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Lorsque le document est trop grand pour dtre reproduit en un seul clichd, il est film6 d partir de Tangle sup^rieur gauche, de gauche d droite, et de haut en bas, en prenant le nombre d'images n6cessaire. Les diagrammes suivants illustrent la m^thode. by errata ned to lent une pelure, facon d 1 2 3 32X 1 2 3 4 5 6 ' " " > '■ /■ VENlItl MKHCnRY I I t . m i . 11" ; ■p*TT*?"*^^'^r* MARS EL UKANU8 H-ontiapiec/i TtLUSCOPIC VIEWS OF TUE PLANETS. ELEMENTS OF ASTRONOMY BY SIMON NEWCOMB, Ph.D., LL.I). FORMBBLT PROFK880K OF MATHKMATK'B AND ASTKUNOHY JOHNS HOPKINS 1IN1VKKHITY -•o^Koo- NEW YOtlR^^:. CINCINNATI:. CHICAGO AMERICAN BOOK COM^rANY ^ 57926 IJbrarv ol' Cuii^reHH Two Conts RtCEivFD OCT 8 1900 FRS1 COfV. 3M Uyy IMkMo* to ORDUi DiMIStun OCT 15 1900 rin r/ - c? o COPYBIGHT, 1900, BT SIMON NKWIOMB. EL. ur ABTBON. W. P. I »• n o n qlete training in at least the elementary branches of matheji atics. But this training is not essential to him who desires only a command of general ideas, without proposing to make technical applications of the science. What is really essential are those conceptions of motion and form which one may derive from everyday observation, and the understanding of a few elemen- tary definitions in geometry and physics. Our mfxlern system 6 f , PUF.FACK of education wis^'ly endoiivorH to implant Huoh conceptions and to tcacli the correspondinj,' dctiiiitiona at an earlior uk'c tluin that wlitMi the growinj,' youth is expected to coinnienco a course of formal mathematics. The author hopes that the early chapters are the only ones that will offer any ditticulty to an intelligent pupil prepared for a high school course. Here it is iMilieved that every diffi- culty may be overcome by two very simple measures on the part of the teacher. One is to point out, approximately, the actual position of the celestial poles and equator and the appar- , ent diurnal courses of the sun and stars, as they might be seen in the mind's eye from the schoolroom or the field. The object of this is that the learner may conceive the phenomena he is studying as if seen in the sky. The other is to see that the learner correctly apprehends the meaning of the figures rei)re- senting points, circles, and motions on the celestial sphere; especially, that he always imagines himself looking at the objects represented as if he were at the center of the sphere. For this last suggestion and for other valuable hints, the . author takes much pleasure in acknowledging his indebtedness to Mr. Edward P. Jackson, teacher of Physics in the Boston Latin School. 1 i and than )IIIN0 iSfti ones •ared (litti- 1 the , the ppar- spen bject he is t the epre- lere ; ; the re. , the Iness Qston CONTENTS I. Uklation «k tub Kartii to tub IIbavknh . . . . 1. liitioduclion. 2. Ideas of Motion. 3. The Karth. 4. Tiu! Celt'Htial Sphere. 6. I'erapeclive of Piano and Line. 0. AnKiilar Meiwuio on the CeleHtial Splierc. 7. Tlie Kela- tion of llio ll(.rizon to the Celestial Spliere. H. Tlie Diurnal Motion. 1>. CeleHtial Kquator and Poles. 10. The Meridian. 11. Diurnal .Motion in Different Latitudes. 12. Higlit Amen- sion and Declination. L'J. Correspondence of the Terrestrial and Celestial Spheres. PAfl« IL III. IV. VI. Tub Ubvoliition ok thb Earth round thb Sun 1. The Karth as a Plam-t. 2. Annual Motion of the Earth round the Sim. 3. How the Sun shines on the Earth at Dif- ferent Seasons. 4. Apparent Motion of the Sun — Tlie Zodiac. 5. Seasons in the Two Hemispheres. «. The Solar and Sidereal Years. 7. Precession of the Equinoxes. Mean and 3. Local Of Timb L Diurnal Motion of the Sun and Stars. 2. Apparent Time; Inequality of Apparent Time. Time and Longitude. 4. Standard Time. OllSBUVATU)N ANI> MeAHIIRBMKNT OF THE IIbAVBNS 1. Refraction of Light. 2. Lenses and Object CJlasses. 3. The Uefracting Telescope. 4. The Equatorial Tele8C0j)e. 5. The Keflectirtg Telescope. 6. Great Telescopes. 7. Me- ridian Instruments. 8. The Spectroscope and its Use. 0. Semidiameter and Parallax. 10. The Aberration of Light. Gravitation , „ ■ . ' 1. Force. 2. Tlie Laws of Motion. 3. Universal GravitSr tion. 4. Weight and Mass. 5. How the Attraction of the Sun keeps the Planete in their Orbits. 0. Centrifugal Force. The Earth , • , ," j 1. Figure and Magnitude of the Earth. 2. I^atitude and Longitude. 3. Length of a Degree. 4. How the Earth is measured. 6. How Latitude and Longitude are determined. 0. Density of the Earth, Gravity, etc. 7. Condition of the Earth's Interior. 8. The Atmosphere. ». The Zodiacal Light. VII. The Sun , • ,• , • „ ' 1. Particulars about the Sun. 2. Heat of the Sun. 3. Spots and Rotation of the Sun. 4. Corona and Promi- nences. 6. Source and Period of the Sun's Heat. 7 32 48 66 80 00 103 J CnSTKSTS I'llAI'TKH VIII, Tub MiioN and Kci.iphkh , . , . , I. DiMtaiu'c, Size, (iiid ANiM'(^t of thn Mdon. 2. I'liii Mdon'M Rcvoluiioii. :). Till! Mdoii'iH I'liiiHfH mill UDiiitiiin. 1. 'I'lio TldcH. r». Ki;li|)8t'N of thiMMiiiiii. «. TliH Mooii'h Orbit and Nodus, 7. Kcliimi'H of tho .Sun, 8, Itecuntiiuu) of Kclii)Mi'H, IX. TiiK Cai.kniiak 1. UnitM of Time, 2. The .Fulliin Calendiir, .'t. Tlie Oio- Rorian I'aiondar. 4. Tlie Year. 5, Kuaturt-i* of the CImrch Calendar, 0. The lIoiirH, X. (JkNKMAI. I'l.AN OK TIIK Soi.AU SVHTKM . , . 1. OrbitHof tliorianetM. 2. Keplur'n I.awH. .1. SlniiUire of the .Solar .SyHteni. 4. DlHtaneeH of tlie I'lanctH ; Uode'8 Law, 6, Anpectw of the I'lanets. 0. Apparent Motions of thu PlaneU. 7. Perturbations of the I'lanots, XI, ThK InNKU GliOlIP Of I'l.ANKTS 1. The I'lanet Mercury, 2. The rianet Venus ; Aspeets of Venus. 3. The Planet Mars ; Aspects of Mars, 1. Tho Minor Planets or Astcruiils. ■■Aiir, 112 u:i 140 151 XII. XIII. XIV. 102 TiiK Four OrTuii Pi.anicts 1. The Planet Jupiter, 2. Tlu' Satellites of .nii)iter. 3. Tho Planet .Saturn. 4. The Hinns of Saturn. .'>. Ilie Satellites of Saturn. 0, U:ui»U8 and its Satellites, 7. Nep- tune and its Satellite. Comets ani> Mkteors no 1. Appearance of a Comet, 2, Comets belong to the Solar System, 3. Orbits of Comets, 4. Remarkable Comets. 6. Constitution of Comets. 0. Meteors. 7. Meteoric Showers. The Constem. vtionr 1. About the Stars In General. 2. How the Constellations and Stars are named. 3. Description of the Principal Con- stellations. 4. Constellations Visible in the Evenings of February and March. 5, The Early Summer t-onstella- tloiis. 0, The Au^'ust Constellations. 7. The November Constellations. XV. The Stars and Neiui,,*; 1 . The Stars are Suns. 2. Proper Motions of the Stars. .3, Motion of the Sun. 1. Motions in the Line of Sight. 6. DLstancps of the Stars. 0, Varial)le Stars. 7. Double Stars. 8. Clusters and Nebuhe ; Clusters of Stars. XVI. A Brief IIistorv or Astronomy Index 101 200 226 287 ] Ui( otii r lar far it \ atl sta oui ] us bod I Ilea i»B not apF liov pre! thei stai I idii t (■Aim 112 laa ASTRONOMY 140 "5*:o 151 1U2 170 101 206 225 237 CIIAPTKR I RKLATIDN OF TUB EAUTII TO TIIK HEAVENS I Introduction When we look at tlie sky by day we see the smi; by niglit we see tlie moon and stars. '1 hese, and all other objects whieh we see in the heavens, are called heawnbi htxiit's. AHtrnnom>i is the scjience which treats of these bodies. The heavenly bodies are all of immense size, most of them larger than the earth. They look small because they are so far away. If we could fly from the earth as far as we please, it would look smaller and smaller as we went farther, until at a distance of many millions of miles it wouM anjHar as a little star. If we kept on yet farther, it would at last disappear from our sight altogether. If we lived on one of the heavenly liodiea, it would be to lis as the earth, and the earth would be swn as a heavenly body. In trying to think of the relation of the earth to the heavens, we may liken ourselves to microscopic! insects liv- ing on an apple. To them the api)lMV 2. Ideas of Motion. — If we think oiirefully, wo shall see that we ean iiev<'i' know that any object is in motion except by comparing its position with that of some other object supposed to be at rest. Inside the cabin of a ship on a smooth sea we are not able to decide whether we are m rest or in motion unless we can look out on the ocean which we suppose to be at rest. Even then water, ship, and everything on the shij*, might be carried along l)y the Gulf Stream without our know- ing it. This general fact is expressed by saying that all motion, so far as we can define or know it, is relative; that is, it is referred to some object supposed to be at rest. It follows from this that the motion of an object may be very different according to the body to which it is referred. Suppose, for example, that a man walks from the front to the rear of a railway car running eastward iW miles an hour. A fellow passenger would say that the man was walking westward at the rate of three miles an hour, because liis motion would be referred to the car as if the latter were at rest. Rut if we refer it to the surface of the earth, he would be going east at the rate of 47 miles an hour. Hence, in speaking of the motion of a body, there must always be some other body, or some position, to which the motion is referred. In everyday life we commonly refer the motions of things around us to the surface of the earth. In astronomy motions are sometimes referred to the center of the earth, or to the sun, or even to the stars. 3. The Earth. — Some of the following facts are taught in geography, but they are of equal importance in astronomy : — 1. The earth hiis the form of a si)heroid. Its tigure is so near that of a globe that the eye could not see any deviation from the spherical form. Hence, we commonly speak of the earth as a globe. 2. We live on the ro)Mid surface of this globe. 3. Our bodies and everything else on the earth's surface are drawn toward its center by a force called rjravity. Were it see I'y sed we iun be lip, j\v- all bat be •ed. the »ur. ing his I at uld in ime 1. ngs ans un. in so ion the are ^ it HELATION OF THE EARTH TO THE HEAVENS 11 not for gravity, objects on the earth would have no tendency to stay there. 4. It follows that the direction we call downward is not the same in any two places, because it is everywhere nearly toward the earth's center. Dwellers on the o[)posite side of the earth stana with their feet pointing toward us, and are therefore called our antipodes. 5. The earth turns continually from west to east on an imaginary line passing through its center, and called its axis. The two opposite points in which the axis intersects the surface of the earth are called poles. One of these is called the north pole, the other the sonth pole. The time required to make a revlution is called a day. 6. An imaginary circle passing round the earth, equally dis- tant from the two poles, is called the earth's equator. 7. The motion of the earth on its axis is so smooth and uni- form that we are entirely unconscious of it. Hence it seems to us to be at rest while the heavenly bodies seem to move in the opposite direction, from east toward west. 4. The Celestial Sphere. —When we look up from the earth, the stars seem to be set in a blue vault or dome, which we call the sky. The sky seems to rise liigh over our heads, and to curve down on every side toward the earth, on which it seems to rest. The sky is not a real object, but only an appearance produced by the blue light reflected from the air to our eyes. There are as many stars in the heavens by day as by night. The reason we do not see them by day is that our eyes are dazzled by the light of the sky, which is really light reflected by the air. If we could mount above the air, we should see no sky, because there would be no air to reflect the light, and we should see the stars all day as well as all night. The stars surround us in every possible direction, l)elow oui feet as well as above our heads. The earth is in the way of our seeing them when they are below us, but they are then visi- ble to our antipodes. I 12 ASTRONOMY The heavenly bodies are really at very different distances. They appear to us to be at the same distance because our eyes cannot distinguish their distances as less or greater. Hence we fancy them to be on the surface of a hollow sphere, in the Fio. 1. —Showing how stars p, q, r, «, etc., are seen by an observer at O as if they all lay on a sphere at the respective points P, Q, R, S, etc. The three stars marked t are seen as if they were a single star in the position T, because, being in the same straight line from the observer, they cannot be distinguished. center of which we stand. Although this sphere is imaginary, it will help our thoughts to think of it and talk about it as if it were real. It is called the celestial sphere. We must imagine the celestial sphere to be so vast that the earth in its center is a mere point in comparison. 