituted the first rude apparatus for measuring the angular distance between' two points on the celestial sphere. Thus the sun's elevation above the horizon might be ascertained, by placing one arm of the rule on a level with the horizon, and bringing the edge of the other into a line with the sun's centre. The common surveyor's compass affords a simple example of angular measurement, Here, the needle lies in a north and south line, while the circular rim of the compass, when the instrument is level, corresponds to the horizon. Hence the compass shows the azimuth of an object, eol how many degrees it lies east or west of the meridian. It is obvious, that the larger the graduated circle is, the more minutely its limb may be divided. If the circle is one foot in diameter, each degree will occupy one tenth of an inch. If the circle is twenty feet in diameter, a degree will occupy the space of two inches, and could be easily divided into minutes, since each minute would cover a space one thirtieth of an inch. Refined TELESCOPE. 31 astronomical circles are now divided with very great skill and accuracy, the spaces between the divisions being, when read off, magnified by a microscope; but in former times, astronomers had no mode of measuring small angles but by employing vei'y large circles. But the telescope and microscope enable us at present to measure celestial arcs much more accurately than was done by the older astronomers. In the best instruments, the measurements extend to a single second of space, or one thirty-six hundredth part of a degree,-a space, on a circle twelve feet in diameter, no greater than one fiftyseven hundredth part of an inch. To divide, or graduate, astronomical instruments, to such a degree of nicety, requires the highest efforts of mechanical skill. Indeed, the whole art of instrument-naking is regarded as the most difficult and refined of all the mechanical arts; and a few eminent artists, who have produced instruments of peculiar power and accuracy, take rank wvith astronomers of the highest celebrity. I will endeavor to make you acquainted with several of the principal instruments employed in astronomical observations, but especially with the telescope, which is the most important and interesting of them all. I thinik I shall consult your wishes, as well as your improve ment, by giving you a clear insight into the principlea of this prince of instruments, and by reciting a few pai ticulars, at least, respecting its invention and subsequent history. The Telescope, as its name implies, is an instrument employed for viewing distant objeccts. It aids the eye in two ways; first, by enlarging the visual angle under which objects are seen, and, secondly, by collecting and conveying to the eye a much larger amount of the light that emanates from the object, than would enter the naked pupil. A complete knowledge of the telescope cannot be acquired, without an acquaintance with the science of optics; but one unacquainted with that sci *4 From two Greek words, 92, (tele,) far, and oxosrErw, (skopeo,) to see 32 LETTERS ON ASTRONOMWY ence may obtain some idea of the leading pitinciples ot this noble instrument. Its main principle is as follows: By means of the telescope, awe first fortm an image of a distant object,-as the moon, for example, —and then magnify that image by a microscope. Let us first see how the image is formed. This imay be done either by a convex lens, or by a concave mirror. A convex lens is a flat piece of glass, having its two faces convex, or spherical, as is seen in a common sun-glass, or a pair of spectacles. Every one Who has seen a sun-glass, knows, that, when held towards the sun, it collects the solar rays into a small bright circle in the focus. This is in fact a small image of the sun. In the same manner, the image of any distant object, as a star, may be formed, as is represented in the following diagram. Let A B C D, Fig. 4, represent the tube of Fig. 4. the telescope. At the front end, or at the end which is directed towards the object, (which we will suppose to be the moon,) is inserted a convex lens, L, which receives the rays of light from the moon, and collects them into the focus at a, forming an image of the moon. This image is viewed by a magnifier attached to the end B C. The lens, L, is called the object-glass, and the microscope in B C, the eyeglass. We apply a microscope to this image just as we would to any object; and, by greatly enlarging its dimensions, we may render its various parts far more distinct than they would otherwise be; while, at the same time, the lens collects and conveys to the eye a much greater quantity of light TELESCOPE. 