5. Perspective of Plane and Line. — You doubtless know that a plane is a flat surface which may be supposed to extend out all round us as far as we please. When we represent a plane on paper we have to give it a boundary because there is not room enough on the paper to show it extending out without 1 .^^b Lar he if he at ut ne ot ut RELATION OF THE EARTH TO THE HEAVENS 13 limit. We are not to suppose that the circle or other bound- ing line on the figure is really the boundary of the plane ; the latter need not have a boundary. Let us see how we may represent a plane seen in differejit ways. Figure 2, a, shows a small portion of a plane, bounded by a circle, on which we are looking perpendicularly. This plane coincides with the plane of the paper. Figure 2, b, shows the plane seen obliquely. We must conceive this plane as passing through the plane of the paper. Figure 2, c, shows the same plane seen edgewise. It then looks like a straight line, but must still be con- ceived as a plane, and as being pei-- pendicular to the plane of the paper. f 6. Angular MeMure on the Celestial Vm. 2. Sphere. — When we speak of the dis- tance of two heavenly bodies from each other, the word dis- tance may have either of two meanings. The real distance is the length of the line from one body to the other. Hence this is also called linear distance. The apparent distance between two heavenly bodies is their distance apart as it appears to us. This is not a line, but the angle between two lines from the observer's eye, one going toward one body and one toward the other. In astronomy we commonly use words expressing distance in this sense; thus we say that the moon and a star are together, or that a star is alongside the moon when they look so to our eyes, although the star is in reality millions of times farther from us than the moon. Two heavenly bodies appear together when they lie in the same line from the observer. The apparent distance being an anpie, is measured as angles i^'» 14 ASTRONOMY are measured in other cases, the position of the observer being the vertex of the angle. Imagine a circle on the celestial sphere with the eye of the oliserver in the center as shown in figure 3. Fio. 3. — Showing the ahgle between two stars, a and 6, as seen by an observer. In the figure this angle is about 10°. Tlie figure also shows how degrees are counted round the circle, passing from the line going horizontally to the right. A right angle, or 90°, measures from the horizon A to the zenitli D. Two right angles, or 180°, bring us to the horizon on the left ; tlie third riglit angle, making 270\ takes us to the iwint D below, and the fourth one will can-y us round to A, where we started. Then this circle is divided into four arcs, AB, BC, CD, and DA, each of which measures a right angle at the center. P«B tlELATlOX OF TlIK EAHTIl 7'6 TIIK UKAVIJNS 15 al in The ni,'ht angle is subdivided into 1)0°, wiiicli may be done by dividing the arcs Ali, etc., into DO equal i)aits. Kacih (h^gree is divided into (50 minutes, and each minute into 00 seconds. , Any little arc on the sphere is then said to subtend the angle formed between the lines drawn from the observer's eye to tlie ends of the arc. To give an idea of the magnitude of angles, a foot rule at the (hstance of 57 feet from the eyt^ subtends an angle of about 1°. Tlie diameters of th(^ sun and moon subtend an angle of a little more tlian half a degree. The diameter of the smallest round object that an ordinary eye can distinctly see subtends an angle of 1'. More exactly 1' is the angle subtended by a nickel at a distancie of ."JL'O feet. 7. The Relation of the. Horizon to the Celestial Sphere. — In ordinary language the line around us where the earth and sky seem to meet is called the visible hnrizon, or simply the horizon. On a ship at .sea the visible horizon is a circle, extending all round the observer. It is called the sea horizon. an ws "g he to us A, er. Fio. 4. — The clip of the horizon from the deck of a ship. The curve is the rounded ocean ; ,S"/' is a horizontal line which does not strike the ocean at all ; //is a point of the sea horizon, seen from the ship in the line SIl. The angle TSII between the horizontal line from the observer's eye and tiie s« .i horizon is the dip of the horizon. If we regard the earth as perfectly round and smooth, a plane resting on it at a point where we stand is called the plune of the horizon, or the horizon plane. As wo look around, we must imagine this plane to extend out indehnitely on all sides of us. In figure 4 we show the relation of the horizon plane to the 10 ASTRONOMY sea horizon. The observer's eye being hiRhor than the water, we see from this iigure that the sea horizon will ai)i)ear a little below the horizon plane. The angle by which it seems below is called the dip of the horizon. The higher the eye is above the sea, the greater the dip. I Fig. ft. — Showing the altitude of a body above the horizon. Iv is the angle between the body and the horizon /f as it appeara to an ob- server. Tlie zenith distance is the angle between the line, iu which the body is seen, and the zenith %. The point in the heavens over onr head (B, fig. >'>) is called the zenith ; the point in the celestial sphere below our feet (D, fig. 3), which we cannot see, is called the nudir. The altitude of a heavenly body is the angle which its direc- tion altitv Th stanii perp( the [] Th from Ze whol 90". Chi celesi figun of ai ing o not d in pr than On t Th sphej is ca We I horiz into Sti sphei hemii becai hemii Wi to an fore ; will exaui I -Jfc a ms is the ob- hicli lied feet ireo- RELATTON OF THE EARTH TO THE HEAVENS 17 tion makes with the plane of the horizon. The greater its altitude, the higher it seems to us to be. The line joining the zenith or nadir to the point where we stand is called a vertical line. It is evident that such a line is perpendicular to the horizon plane. Its direction is that of the plumbline. The zenith distance of a body is its apparent angular distance from the zenith. Zenith distance and altitude added together make up the whole arc from the zenith to the horizon, which measures 90°. Change of Horizon as we Travel. — Now let us conceive the celestial sphere with the earth in ''■s center. Let the globe in figure represent the earth, and let APB be the horizon plane of an observer standing at P. We imagine this p.ane extend- ing out all round until it meets the celestial sphere. We can- not draw this si)here round figure G because, to be large enough in proportion to the earth, we should have to make it bigger than a house. So we draw it on a smaller scale irt Hgure 7. On this scale the earth is a mere point in the center. The plane of the horizon at P in figure 6 cuts the celestial sphere in a circle AB, seen edgewise in figure 7. This circle is called the celestial horizon of the observer at P in figure 6. We must imagine it extending all around us, like the visible horizon. 'Ihe celestial horizon divides the celestial sphere into two hemispheres, ACB and BDA. Standing at P in figure (5, we can see the stars in the hemi- sphere ACB, figure 7, which is therefore called the visible hemia})herc. We cannot see the stars in the hemisphere ADB because the earth is in the way, so this is called the invisible hemisphere. We see in figures G and 7 that, as we travel from one place to another, the horizon changes its direction, so that stars, be- fore invisible, will come into view on one side, and visible ones will seem to sink below the horizon on the other side. For example, if the observer travels to the point Q (fig. G), his KEWCUMIl's AMTKO.N. 2 ^ f ; 18 ASTROyOMY horizon piano will be CD, in figure 7. Then the stars in the refjion ^V, between B and I), will come into view, and those in the region ^f, between A and C, will seem to sink below the horizon. Fio. 6. — Showing how the horizon changeR an an observer travels from one point of the t-arth to another. The straight lines Alt and CD toucli- ing the earth represent planes seen edgewise. These are planes of tlie horizon, or h( .izon planes. Every different point of the earth's sur- face has a different horizon plane. For example : when the observer is at the point P, his horizon plane will be AB, which is represented as a line because we see it edgewise. If he travels to the point Q, his horizon plane will turn round into the position CD. Tlius his horizon alwivys clianges an he travels. These horizon planes must be supjxised extended out on all sides until they cut the celestial sphere. As the scale in this figure is too large to represent the celestial sphere, we make another figure on a much smaller scale to show the continuation of the horizon planes. It is very interesting to watch this change during a long ocean voyage from the north to the south, or vice versa. As we steam along the rounded surface of the ocean, we see the constellations behind us sinking lower and lower every night, till tliey disappear below the ocean horizon, while new ones seen cans Fi, in figure 7. Thus the stars in tlie region M, which are visible when the observer is at 7', are invisible when he is at Q, while the reverse is true in the region .V. The i)08i- tion of the zenith on the celestial sphere also changes from Z\ to Z^, as the observer travels from P to Q. ocean from Europe to America, and less than 24 hours long when we cross from America to Euroiie. I?^ i^-:'^ r" 20 ASTllONOMr 8. The Diurnal Motion. — If, in our latitudes, wo look at any star Houth of the zenith, and watch it for an hour, we hIuiII Hnii it moving from the left toward the ti^'ht. If we watch it a few liours, we shall see that it moves yet farther and fartlier toward the right and at length sets in the west, like the sun. If we look at a star in the east, we shall find it rising upward and moving toward the south as the sun does every day. Thus the stars in the south rise and set like the sun. We know that this is becrause the earth turns on its axis. To us, the appearan(!e is the same whether we sujtpose the earth to turn and the stars to stand still, whit'h is the truth, or whether we suppose the earth to be still and the stars to re- volve round it, which is not the truth. In order to describe how things look, it is sometimes easier to suppose that the earth is still, as we may fancy it to be, and then to tell how the stars seem to move. We call any seeming motion of the heavtmly bodies, sun, moon, and stars, their apparent motion, which means their motion as it appears to ns. The apparent motion which they have in consequence of the earth turning on its axis, is called the diurnal motion. Hence, the motion by which the sun, nxoon, and stars rise and set every day is the diurnal motion. Tl inter 'IM (•eb:sl the , Tl intei 9. Celestial Equator and Poles. — Figure 8 shows the earth with its axis passing through its center, from the south to the nortli pole. We must imagine this axis continued as far as we please in Iwth directions. Imagine a plane EQ passing through the center of the earth, at right angles to its axis. This is called the j)kun' of the equator, l)ecause it intersects the earth's surface all round on the equator. We must conceive this plane to be extended out as far as we please. Now we draw figure 9 on a small .scale. The outer circle represents the imaginary celestial sphere with the earth as a black dot in the center. Fio. the circ ti(m wes zeni hall »y m\ ow ler nil. ird 1U8 as. ;he or le- ibe the ow 111), eir the ce, set rth the the LDCI we ter rth RELATin.V or rilK earth to TIIK n HAVENS 21 'Iho points iV/'and .ST, in which the line of the earth's axis intersects the ('(^hmtial splu-re, are called the cclestis. If wv iiiiiiK'inu tiiis plaiM! I'xtcmlcd ii|iwiinl, so as to intorscrt till' roU'stiiil sphert', the circUi of intiTscctioii is ciillfd tho I'flcsliiil iiii'riiliiiii. The celestial meridian [lasses tlll•uu^h tho eeh'stial poles and the zenith of the phu-e. In tigiire *.) the plane of the paper is the plane of tlie niorui- ian, and tho outer cinde of the figure is the (-(destial meridian. The points N and S in which it intersects tiio plane of the horizon are the north and south points of the horizon. Meiufo if one stands fiu-ing the south, Ids meridian rises perpen- di(udarly from tho .south hurizuu to tho zonith and cuntinues to tho celestial polo. IT at O. :f(, illlil nppai'i- L-flestiiil are on tver tliu litli. called Han of icircles 11. Diurnal Motion in Different Latitudes. — The apparent diurnal motion takes pl.'u'o as if the two celestial poles wore juvots on which tho celestial s[»here is continually turning. Sui»pos(t a person to stand at tho north pole of tho earth. Then the celestial sphere will ajipear to him as it is represented in figure 10. The north celestial polo will be over his head, that is, in his zenith. The jdauo of his horizon will be ^fX, which you see is parallel to the piano of the ecpiator. When the earth becomes a mere point, these planes are so close together that we cannot distinguish between them. Menco : — To an observer at the noitli pole, the north celestial pole is at the zenith, the celestial equator is in the horizon, and the south pole is at his nadir. The poles being pivots, the diurnal motion of each heavenly body will seem to go on in a horiz(nital circle, from left to right, so that none of thc.?e bodies will either rise or set. When the observer is iiear the pole, the motion will be nearly horizontal. || 24 ASTnONOMY Suppose the observer travels to latitude 40 degrees, which is nearly that of New York and I'liiladolphia. We have seen in figures 6 and 7 how his horizon i)lane turns round as he travels. As it turns, the celestial pole will appear to move over to a position where it will be 40 degrees above the horizon and 50 degrees from the zenith. ;- ZP Fig 10. — Showing the equator, horizon, and direction of the diurnal motion a.s Heen by .an observer at the nortli pole of the eaixh. The earth is an invisible point in the center at A'. 'I'lie outer circle is the celestial sphere as seen by the observer. Round his horizon o'- the celestial sphere is the celestial (^uator, whicli in this particular case is the same as the celestial horizon. The other horizontal circles show the diurnal motion of the heavenly bodies, which seem to make their revolutions round and round in horizontal circles, without either rising or setting. In this position the northern heavens appear as in figure 11. The celestial pole is marked in the center of the map. There is a fairly bright star so near the pole that it is called the Pole- star. If one has an exact north and south line, he can find the polestar by looking nearly halfway between the horizon and the zenith, toward the north. If not, he must find the .^ l«i t £t i li i ¥ . .i i RELATION OF THE EARTH TO THE HEAVENS 2.') liich is seen ill ;nivols. or to ti mill 50 diurnal h. Thf circle is rizon o'- irticular .1 circles ieein to without live 11. There 16 Pole- m find lorizon iid the constellation Ursa Major, commonly called the Dipper. The two stars which form the outside of the dish of the Di])per point very nearly at the polestar, as we see by the dotted line Fig. 11. — The circle of perpetual apparition to an observer in latitude 40", showing the brightest stars of the northern constellations, includ- ing the pointers in Ursa Major pointing at the north star. To see how the stars will look at half-past eight o'clock on any evening in the year, liold the figure with the month at the bottom and look at the stars at the corresponding hour. The hour K'ven is for the middle of the month. To find the positions at other hours, notice that the diurnal motion takes place in a direction the opposite of those of the hands of a clock, as shown by the arrows, and turn the figure accordingly. in the figure. The Dipper can be seen almost any evening in the year, but in the evenings of autumn 'X will be low down i -"■». iil!riiiilWlf|frtlijf»<<»n>illW»l!»^ 2^" ASTRONOMY near the northern liorizon. If the pointers cannot be seen, there is only one other star that there is danger of mistaking for the polestar. 'IMiis is lietn Ump, Minorin, or Beta of the Little liear, which is about In degrees from it and is of the same briglitness; but it ean still be distingnished by its being a little redder and by the two stars between whieh it is situated. Having found the jiolestar, imagine a eircle to be drawn in the heavens, having the pole as a center, and at such a distance as to graze the northern horizon. Then studying figure 11, you will see that, as the celestial sphere appears to revolve around the pole as a pivot, all the stars within this circle will merely turn round and round the pole, as shown by the arrows, but none of them will ever rise and set. Hence this circle is called the circle of perjtettial apparition. It will readily be seen that the stars but a little outside this circle dip only a short distance below the horizon and are but a short time below it. They are above the liorizon most of the L ■>M RELATION OF THE EAIiTH TO THE UEAVENS 27 seen, iking f the f the being it is time, but not all the time. The farther they are from this cir- cle, the longer they are below the horizon. Next let us study iigure 12. Tiiis sliows the celestial sphere as if we were looking at it from the east, so that we see the western portion of it. Here you see that the etjuator EQ is one half above the horizon Hit, and one half below it. Hence vn in tance i r \'A> ^ ('- i '' W M A West \ / \ * 1 A /I /r '^1 South r- 1 ; / /It tl \ : \ ■' St ■ ~TH fefizon 1 \ y V * ' M */ » 7 \ \ ;• / \ / \ A U * / ' North •/*■ T .1 1, you .round nerely '8, but cle is le this re but of the Fio. 13. — Showing the diurnal motion as seen by an observer at the equator. The earth is .a point in tlie center at E. Tlie celestial poles are in the north anil .--outh horizon, and the (Hurnal motion takes place in vertical circles shown in the figure. All the heavenly bodies are as long above the horizon as below it, except for a small effect of refraction, which will be hereafter explained. a star on the equator is 12 hotirs above the horizon and 12 hours below it, in the course of each apparent diurnal revo- lution. The farther south the star is situated, the shorter the time it is above the horizon and the longer the time it is below it. At :| h . il|Pi««B«1M 28 ASTRONOMY 60 degrees south of the equator, the star will barely appear on the horizon and will immediately sink below it again. Now notice the youth celestial pole, as shown in figure 9. We see that, as the sphere seems to turn on it as r. pivot, if we imagine a circle drawn so as to touch our southern horizon, the stars within this circle will never seem to us to rise at our latitude. Hence this circle is called the circle of perpetual occulhUion. If we travel yet farther south, say to the Gulf of Mexico, the north celestial pole being nearer to the horizon, the circle of perpetual apparition will be smaller. The circle of perpet- ual disappearance will also be smaller. If we travel to the equator, the two poles, or seeming pivots, being in the north and south horizon, all the stars will rise and set, each being as long above the horizon as below it. Hence there will be no circles of perpetual apparition or occultation. If we go on into the southern hemisphere, the south celestial pole will rise above our horizon, the circle of perpetual appari- tion will be around it, and that of perpetual disappearance will be round the north pole, now below the horizon. Because a meridian line is fixed on the earth's surface, it follows that all the meridians revolve with the earth. Hence one result of the diurnal motion is that all the heavenly bodies seem to pass the meridian every day. Really, the meridian passes them, but, to our senses, they seem to pass the meridian. ■ n tl r< ', 12. Right Ascension and Declination. — The Declination of a heavenly body is its apparent distance from the celestial equator. To measure it we imagine a circle to pass from the pole through the body S, figure 14, to the celestial e(iuator. This is called an hour circle. The arc SR of this circle, be- tween S and the equator, is the declination of the body S. It is measured in degrees, minutes, and seconds. When the body is north of the equator, as at S, it is said to be in North Declination; when south, in South Declination. *^*»*T'! miles ; we scarcely know whether a little greater or a little less. Some idea of this distance may be gained by saying that a railway train running <'»() miles an hour, and making uo stop, would re(piire more tlian 160 years to reach the sun. Five generations might be born \ipon it liefore the journey was completed. NEWCOMII'S ASTUON. ',i ' r, ■! 84 ASTRONOMr Tho most markod dilTfioiiofl botwcpn tho sun anrl tlip i>liiiiets is tliiit tlic sun sliincs l.y its own li;r|it, wliil,. tli.' i.liuu-ls sliine only by tlie liK'lit that Ciills on them from tlie sun. 'I'lius, so far as moans of seoiuf,' arc coucorncd, the sun is like a candle in an otheiwiso dark room and tho planets aro iiki' little Itodios soon by tho lijj;ht of tho oandlo. Wo havo said that tho l.odi<'s of tho sohir system form a Kroup by tliomselvos I^ookin^ down from tho height we liavo supposed, wo should see this very olearly. Tho stars wliioh stud tho heavens would be seen just as tvo si'o them from the eartli, in every direction. Their distanees are so vast of.mparod with the size of tlie solar system that oven the latter, numenso though it is, is but a speck in oomj^irison. Wo may, if wo ])loaso, call them suns. Most of tl . -u aro brighter than the sun. They look snuill and dim beci.iso they are so inueh farther away. Thus, having oxpaiuled our ooneoptions so that the earth shall be but as a point in tho .solar .system, w(! must again ex- pand them .so as to think of the wh,(M)(> times that of thi! earth ilself. So we have to draw lif^mes, as Ix'fore, on two very dilTfient scales. I''i,i,'iire K! shows the orliit of the earth seen nearly e(lf,'ewise. The |ilane eontaininf,' this orliit is called the jiUtin' of tin' I'di/ifii: On a trm* scale the earth in this li^Mii'i' winiid he an invisihlc dot. so we make it larger, and then represent it on a still larger scale in ligure 17. Vui. 17. — Sliowiii^ how tli(! Klin Mliiiicsnii the earth in Junr, illiiininaliiiK the wlioif (if tlic Arrtic circle, wliiit' the wliole of the Antarelit; eirdu is in tlarkiiosH. In figure 1(5 the direcitioii of the axis is shown by the inclined line SS. An iiniiort.'int law of the earth's motion is this: As the earth moves round the sun, the direction of its axis remains almost unchanged. Tliis direction is not quite i)er[)endioular to tlie ecliptic, but is inclined to the perpendicular l)y 23^°, or a little more than one fourth of a right angle. This angle is called the obliquity of the ediptic, because it is equal to the angle which the plane of the equator makes with the plane of the ecliptic. ■*■«? ■ riti.ii r i. r ^tfc.. ^, .^,^ ^ I ii sc AfiTnONOMV 3. How the Sun shines on the Earth at Different Seasons. — Lot us now Hcti how tlio Hiin Hhiiics on tlm fiutli at ditlorcut tiiiit'S of th»' year. Spring Position of the Earth. — Ahont llio L'Ist, of .ManOi of fach year tlus eartli is in tlio position A. tiKiiro I«J, wlicrti tlio line from tlie sun to tlie earth is at rijjiit auKlcs to tin- earth's axis. The sun tlien illuniinutes tiie wliohi iieniisiiiiere of the earth whieli is turned toward it, from polo to pole. The chiyH and nights are equal all over the earth. This time is (tailed that of the ]Wual J'J th w ai al di ttKVOLVTJOS Of TIIR EAUTU nnrVT) THE sf'V 37 TliP fmthpr north wi' ^o from tlic fiiuiitor, tlio liirf,'«'r the fnwtioii of a fircU" of hititiKh- roiii I tlm t^iiitli whirli will Iw ill sen shiiio. Ilfiuroon llit^ -Mst of .liiiic the (hiys aro longer 1111(1 liu' iiiKhtH shorter as we p) toward the north. Hoiith of the equator tlio ihiys get shorter anil tli« nights longer, as we travel south, until we reaeh llu' antarrtic circle, when the sun will simply show himsull' on tho horizon ut nuon. Autumn Position of the Earth. — At (', the itlmio of the eiiuator again passes tiiroiigh the sun, and the latter shines over one hemisphere of the earth, from the north to the south pole. At this time the days and nights are again ecpial the world over. This is callcil the Aiitumind TCt/iiinoj; because tho days ami nights are again etpial and the s«ia.son is autumn. Winter Position of the Earth. -On Decendier LM the earth is in the position I), with the north end of the axis tipiM>d away fron\ the sun, and the south end tijiped toward it. Now day and night are the reverse of what they were with the earth at li. Kigiire 17 will still answer for us, only it is now night where it is represented as day in the figure, and vi'cp verm. All the region within the; 'iretio cirtdc* is in darkness, all that within the antarctic (drcle in the sunshine. North of the etpiator the nights are longer than the days; south of it the days are longer than the nights. The sun passes through the zenith of every place! in latitude 23i° south at noon of this day. This circle of latitude is called the Tropic of Capricorn. 