33 than would proceed directly from the body under examination. A very few rays of light only, from a distant object, as a star, can enter the eye directly; but a lens one foot in diameter will collect a beam of light of the same dimensions, and convey it to the eye. By these means, many obscure celestial objects become distinctly visible, which would otherwise be either too ninute, or not sufficiently luminous, to be seen by us. But the image may also be formed by means of a concave mirror, which, as well as the concave lens, has the property of collecting the rays of light which proceed from any luminous body, and of forming an image of that body. The image formed by a concave mirror is magnjfied by a microscope, in the same manner as when formed by the concave lens. When the lens is used to form an image, the instrument is called a refracling telescope; when a concave mirror is used, it is called a refiecting telescope. The office of the object-glass is simply to collect the light, and to form an ilage of the object, but not to magnify it: the magnifying power is wholly.in the eye-glass. Hence the principle of the telescope is as follows: By means of the object-glass, (in the refracting telescope,) or by the concave nmirror, (in the reflecting telescope,) we formt an, irmage of the object, asnd magif/y that image by a microscope. The invention of this noble instrument is generally ascribed to the great philosopher of Florence, Galileo. He had heard that a spectacle maker of Holland had accidentally hit upon a discovery, by which distant objects miight be brought apparently nearer; and, without further information, he pursued the inquiry, in order to aseertain what forms and combinations of glasses would produce such a result. By a very philosophical process of reasoning, he was led to the discovery of that peculiar form of the telescope which bears his name. Although the telescopes made by Galileo were no larger than a common spy-glass of the kind now used on board of ships, yet, as they (gave new views of the 34 LETTERS ON ASTRONOMrY. heavenly bodies, revealing the mountains and valleys of the moon, the satellites of Jupiter, and multitudes of stars which are invisible to the naked eye, it was regarded with infinite delight and astonishment. Reflecting telescopes were first constructed by Sir Isaac Newton, although the use of a concave reflector, instead of an object-glass, to form the image, had been previously suggested by Gregory, an eminent Scotch astronomer. The first telescope made by Newton was only six inches long. Its reflector, too, was only a little more than an inch. Notwithstanding its small dimensions, it performed so well, as to encourage further efforts; and this illustrious philosopher afterwards constructed much larger instruments, one of which, made with his own hands, was presented to the Royal Society of London, and is now carefully preserved in their library. Newton was induced to undertake the construction of reflecting telescopes, from the belief that refracting telescopes were necessarily limited to a very small size, with only moderate illuminating powers, whereas the dimensions and powers of the former admitted of being indefinitely increased. Considerable magnifying powers might, indeed, be obtained from refractors, by making them very long; but the brightness with which telescopic objects are seen, depends greatly on the dimensions of the beam of light which is collected by the object-glass, or by the mirror, and conveyed to the eye; and therefore, small object-glasses cannot have a very high illuminating power. Now, the experiments of Newton on colors led him to believe, that it would be impossible to employ large lenses in the construction of telescopes, since such glasses would give to the images, they formed, the colors of the rainbow. But later opticians have- found means of correcting these imperfections, so that we are now able to use object-glasses a foot or more in diameter, which give very clear and bright images. Such instruments are called ctchromnatic telescopes,-a name implying the absence of prismatic or rainbow colors in the image. It is, however, far more TELESCOPE. 3b difficult to construct large achromatic than large reflecting telescopes. Very large pieces of glass can seldom be found, that are sufficiently pure for the purpose; since every inequality in the glass, such as waves, tears, threads, and the like, spoils it for optical purposes, as it distorts the light, and produces nothingl but confused images. The achromatic telescope (that is, the refracting telescope, having such an object-glass as to give a colorless image) was invented by Dollond, a distinguished English artist, about the year 1757. He had in his possession a quantity of glass of a remarkably fine quality, which enabled him to carry his invention at once to a high degree of perfection. It has ever since been, with the manufacturers of telescopes, a matter of the greatest difficulty to find pieces of glass, of a suitable quality for object-glasses, more than two or three inches in diameter. Hence, large achromatic telescopes are very expensive, being valued in proportion to the cubes of their diameters; that is, if a telescope whose aperture (as the breadth of the object-glass is technically called) is two inches, cost one hundred dollars, one whose aperture is eight inches would cost six thousand four hundred dollars. Since it is so much easier to make large reflecting than large refracting telescopes, you may ask, why the latter are ever attemnpted, and why reflectors are not exclusively employed? I answers that the achromatic telescope, when large and well constructed, is a more perfect and more durable instrument than the reflecting telescope. Much more of the light that falls on the mirror is absorbed than is lost in passing through the object-glass of a refractor; and hence the larger achromatic telescopes afford a stronger light than the reflectIlg, unless the latter are made of an enormous and unwieldy size. MIoreover, the mirror is very liable to tarnish, and will never retain its full lustre for many years together; and it is no easy mat