4. Apparent Motion of the Sun. — The Zodiac. Having explained how the earth turns on its axis and revolves round the sun, while, to us who live on it, it seems to remain at rest, we shall now explain how the sun seems to us to move. The apparent motion of tho sun is based on these hu'.ta : — 1. Each fixed star is really in the same direction from us all day and all the year. The stars seem to m to change their direction only because we live on the moving ea.rth. •• IKff '•■Triif^'iiiir.iMiitrm-TlHtfi,?: -88 ASTRONOMY 2. Tlio sun is nearly, but not oxactly, in the same direction from ns all day, from its rising' lo its setting. Uut this direc- tion changes during the year in conse(]uence of the earth revolving round it. Fir.. 18. — Sltowinj; how, in consequpnee of tlio partli moving around the sun, Lhe sun seems to us to make an annual revolution round the celestial sphere among the stars, passing througli the Iwelve signs of the zodiac. Let us study figure 18, which shows the earth's orbit, AliC, with the sun in the center. Far outside the orbit lie the stars. To make a figure on the right scale, we should have to place the stars several miles away ; as wo. cannot do this, we repre- sent their positions as in tlie figure. RKVOLVTIOS OF THE EAIITII liOVM) TllK SUN 39 % Now suiJposo we coukl Hy a few thousand miles above the earth and aiconipany it in its eourse round the sun. Then, hjokinj,' down, we shouhl see the earth turning on its axis, and bringing its oceans and continents into view, one alter the other. Looking at the stars, we should see them at rest. They would neither rise nor set, nor even change their direction by any quantity we could perceive. Next, let us see how it will be with the sun. When the earth is at the point A, we shall see tlie sun as if it were among the stars at the point a. A month later when the earth has got to the point B, the sun will appear among the stars at b. In another month, with the earth at C, the sun will be seen as if at c, and so on through the year. As the earth goes through its revolution round tlie sun, the sun appears to move around in a circle among the stars, until the earth gets back to the position A, when the sun will again appear in the position a. Henee : — The sun appears to us to describe a complete circle around the celestial sphere, among the stars, every year. The circle thus described by the sun on the celestial sphere is called the ediptic. The zodiac is an imaginary belt in the heavens, extending 8° on eacli side of the ecliptic, and passing all round the celestial sphere as the ecliptic does. The ecliptic is its central line. If the axis of the earth were perpendicular to the ecliptic, the plane of the earth's equator would always pass through the sun, and the sun would always be seen in the celestial equator. Because of the obliqinty of the ecliptic^ already des(!ribed, the ecliptic is inclined t(j the equator by an angle of 2;5^°, cutting it at two points called the Vernal and Autunnud equinoxes, as shown in figure 10. To make this clear, we show in figure 20 how, if we could see the stars around the sun, and the eclijjtic and ecpuitor marked on the celestial sphere, we • hould, day by day, see the sun moving from west toward cast, among the stars. ; ?/» 40 ASTRONOMY In very ancient times men mapped out the apparent course of the sun round the celestial sphere, as shown in figure 19. They divided it into twelve parts, each 30° in length, and w tl m Taurui Fio. 10 Showing how the celestial equator and the ecliptic span the celestial sphere among the stars, the two being inclined at an angle of 23i°. Not only the earth, but the whole solar system, must be conceived as a point in the center of the figure. We must imagine ourselves looking out from this center. Then if we could see the stars around the sun we should see the latter appearing to pass around the ecliptic through the signs of the zodiac, as marked in the figure. Fig. 20. — The sun crossing the equator about March 20. named each part after the constellation in which the sun would have been seen had the stars been visible. These parts F e o h a r s imi^ifcttay sin ' in ' miiiiwiMtimmv-u.^ :^lk^ course ire 19. h, and 3&n the n angle nceived irselves around ecliptic le sun ! parts * UEVOLUTION OF THE EARTH ROUND THE SUN 41 were caiUed signs of the zodiac. The sun enters a sign about the 2l8t day of each month. The names of the signs and the months wlien the sun enters each are as follows : — Aries, The liaiii Taurus, The Bull Gemini, The Twins . Cancer, The Crab Leo, The Lion . Virgo, The Virgin Libra, The Balance . Scorpio, The Scorpion Sagittarius, The Archer . Capricorn us, The Goat Aquarius, The Water Bearer Pisces, The Fishes . March April Mivy June July August September October November December tJanuary February When the sun is at tlie Vernal Equinox, it appears in the celestial equator, rises exactly east, and sets exactly west. In figure 19 we see that during the six months the sun is passing from Aries to Virgo, it appears north of the celestial equator. It is therefore in north declination ; it rises north of east and sets north of west. At this time, in the northern hemisphere, the days are longer than the nights. See the apparent diurnal course of the sun as shown in figure 22. When the sun passes from Gemini into Cancer, it has reached its greatest north declination, and now begins to move south again. This point is called the Summer' Sohtice. When the sun reaches Libra, it again crosses the equator toward the south. This point is called the Autumnal Equinox. During the remaining six months, while the sun is passing from Libra to Pisces, it is in south declination ; it rises south of east and sets south of west. In the northern hemisphere the nights are then longer than the days. When the sun passes from Sagittarius into Cai)ricornus it has reached its greatest south declination, and begins to return toward the equator. This point is called the Winter Solstice. |;: 42 AsriKtythvy 5. Seasons in the Two Hemispheres. — Tho reason that suni- iiu'r is liotlcr tlian winttT is tliat the sun when nortli of tlie equator, not only sliincs longer upon us every day, but is nearer the zenith at noon. Thus more of its heat falls on any given s\;rfaee — a scjuare mile, for example, as shown in figure L'l. As the sun moves south in deeliuation, its rays fall upon our portion of the t\'irth at a greater ohliijuity, so that every squaie mile of our eountry r«>oeives less heat day by day. Ki(i. 21. —Showing how a 8(iuare mile of the earth receives less heat, the nearer the; sun is t>> tlie horizon. When tin; sun is in the zenith, tlie region 7JC receives as many of iiis rays as the region AV, twice as large, receives when the altitude of the sun is 30°. I tl ni tl di The greatest amount of heat is re(!eived at the time of the summer solstice, about frune 21, and the least at the winter solstice, December 22. But the highest average temperature does not occur till .Inly. This is because the sun's rays re- quire time in order to warm up the air and the surface of the land and sea, much as it takes time for a fire to warm up a room. The lowest temperature does not occur till January, because earth, air, and ocean retain for some time the heat radiated to them during the preceding months. Fi J% it sum- of tlie but is ills on Dwii in ion our squaie B» eat, the lith, the -wico as of the winter ;raturi! lys ro- ot tlu) a up a nuary, e heat HEVOLITION OF THE EARTH liOVNI) THE SUN 43 I'lUt in the southern hemisphere t.lie seasons are reversed. When the sun is in soutli declination, as at the winter solstice, the sonthcrn heinis|ih('re has the loni,'est days and the shortest nit?hts. Ileni-e, durinj^ our winter in the northern hemisphere, the southern hemisphere has its summer, and it has its winter duriuf^ our summer. ScufA A/br/A \\ \ WW v.J Fni. 22. — Showing the apparent (liurnal course of the .sun, as we .see it in our latitudes at different times of the year. Vou must fancy youi-self standing in tiie center of the hmdseape. Then in sinnnier you will see tlie ,sun rise eonslderal>ly nortli of east, pass not. far soutii of tlie zenith at noon, and set to tlie nortli of west, as shown in the right hand circU? of the figure. During the night it is completing that part of the circle which is helow the horizon. During the remaining months of the year it seems to i)ass, day by day, farther and farther toward the scmth until December, when it seems to describe the left liand circle. It then rLscis .south of east and sets .south of west. Wo .see that at this time the greater part of the circle is below the horizon, while in .lunc the greater part is above the horizon. fir i i 44 ASTIiONOMY We know that on a general average the hottest climates are within the tropics, and that the temperature is lower toward either juile. This is because the ohlirjuity of the sun's rays increases toward the poles. At the poles the sun shines only half the year, and then is never more than 23,J° above the horizon. 6. The Solar and Sidereal Years. — There are two ways of linding how long it takes the sun to complete its apparent rev- olution in the heavens, or, in other words, how long it takes the earth to make a complete revolution round it. One of these consists in observing the exact time at which the sun reaches the equinoxes. In ancient times astronomical observers were able to do this by noting the days when the sun rose exactly in the east or set exactly in the west. By observ- ing the rising and setting from day to day, they could find not only the day, but almost the hour in which the sun was on the celestial equator. Of course, with our more exact instruments, we can get this time with still greater pre -ision. The period between two returns of the sun to the same equi- nox is called the solar year or eqniuoxial year. The other way of finding the length of the year consists in observing the interval of time between two jiassages of the sun past the same star in the heavens ; for example, the period between two of its passages past one of the stars shown in figure 20. This method seems to involve the great difficulty that we cannot see when the sun is near the star. But the astronomer has methods of knowing exactly where a star is by day as well as by night, and can determine the moment at which the sun passes it. The ancient astronomers got the same result by using the moon as an intermediate object to measure from. The moon could be seen before sunset and its distance from the sun de- termined. Then, when the star appeared after sunset, the distance from the star to the moon was measured. Allowing >.jmii^emmm»^.- tasm ;es are oward } rays s only .^e tlie lys of it rev- takes which )mical le sun bserv- 1(1 not m the nents, ! equi- sts in le sun period wn in at we Qomer s well le sun ag the moon in de- t, the owing REVOLUTION OF THE KAHTll ROUND THE SUN 46 for the motion of the moon during the interval, the apparent distance between the sun and the star i-ould thus be learned from day to day. In this way it could be found how many (lays it was between the times at which the sun was at the same distance from any given bright star. This would be the period of apparent revolution of the sun in the celestial sphere, or, as we now know it to be, the [icriod of one revolu- tion of the eaith in its orbit. This period is called the sidereal year, because it is fixed by tlie stars. Hipparchus, who flourished about 150 n.c, was the first to make exact observations of the length of the year. Ptolemy, who flourished about 300 years later, made similar ones. They found that the length of the year, as determined in these two ways, was not the same, and that the solar year, as determined by the equinoxes, was several minutes shorter than the side- real year determined by the return of the sun to the same star. With our exact modern observations we have found the lengths of the years to be : — Solar year, „„„ Sidereal year, .^Cj Difference, 365 d. 5h. 48 m. 46 8. ""' 6 9 20 m. 23 s. This difference shows that the position oi the equinoxes among the stars is changing from year to year. Hipparchus and Ptolemy estimated the change to be about one degree in a century. We know it to be gieatc than this,— nearly one degree in 70 years. 7. Precession of the Equinoxes. — The motion of the equi- noxes which causes the difference between the solar and side- real year is going on all the time. It is called the Precession of the Equinoxes. The nature of precession is now to be explained. The equi- nox is the point where the sun crosses the celestial equator. The position of the celestial equator on the celestial sphere 46 ASTHtfNoMV is (letennined by tlie dirtH'tioii of tlie eartli's .axis, l)e('iiu.se tlie celestial etiuuUjr is IK)° t'loiii either celestial pole. The precession of the equinoxes arises from the fact that the direction of the earth's axis in space is slowly changing. Next, let us see how the change goes on. 1 'lagiiie a line passing through tin; sun pei'pen(li<-ular to the plane of the ecliptic. The point in whidi this line, when continued to the stars, meets the celestial sphere, is calUid the /'(Ac of the IJclip- tic. It lies in the coustellation Draco, the Dragon, but there is no bright star near it. Fio. 23. — Showing how tlie uiiuiiioxes are gradually shirting in conse- quence of the motion of the celcslial equalor among the stars. One of the brightest stars in the figure, which was south of the e(iuator two thousand years ago, is now north uf it. You will readily see that the angular distance between the pole of the ecliptic and the celestial pole, corresponding to the direction of the earth's axis, is equal to the obliquity of the ecliptic, 23^°. Now, the law of precession is that the celestial pole is in motion, and niak.is a complete revohition round the pole of the ecliptic in about 25,700 years. This motion is very slow to ordinary '. ision; it would take a century for the naked eye to notice it, even by careful observation. But the exact obser- MrlimffiWillllltWMIIWlMlllfllBHIfWI tmmmtimSBi nEVOI.UTWN OF TIIK EAtiTII HOUND TItE Sl'.W 47 Q the it the , line f tlio () the theie One luator ti the ng to ity of is ill )le of 8h)W (I eye obser- vations made by .'wtronoiners witli the meridian ciniie make it evident month after month and year alter year. Owing to tliis motion of tlie celestial pole ilie eelestial eiiuator moves also, continually sliding along tht! ecliptic, and carrying the ecjuinoxes with it, as shown in ligure I'.'t. This is why the eciuinox moves among the stars. The rate of motion is a little more tiian AO" in a year, (»r nearly 14° in KWX) years. Motion of the Ecliptic. — If the plane of the ecliptic were absolntely fixed, the obliquity of the ecliptic would be always the same, and the motion of precession would go on forever at the same rate that it now does. Hut the attnuition of the other planets on the earth produces a very slow change in the cclipti(^ itself, about -g\ the (ihange of precession. In conse- (pience of this change, the revolution of the celestial j)ole round the pole of the eclii)tic does not take place at an exactly uniform rate, nor will it always be comideted in exactly the same time. For the same rea.son tlu^ obliquity of the ecliptic slowly changes. It is at the present time dimin- i.sliing at the rate of about 4(>" in a century. Results of Precession. — One result of precession is that the celestial i)ole wa.s not so near the polestar in former times a,s it is now. In ancient times it was so far away from that star that the latter could not be considered as a polestar at all. It has been continually coming nearer, and is still approaching it. About the year 2110 it will pass by tlie polestar at a dis- tance of only 24'. Continuing its course, the celestial pole will pass some ,')'' from the star Alpha Lyr*. about 1 1,000 years from now, and will continue its circuit until it gets back to where it now is in about 25,700 years. The two equinoxes will make a revolution round the equator in the same period of time, being carried along by the earth's equator, which is always at right angles to the earth's axis. CHAPTER III OF TIME 1. Diuraal Motion of the Sun and Stars. — We now know why it is that we do not see the same stars every eveninjj all the year round. A star which, at any time, is seen in the west after sunset, will, evening after evening, be seen nearer and nearer the sun, until it is lost in the sun's rays. Then, when the sun has got eon^iderably past it, we shall see it in the morning before sunrise. Fi«. 24. Imagine ourselves seeing the sun pass the meridian to-day. Suppose any star above, it passing the meridian at the same moment. To-morrow, at noon, the sun will have moved a little east of tlie star (figure 24). Henoe the star will pass the meridian before tiie sun does. Next day it will pass earlier than the sun by a yet greater amount, and so on through the entir*^ year. At the end of the year they will ag.ain pass the meridian together. You see from this that the star, in its 48 ap ah( en( Bta tlu tio fol re^ gi^ da; Su rei 4r thi eai m( t.K no on a { thi tlu se< coi tin ck ha na th( thi I * tarn 3W know enin;^ all the west sarer and en, when it in the ,n to-day. the same ed a little pass the ss earlier ouf^h the I pass the :ar, in its OF TIME m apparent diurnal revolution, has been continually running ahead of the sun and has caught up to it from behind at the end of the year. It follows that, in the course of the year, the star will have risen, crossed the meridian, and set one time more than the sun. The sun makes '>iG5\ apparent diurnal revolu- tions around the earth, there being one revolutiun a day. It follows that the star will have made 3G(>^ apparent diurnal revolutions. If we divide the number of seconds in a day by 365J, it will give us the time by which the star has gained on the sun every day. We find the (piotieut to be 237 seconds, or .') m. 57 s. Subtracting this from 24 hours, we find the apparent diurnal revolution of the stars to be made in 2',i h. r»oint liis Ic1(-hco|>u at any .slur hu wunta to see by day as well uh l»y ni^iit. 2. Mean and Apparent Time ; Inequality of Apparent Time. — The iiH'asnrc of tinit' wliiitii we use in daily life is called n'ril tiiiii'. Tim moment when the sun crosHcs our meridian we call vooii. Hilt there is a ditt'eiijty in iisini,' the true noon as 12 o'clock, owin},' to the oltliquity of the uclipliu and the unequal motion of the earth round the sun. Fio. 25. — Showing tlio reason of the b()uati(>n of time. The earth moves a little faster in its orbit in our winter than it does in our summer. Hence the sun seems to move along in the ecliptic a little faster in winter than in summer. Owing to the obli(iuity of the ecliptic the earth sometimes lias to turn farther in order that a meridian miiy catctli iip to the sun, than it does at other times. "Figure '2~> shows this. Suppose thtat (m .some day at noon near the vernal ecjuinox we see the sun at li. Next day the point 72 being fixed among the stars will pass the meridian .'J m. 57 s. before noon, and the sun will be at S, having moved obliquely toward the north. Tn order that the sun S may reach the meridian TJi, it will have to pa;s over the distance ST hy its I lolative ho WUIltH Time. — 11<'(1 n'ril II we rail oil a.s \'2 uuequul iter than ! ah)iig in Owing i8 to turn mn, tlian loon near the point ',) m. 57 H. obliquely •each the S^rby its OF TIME at apparent diurnal motion; or, in othiT wohIh, the nu'ridian Tit will have to pass over the diHtanee TS to be at the huh. H'lt tlie lint' .S'7' is shorter than Sit. Hence it will take the sun less than .'t in. 'u s. to i)ass from .S to 7', so tliat it will be on tile meridian a littltt earlier than it was tlie day before. Next HU|tpose the muii near the Hummer solstice. ( Mi iMfcoiint of the convergence (d' tlie meridians from tlie cnuator toward the north pole, the sun will pass over nmie than i! m. oT s. of right asceiKsion near the solstices, and ho will pass the meridian lat^T eacii day than the day before. In eonscipience of these two iiuMiiialities, the sun, at certain times of the year, falls behind, little by little, day after day, and at other times it catches up again, making the timen iH'tweeu iii..)iis longer at some seasons than at others. Hence, if we used time measured by the true position of th(^ sun, our hours would be of sligiitly unerpial length. This uiief[ual time, measured by tiie true sun, is called ((/ifiiu't'iit timr. The moment when the real sun is on the meridian is called iipixtrciit unmt. In former times, when people did not have good watches or docks, and the exact time was not important to know, tliey generally went by the sun in setting their timepieces. T.ut owing to the inecpuility of the intervals lietwecu two apparent noons, a timepiece will not keeji apparent time. Mean Time. — To make the hours of ecpial length, we fancy an imaginary sun to move round the celestial Cipuitor at a uniform rate, so that the true sun shall be sometimes ahead of and sometimes behind the imaginary sun. The latter is called the mcdn aim. When the mean sun passes our meridian, it is called meUH noun. Time measured from mean noon to mean noon is called viean time. This is the only kind of time we can measure with a clock, and it is the only kiiitl now in geueial use. Equation of Time. The differeiuui between apparent time and mean time is called tlie eqmiHim of time. It is greatest early in November of every year, when the true sun crosses the meridian about IG minutes befori;! mean noon. In February, I ! I 08 ASTRONOMY the true sun is nearly as far aliead of tlie mean sun and crosses the meridian about 14 minutes after mean noon. Thus the greatest mistake we should make in measuring time by the true sun would be about a quarter of an hour. Some almanacs give the etjuation of time for every day in the year, or, which amounts to the same thing, the time to which you should set your clock every day at the moment when the sun is on the meridian. The following examples will make this clear : — February 11, April 15, May 14, June 14, July 26, September 1, November 3, December 2-5, sun on meridian at 12 h. sun on meridian at 12 sun on meridian at 11 sun on meridian at 12 sun on meridian at 12 sun on meridian at 12 sun on meridian at 11 sun on meridian at 12 14 m mean time mean time 56 mean time mean time 6 mean time mean time 44 mean time mean time We see that there are four days in the year when the sun is on the meridian at mean noon, so that the mean and apparent time are then the same. 3. Local Time and Longitude. — As the earth revolves on its axis, all its meridians in succession pass the sun, or, as it appears to men, the sun passes all the meridians in its apparent diurnal motion round the earth. Because it is noon when the sun is on the meridian of a place, we see that noon is contin- ually traveling round the earth, getting back to the same place in 24 hours. The circumference of the earth being 360°, we find, by division, that noon travels round the earth at the rate of 15" in 1 hour of time 15' in 1 minute of time 15" in 1 second of time In the latitude of the middle states, 1' of longitude is about 4800 feet. Hence, 15" is about 1200 feet. Thus we see that, in our latitude, noon travels from east to west at the rate of t I hi L crosses hus the the true f day in ;o which hen the 11 make ne ae ne ue ne ne ne ne e sun is pparent s on its ir, as it pparent hen the contin- ue place \m°, we the rate is about ee that, I rate of OF TIME 63 about 1200 feet a second. It requires between 4 and 5 seconds to travel a mile. Hence, tw and thrown upon a white screen or wall. We may then dis- tinguish five very brilliant colors, red, yellow, green, blue, and violet, as well as some intermediate shades between these. If we notice how these colors are phuu'd, we shall see that the Fio. 20. — Sho... how ii._ [ light are refracttd iii prism. ...g through a red light is refracted from its course the least of all, yellow more, green yet more, and so on. This shows that the white light of the sun is a mixture of light of countless different kinds, each kind being refracted differently from the other kinds. 2. Lenses and Object Glasses. — When rays of light from a distant object pass through a convex lens, the curvature of the surface cau.ses the rays to be moie refracted the nearer they pass to the circumference of the lens. The result is that the rays coming from any one point of the object all converge very Flo. 30. — Showing how parallel rays of light are brought to a focus iit F by passing through a convex lens. An observer holding his eye at F and looking at a light, however small, in the distance, would see the whole lens ilhuninatt;d by the light. nearly toward a certain point, fVfignre 80), which is called the f)K'UH of the lens, aiul then diverge again as if they were emitted by the focus. The effect of this can easily be seen bv liolding a ■ JifciBBi'iMiiarttJ 60 ASTRONOMY common reading glass or magnifying glass perpendicular to a window on the other side of a room. If you then hold a piece of white paper at the proper distance beyond the glass, you will see a little picture of the window on the paper. A picture thus formed by a lens is called an image of the object emitting the light that forms it. The lens may form the picture in the air when there is no surface on which the light may fall. The focal length of a lens is the distance from its center to the image of a distant object formed by it. The ordinary lenses which we use have convex surfaces and are called convex lenses. But a lens may be made having one or both surfaces concave. It is then called a concave lens. Fio. 31. — Showing how rays of light are made to diverge by a concave lens instead of being brought to a focus. I i When rays from an object fall on a concave lens, they are not brought to a focus, but, on the contrary, are made to diverge by the refraction of the glass, as shown in figure 31. An ordinary lens does not bring all the light actually to the same focus, on account of the dispersion of rays of different colors just described. The image of a star, instead of being a point, is a little colored circle near the focus. This disper- sion is called chromatic aberration, and results in indistinctness of vision. But two lenses of different kinds of glass may be so formed that, when joined together, the rays passing through them shall all converge almost exactly to the same point. One of the lenses must be convex, the other concave. The convex lil -tsiSSSi^- ir toa i piece »u will e thus ng the he air iter to »s and ig one IS. oncave re not •geby ;o the ferent being isper- ;tness lay be rough One juvex alhL OBSERVATION AND MEASUREMENT 61 lens is commonly macle of crown glass, the concave one of flint. The property of the.se kinds of glass is that flint refracts light about as much as crown, but disperses the rays nearly twice as much. The dispersive powers of the concave and 'convex glasses act against each other, so that the rays leave the last lens with- out dispersion and so come to the same focus. Such a com- bination is called achromatic or free from color (figure 32). Objective _ Fio. 32. — Section of an achromatic objective, showing the form of the flint and crown lenses. The crown lens is always convex ; the flint has at least one surface concave. 3. The Refracting Telescope. — A refracting telescope is one in which the image is formed by a lens or achromatic combina- tion of lenses called the object glass or objective of the telescope. When the telescope is pointed at a heavenly body or other dis- tant object, the rays passing through the object glass come to a focus, and form an image of the object. This image is to be seen by the aid of an eyepiece, which is a combination of two small lenses so arranged that the observer can get as good a view as possible of the image. BIagnlf3rlng Power. — The magnifying power of a telescope is the number of times that it makes the linear dimensions of an object seem longer than they do to the naked eye. For example, the apparent diameter of Jupiter is commonly about 20". A magnifying power of 60 would make it appear 1000", or 16' 40" in diameter, and therefore larger than the sun or moon. The law of magnifying power is that it is equal to the quo- tient of the focal length of the object glass divided by that of the eyepiece. Thus, with a telescope of eight feet focal length we I •m \ I 62 ASTHOMtMY should get a magiiifyiug power of Ofi l»y using an eyepiece of one inch focus, and a power of l'J2 by using one of half an inch focus It follow;; iluit, with any telescope, as liigh a jtower as we wish can he prodiu'cd by using an eycpi(>cc small enough. But a limit is soon reached beyond whiidi a higher [K)wer will not cnai)lc us to see any more, bet^ause the light btfcomes fainter and tlie object more iiidislinct. Commnnly an eyepiece between i and \ an inch in focal length will show all that can be seen with any telescope. As mucdi ol llm sky as we can see in a tcdescope at one time (as magnified in the telescope) is «'alled i\wfidd of view of the telescope. In the teles(!0])e, tiie field of view commonly looks very large, but tlie iicluai portion of the sky which it lilk^^s in is very small. The higher the magnifying power, the smaller it is. The line wliich I'oltiiS thecenllttl itxi» of the tube of the lele- scope, or which passes through the centers of the objective ami eyepiece, is directed toward the center of the Held of view. It is called the //«« o/siijIU of the telescope. 4. The Equatorial Telescope. — If we jioint a telescope at a star, and ilo not move it, we shall see the star nujve rajjidly across the iield of view and disapi)ear. This is because the telescope stands on the revolving earth, and turns witli it. 'J'he ajjparent diurnal nujtion of a star, when seen in a teles«!oi)e, is niultii)lied as many times as the telescope magnities. Hence the higher the magnifying power of a telescope, the more rapidly thfi star will seem to move across the field of view. If we wish the telescoi)e to stay pointed at a star, we must move it in the opposite direction to that in which the earth turns. This is done by supporting the telescope on axes on whicdi it can revolve. The nia^ddnery by which the telescope is made to revolve, and the handling of the telescope made possible, is called the mounting of the telescope. A telescope mounted so as to follow a star in its diurnal motion is called an efjuutorial telescope, or simply an equatorial. I< axil in t Thi JH>1 teh '1 end axi a s tail axi cal nut (!au the it, 1 l)e cir( tioi ] tel( tW( poi pai ent it< at oul it, mi wi so we Th dc im wmmm iece of ulf an UH we . I^it ill not Fiiiiiter Hween a seen Vi time of the ' luokH •N ill is T it is. le lelu- ve and w. It a star, acrroHS escope parent tii)]ied hij,'her lift star sh the in the rhis is it can ade to ible, is ited so latoricU OliSKUV Alios AND MKAHVUKMKST 08 Figure .3.'i shows the iiioiintuif,' of an erpiatorial. In /* is an axis parallel to the axis of the earth, the upper eiiil of which, in the northern hemisphere, points to the north celestial pole. This axis is therefore oblitpie to the horizon, it is called the palnr iirix of the telescope. To the njiper end 'jf the jiolar axis is fastened a sheath I), eon- tainiii},' another axis. This is called the th'cli- uatiun nxis, hc- cau.se by tiirniiifj the telescope on it, the latter may lie pointed at any eiride of declina- tion. By turning the telescope on these two axes we can point it to any part of the heav- ens. If we wish it to stay pointed at a star with- out our touching it, the telescope must be 8upi)lied with a clockwork so made as to keep the telescope turning from east toward west, exactly as fast as the earth turns from west toward east. Then by pointing the telescope at a star, and starting the clockwork, the star will remain in the field of view. Fio. 33. — A small ctiuatorial U'lescopc. 64 AHTRONOMY 5. The Reflecting Teleecope. — Rays of light from a lioavenly botly luuy be brought to a focus by a concave mirror as well aa by a lens, as shown in figure 34. On passing through the focus - " -^?i^v- ::: - Fio. 34. — Uhowliig how parallel rnya falling oii a concave mirror are brought to a fixnw at F. they will diverge again, as they do after passing through the focus of a lens. Hence an image of a heavenly Inwly may be formed in the focus of a concave mirror. A reflecting tele>icoj)e is one in which the image is formed by a concave mirror. Such telescopes can be made of larger size than refracsting telescopes, but they are not so convenient to use. The observer, to view the image directly, would liave to stand in front of the mirror, and tluis be in the way of the light from the body to the mirror. The best way of avoiding this is to put a small diagonal reflector in the middle of tlie tube, near the focus, as shown in figure .S5. Then the observer looks in sidewise near the end of the telescope where the eye is shown in the figure. The small mirror and its supporting piece cut off some of the light, but not so much as the observer's head and shoulders would cut off if he looked directly at the image. 6. Great Telescopes. — Large telescopes are objects of so much interest, that a short history of their growth will be given. The object glasses of the first telescopes, namely, those made by Galileo and his immediate successors l)etween 1610 and 1750, consisted of only a single lens. Such a lens, as we have already seen, refracts the light of different colors to different foci. For this reason distinct vision was impossible with these inst pro' said lon^ mm of \ 'I frac Nfv ing mat iuHt littl VVil was and ing thei f«!Cl thci ror, wci fori rity his sniii A the mei glaf site glas to I] to 1 off pur -^mmmamm 0«.s Kli VA TION AND MKA SVll F.yiUNT ♦55 ivenly ^ull as focus ror are ;li the nay be led by 'a(;ting ) stand t from i is to B, near uks in shown ce cut i head image. I much given. I made and e have tferent 1 these instruments. Vision was, howevci', iin- provjul l)y nuiitin^' tluMii very long. It is said that Honic wore 1(K) feet or more in hdigth, lint tht'sc jirovt'd to Iw (|uite nn- manaKcabio and were probably of very little use. Tliis (limculty with the re- fracting teles<'o|)t' led Sir Isaao Newton to propose the use of the reflect- ing telescope, which was free from chro- matic aberration. He made sonic small instruments of this kind, but they were little more than toys until the time of Sir William Hcrschcl. This great astroiioiiier was at the height of activity between 1770 and 1800. He acipiircd siuOi skill in mak- ing reflecting tehf.scopes that Im carried them up to two feet, and in (me case, four feet in diameter, lint the difficulty was thin encountered that the refleeting mir- ror, when large, wonld liend under its own weight, so that a good image conld not be formed. Thus, m)twithHtan . *■ ; ! i .i- «»;i »iw^ «p mw" ee ASTRONOMY large quantity of load, and the lead would sink down to the bottom of the pot in which the ghiss was melted, and thus make the glass heterogeneous and unfit for use. Thus, a hun- dred years ago, a refracting telescope four inches in diameter was considered large. About ISIO, Uuinand, a Swiss glass maker, found a method of nuiking disks of glass much larger than had before b.'en possible. At the same time rose the celebrated Fraunhofer, a (Jerman oi)tician, who acquired remarkable skill in grinding and figuring the lenses of object glasses into exiUit shape. He .and his successors in Germany carried refracting telescopes xip to lo inches' aperture. In 1845 a telescope of this size was made for the Harvard Observatory in Cand)ridge, Massachu- setts, and became, in consequence of its size and excellence, one of the celebrated instruments of the world. About the same time, Lord llosse of Ireland made his cele- brated reHecting telescope, six feet in diameter. This is still, in size, the greatest telescope ever constructed, lint the impos- sibility of keeping the mirro: in proper polish an. — Great 4(>-iiicli telescope of the Yerkes Observatory. Next, a telescope of 30 inches' aperture was made in France by the lirothers lienry, and is now mounted at the observatory of Nice oil the coast of the Mediterranean. wmkmm^a^dm 68 ASTRONOMY In 1883 Ml". Clai'k and his two sons made the object glass of another telescoi)e of 80 Indies' aperture for the observatory at Piilkowa, in Kussia. The mounting of this instruiueiit was made by the Repsolds of Hamburg. In 1876 Mr. James Lick, of (Jalifornia, gave money to found an observatory, which was to be provided with the largest telescope that had ever been constructed. The work of making the object glass of the instrument was again intrusted to Messrs. Alvan Clark and Sons, but great difficulty was found in getting disks of glass of the necessary size and purity. At length, after many years of failure, a Frenchman succeeded in the difficult task of making excellent disks of 36 inches' diameter. With these the Messrs. Clark completely finished the object glass of the telescope in the year 1887. The mount- ing was made by Warner and Swasey, of Cleveland, Ohio. Mr. Li(!k's observatory, which is called after him, was built on Mount Hamilton, in California, and the telescope com- menced its work there in 1888. The largest refracting telescope now in actual use is that built at the expense of Mr. Yerkes of Chicago for the univer- sity of that city. The object glass is 40 inches in diameter, and was figured by Alvan G. Clark, the son, and mounted by Warner and Swasey. The Yerkes Observatory, in which it is placed, is near the shore of Lake Geneva, Wisconsin. 7. Meridian Instruments. — A telescope of some sort is an essential part of every instrument intended for exact astro- nomical observation and measurement. One of the most common of astronomical instruments is the meridian traaait instrument. Instead of being mounted like an equatorial, so as to be pointed in any direction, it turns on only a single horizontal axis, having an east and west direction. Thus the telescope turns only in the plane of the meridian, so that jt will show us objects only while they are crossing the meridian. To explain the use of the transit instrument we must recall what we have said about sidereal time. A sidereal clock is set glass itory was ouncl rgest king d to ound At eded ches' shed ount- )hio. built com- that liver- leter, dby it is s an istro- most •a,iHit i\, so ingle ? thr lat jl dian. recall is set nmfmrnm J*L OBSERVATION AND MEASUREMENT 69 running in such a way that its hands shall point at Oh. Om. Os. when the vernal equinox is crossing the meridian. Then as the various heavenly bodies are seen in the transit instrument, crossing the meridian, the tini> shown by the hands on the face of the sidereal clock shows the right ascension of each. If you should h)ok into a transit instrument, you would see one or more dark lines passing up and down across the field of view. These are. fine lines made of spider web, the middle one of which marks the meridian. TIk moment at which a star crosses this line may be not(!d on tin clock within a small fraction of a second. This gives us the right ascension of the star with the same precision. This instnmient also enables us to determine the time of day, or the error of a ciock or watch, with the same exactness. The observer notes the time by the clock at which a star of known right as- cension crosses the meridian ; the difference between the clock time and the right ascension is the error of his clock, for which he can make due allowance at any moment. The moment of noon is sent out by a telegrajihic signal from different observatories to railway offices and elsewhere, so that any one who receives the signal may set his clock exact to a second, if he has the skill to do it and exercises the necessary care. To the transit instrument are sometimes attached vertical circles, which will turn with the instrument. These circles have fine lines engraved all round their circumference, so as to mark off the degrees and minutes of the circle, liy their use the declination of a star, as it passes the meridian, may be observed. Fio. 87. — The threads in the focus of a, transit in.siru- ment, witli a star passing over tliera. " 70 /l.STKOA'OiVr Fill. ."W. — A iiii'i itiiiiii ciiolc seen from the SDUtli. <\ (' art' Hu> friayo so as to touch the glohc. ( )ni' half of this dianiftcr, or either of the angles .lA'C'or lU'JC, is (iitlled the Heiuklid meter. Thus by the Fi,j. l:!. — Apimniil (liiiiiictcr of ilir huh, moon, or oii.?r lipavci.ly body, ius seen l>y an oliMcrvcr at K. The diamelcr is the iiiiKly At^if, sub- K'luUid by tbi' wholi? diaiiietiT of the body, whili; Uio seiuidiuiii- clcr is liic aii;;li^ CIIA or CJiK l«etwwii tiie center and ai>i)arc'iil circuiiiferunue. semidianieter of the sun or moon we mean the angle lu^tween two lines, one of which is drawn to the center of the sun or moon, and the other to its aitparent circuniference. It is evident that the semidiiiineter of a given body is smaller, the farther the body is away. The iHtmlhi.1' of a heavenly body is the difference of tlie directions in which it is seen from two ditTerent points. Let S, figure 44, be the body, and A and U the two points from which it is seen. X B Fi the naked eye, it would be a large object, when measured with the tcles(U)pe. The only way in which the distances of the bodies of the solar system can be directly measured is by their parallax. Two observers on opposite sides of the earth, making exact observations of the direction in whie displaced in the direction toward which the earth carrying the observer is moving at the moment. For example, if the star is in the position S, fig\n-e 46, and the earth at E is moving in the direction of the arrow, then the star will appear as in the position T, in the direction shown by the dotted line. This displacement arises from the 78 AsrnoNoMY (•oiiihiii;i1i( I' the ciirlh'M luotioii with tli(' iiiotinn ol HkIiI. Il- ls nillcil llu) iihrrmUnn of liijfil, ur, lor Hliortiicss, iil,i',ritliii„ Hiiiilily. • T f I I I I Fid. -Id. Fki. 17. To ()Xi)lain iiltorriition hiii)jm)sc .Hi, lipiio 47, to Im- ;i very long and narrow tul)»s and let tlin dotted line W a ray of liiclit from ca star, so tliat, if the tubo were at rcwt, the ray would pass centrally through it, from A to Jl. Then an observer looking through the tube at B would see the star in the central line of the tube. Now suppose the tid)e and observer to be moving in the direction shown by tlie arrow, so that while the light is pass- ing from .1 to li, the tube is carried from the position AB to the i)osition CI). Then, the motion of the tube would cause the ray to strike the side of the tube before getting through it, so that the observer would not see the star. In order that the star may be seen while the tube is in motion, the latter must be inclined in the position .LV. Then, whih the 1 1 light lonki stead Th earth tiie s passi the CI Th Bume whih will ^ direci it rea If strain dnip^ ward «teM irikr- .-jLl^ nnsHUVATins A.\l> MKAsritKMI.WT 71 > wliilt' tlic iiiv til H^jlit is |i;i.s.siiiK rmm ,1 to li, \ho ci tl ,V of till" liilif will !«■ ciinit'd rnun .V Ui //, uinl, in ('(uistMiiKMicc, lli« IIkIiI will liass ci'iitriilly tliroii^,'li Mh' tiilM-, llcnct' tin' (iltHcrvt'i-, iiHikiii^' ill ill A', will now set' llic hImt in tlir diit't'lioii AM iii- Mtciul of till- t-i'iic (lirt'ftion /LI. Mill 1 11' III MM 1 |MI' I'llill' I III- til 11-1 till II 'II \'t 11^ ill' WW! .-1. II IV -n ' i»ii.» • wliili' in motion lias a Hiilo wind Itlowinj,' aKaiiiHt ht'r, tlic wintl will H»M'iii to tilt' iias.sfiimM'H to Mow from a jHiinf, nfart'r tlio iliri'ftioii in wiiicli tlii' ship is KoiiiK tlian tin- oiit^ from wliii'h it rt'ally blows. If one ilrivt's rapidly tliroii>,'li a sliowpv of rain falliii},' strai^'lit down, ami wntflit's tin' tlirt'ction of tlif motion of tlit* ilrops, tlify will lit' seen falling,' oliliipifly as if carrieil back- ward by a wind. r :> CHAPTER V ' ^ i GRAVITATION 1. Force. — The motions of the hoavenly bodies seem so different from the luotions we are accustomed to see on the earth, that for many generations it was supposed that they could not be explained by the same laws. We have now to see how it is that the planets revolve around the sun accord- ing to the same laws which govern the motion of a ball thrown into the air. To do this, we must learn the meaning of certain words. The substance of anything we can see or feel is called Anything .Tiade up of matter, and considered as a thing by itself, is called a hody. For example, a ball is a body ; the rubber, leather, and yam of which it is made are matter. That which makes a body move or stop moving is called forcfi. For example, if you throw a ball, your hand exerts a force on the ball ; it is that force which sets the ball in motion. As the ball flies through the air, the air exerts a force against it ; this force makes it go slower than it otherwise would go. When the ball strikes the ground, the ground exerts a force against it ; this force soon stops it. Friction is a force exerted by one body upon another that rubs against it. If you try to draw a sled on ice, you can pull it along very easily. But if you try to draw the same sled with the same load on a smooth pavement, you will have to pull harder, and on a rough pavement yet harder. This is 80 becau runne Qn fall t( all of stone it did from down 2. have bodie first ( Eve forwa It unsu] straif all m all wi On bring is fri' and I Th railw dista fricti allb: w: move. to m they their iSiltiaSi|iWfiii?iBi)iSifwiilii: m" a bullet seemi> uinuwt straight, while a ball thrown .ii- GRAVITATION ire .d, ^e- ne or ill lis in nil me m by a little child describes a path much more curved than one l)atted by a baseball player. Imagine the earth to be a body thrown like a ball, and attracted by the sun. To learn how it will move, we must note that although the mass of the sun is 333,000 times that of the earth, yet it is so far away that its attraction is only about ttiVt ^^*"'*' *^^ *^'^ eartli on things about us. Hence a ton weight. 0* Ol '"/) lllg ing 'all aw eet we ;1G lis use the iwn Fio. 40. — Showing a small part of the earth's orbit round the sun. In consequence of tiie sun's nttraction, it is continually falling away from the line of its motion, AB for example. Compare this figure with the preceding one, and note that as a ball's path continually curves toward the earth, so the earth's path continually turves toward the sun. or li.^40 pounds, is here on the earth attracted by the sun with a force of little more than one pound. To tind how far a botly like the earth would fall toward the sun in one second, we must divide the distance, 16 feet, or 192 inches, by 1(>50. This is less than \ of an inch. Now in figure 49 let the arc be a piece of a circle around the sun. Draw tlie line AB, touching the circle, and let the earth be thrown in the direction of this line with such speed tliat the curvature [T 8S ASTRONOMY of the patli in consequence of the fall of the earth toward the sun shall be equal to the curvature of the circle. Then, notwithstanding that the earth has coniuienced to fall toward the sun, when it reaches C, it has kept in tiiis circle and is no nearer to the sun than it wsis at A, but is now going in a slightly different direction. In the same way, when it has described another arc, it has got no nearer the sun by its falling, but has only kept in the circle. All that the sun has done by its attraction is to keep the earth from flying off from it al- together in a straight line. It keeps bending the path of the earth from a straight line into the circle round the sun. Thus, instead of either falling into the sun or going away altogether, the earth revolves round and round the sun forever. The idea wd have now to grasp is that the earth is not held by anything, but is flying through space, turning on its axis all the while, with us upon it, as a ball might fly through the air with insects on it. One who knows enough of geometry to be able to make the necessary calculations will find that in order that the earth may thus describe a circle round the sun, the speed with which it is to be tJirown must be about 18.6 miles per second. That is, at the distance of 18.6 miles along the line AB the circle round the sun will be \ of an inch from this line. But the earth does not always go exactly with this speed. \Ve must, therefore, show what hai)i)ens if the velocity should, be a little less than that we have supposed. In such a case, the earth or other planet will fall a little nearer the sun until it gets halfway round. But, in thus falling nearer the sun, it will have accpiired a greater velocity, and, in consequence of this increase of velocity, it will, after going halfway round, begin to recede from the sun until it gets back to the place it started from. Tlius it will go round and round the sun, de- scribing an orbit a little nearer the sun on one side than it is on the other. That is, the sun is not exactly in the center of the orbit. In another chapter we shall see that the orbit is an ellipse. 6. volvi rim t piece in vii . tend are cH arrov Bu ing c upon law ( the s the s Th force from Th great the s great fly a movi ery i piece air ni Th ugal is to than An form the n I (IKAVITATION 89 ml 311, ird nd ill I as by al- he L13 > er, (is lie he th th id. !le id. Id ie, it it of d, it ie- is of m 6. Centrifugal Force. — Let figure r»0 represent a rapidly re- volving wheel. To show the matter (clearly, we suppose the rim to be cut into eight pieces by tin; black lines, so that ea(di piece is fasteneil only by the spoke. Then at every instant, in virtue of the first law of motion, the i)arts of the rim will .tend to fly off in straight lines in the direction in which they are at the moment moving. This direction is shown by the arrow hesvds. But they are kept from thus fly- ing off by the pull of the spokes upon them. By virtue of the third law of motion each part pulls on the spoke with the same force that the spoke pulls on it. This pull is called centrifugal force because its direction is away from the center on every side. The swifter the motion, the greater the centrifugal force. If the speed is increased without limit, the force will become so great as to break the spokes. Then, each piece of the rim will fly away in the straight line in which it is at the moment moving. A fly wheel regulating the motion of heavy machin- ery is sometimes known to break from this cause, and the pieces flying away through the roof of the building and the air may cause great damage. The rotation of the earth on its axis causes a slight centrif- ugal force, which is overcome by gravity. One of its effects is to make all bodies on the earth's surface a little less heavy than they would be if the earth did not rotate. Another effect is to make the earth and planets assume the form of oblate spheroids, as we shall explain, for the earth, in the next chapter. Fig. 50. — Centrifugal force. r CHAPTER VI TIIK KAKTH 1. Figure and Magnitude of the Earth. — 1 f the earth did not rotate, the attraction of every partiele of the matter composing it upon every other particle woukl tend to bring it into the form of a splu^re. V,\\\ the rotation of the earth on its axis gener- ates a centrifugal force which partially counteracts the attrac- tion of gravity at the equator, and thus makes the earth bulge out at the equator, so as to take the form of an oblate spheroid. In this form of spheroid the equator is a circle, and the axis or diameter through the pole, called the polar axis, is shorter than that through the e^piator. The ratio in which the polar axis is less than the diameter at the equator is called the elliiiticity of the earth. Its amount is about ^^Tj ; perhaps a little greater. That is, if we repre- sent the equatorial diameter by the number 300, the polar diameter will be about 299. The elevation of the mountains and continents, as well as the depression of the ocean bottom, make the real figure of the bolid earth slightly irregular. In considering the general fig- ure of the earth, geodesists conceive of it as if the earth had been put into a turning lathe and all the mountains and conti- nents planed off to the sea level. The figure thus formed by the surface of ocean and planed-off land, is called the geoid. The figure and size of this supposed body are taken as the true figure and size of the earth, on which the continents and mountains are regarded as excrescences. That portion of the matter composing the earth which is near its surface is called the earth^s cruat. 90 TtlF EAUTH The cliameters of tlio geoid are : — • K(iuat()rial diiimeter Polar diameter .... 71)20.5 miles. 7lS!»il..') miles. Thus tho (lianu'ter of the earth is about 27 miles less through tho poles thau through thti e(iuator. Tho surface of the geoid tints dct'med is everywhere at riglit angles to the line of gravity, whi i a degree. 2. Latitude and Longitude. — liy the aHtnnimniral latitude of a jioint on the earth's surface is meant the angle which tho plumb line at that point makes with the plane of the eijuator. In figure 51 the latitude of the point /* is the angle Klil\ It is so (tailed because it is determined by astrononncal observa- tions. Pole Plane of Equator Tio. j)l. — Showing the difference between geographic and geometric latitude. The geographical latitude of a place is the same as its astro- nomical latitude, except that certain small deviations in the direction of the plumb line are allowed for. .,;ji:i--uirs*»t--. i{ 92 /! STflOJVOiVr The geocentric' latitmle is thn aii^le which the lino from the center of the earth to tlie phue makes with tin- i>liii»e of the equator. In figure al, the geo<'entri(' latitude of the point P is the angle ECJ\ These two latitudes differ by the angle of the vertical. The geocentric latitude of a place cannot he directly deter- mined, because we cannot sec the center of th(^ ciirtli nor iit we can always determine the direction of the i)lumb lino with suitable instru- ments. Hence on mai)s and for ordimiry purposes the astro- nomical or geographic latitude is always nuule use of. 3. Length of a Degree. — When an observer stands on the equa- tor, say at the \miit E, figure o'J, the jdane of his horizon is at '«ft '4^' ■** ^ d ;\o ■' :^ P/ane of f/ie Equator Fid. 62. right angles to the plane of the equator. If he travels north, we say he has traveled one degree when his horizon has changed its position by one degree on the celestial sj)here. The further north he goes, the slower his horizon will turn as he travels, and conseciuently the further he must go in order that tlie change may be one degi-ee. Hence : — The degrees of latitude are shortest at the equator, and continually grow longer as we approach the poles. ■iHtllllWHWIII m « |fiasSe,b(4tife:'vMM^!eilf^ V] vl ^/ IMAGE EVALUATION TEST TARGET (MT-3) 1.0 |28 1^ - 2.2 I.I 1^ Z 1^ 12.0 im y Photographic Sciences Corporation 1.25 1.4 II <> < 6" — = ► 23 WEST MAIN STREET WEBSTER, N.Y. U5S0 (716) 872-4503 vBSibbtMiw^ »' CIHM/ICMH Microfiche Series. CIHM/ICMH Collection de microfiches. Canadian Institute for Historical Microreproductions / Institut Canadian de microreproductions historiques % ''-:;, mi^-fr I THE EAliTll 98 They are about 68.8 miles in length at the equator and 69.3 miles at the poles. At tlie equator one degree of longitude is a little more than 69 miles. Owing to the convergence of the meridians toward the poles, it continually shortens as we ajjproach the poles, where it becomes nothing, because all the meridians there meet. One sixtieth of a degree — that is, a minute of arc — on the earth's surface is called a nautical mile, because it is the mile used by sailors. The latter usp it in preference to our land mile, which we call a statute mile, because they determine their positions by astronomical observation in degrees and minutes, and they find it easy to take one minute of arc on the earth's surface as a mile. This is nearly a mile and a sixth. In ordinary cases navigators make no distinction between the lengths of a degree of latitude at different distances from the equator. But when we want to speak of the length of a nautical mile with exactness, we commonly take it to mean a minute of longitude at the equator. 4. How the Earth is Measured. — Owing to the obstructions on the earth's surface, and the impossibility of fixing points on the ocean, we cannot measure the distance round the earth as we would measure that round a field by a tape line. The determination of the magnitude and figure of the earth must therefore be made by special methods, in which astronomical observation and measurement of distances on the earth are combined. The operation of measuring large portions of the earth's surface with great exactness is called geodesy. Geodesy requires two operations. One of these consists in determining the exact distance between two points on the earth in meters, yards, or miles. This is done by a process called triangulation. The other operation consists in finding out by astronomical observation what fraction of the distance round the earth, or how many degrees on its surface, is included be- tween two points whose distance is measured by triangulation. ~TffiiM«aiiii»iiiiw^ 94 ASTRONOMY i-f'- Triangulation. — The principle on which triangulation is effected is this: Tlie length of a line, AB, figure 53, is measured as exactly as possible on some nearly level plane. A line thus measured for geodetic purposes is called a base line. The direction of the base line, or the angle which it makes with the meridian, is deter- / mined by astronomical observation. , /\ A distant high point P is then /' \ chosen, on a hill or mountain, which /' » can be seen from both ends of the /'' \ base line. The angles PAB anjj \ PBA are measured as exactly as pos- / \ sible with a theodolite. Then, by *2,f— , \ trigonometry, the sides of the tri- angle BP and AP can be exactly computed. If there are other distant points, like Q, which are visible from the two ends of the base line, tri- angles to them are determined in the same way, and their distance from each other computed. Any of the sides AP, BP, AQ, BQ, or PQ can then be used as a new base line, and the positions of other distant points like R determined by sighting on them. By sights from these points other yet more distant points can be determined, and so on all the way across a con- tinent if necessary. The more mountainous a region is, the easier it is to make a triangulation, because longer sights from one mountain toj) to another can be taken than on a plain. Triangulation on a large scale is carried on by the United States Coast and Geodetic Survey. The latter has made meas- ures across the American continent from the Atlantic to the Pacific Coast. Long networks of triangles have also been measured in various parts of Europe, Asia, and Africa. Fig. 53. — Example of a tri- angulation. 1 1 iriii%i miiiiiiyiriiiiH THE EARTH 06 iangulation is figure 53, is level plane. i called a base angle which it dian, is deter- observation. int P is then ountain, which ;h ends of the les FAB and exactly as pos- ite. Then, by es of the tri- ;an be exactly •e other distant ,re visible from base line, tri- ;ermined in the distance from ', BF, AQ, BQ, used as a new sitions of other determined by iy sights from it more distant y across a con- I, region is, the ger sights from on a plain. by the United has made meas- Atlantic to the have also been I Africa. If the earth were a perfect sphere, the determination of its magnitude by triaugulation and astronomical observation com- bined, would be a simple problem. Suppose we should meas- ure a north and south arc 500 miles in length. By astronomical observation we find the difference of latitude between its two ends to be 7° 12'. Since 360° reach round the earth, we could state the proportion : — 7" 12' : 500 : : 360° : circumference of earth. This would give 25,000 miles as the circumference. We might also get the length of one degree by dividing 500 by 7.2 ; then the circumference, by multiplying the quotient by 360. Dividing the circumference by 3.1416 would give us the ea'„h's diameter. This shows only the principle by which the problem is solved. The actual Avork is a great deal more complicated and occupies the time of many men, year after year. The compli- cations arise not only from the ellipticity of the earth, but from the fact that wherever we go, the direction of the plumb line iy slightly changed by the attraction of hills, mountains, and continents, and also by that of matter of different densities under the earth's surface. Even when these iricgularities are allowed for, it is found that the figure of the geoid has many irregularities which have not yet been well determined. 5. How Latitude and Longitude are Determined. — The second operation of geodesy which we have described requires the determination of the exact latitude and longitude of places on the earth's surface. This determination is necessary, not only for the purposes of geodesy, but in r der that we may make exact maps of counties and states, lay Cown on them the posi- tion of cities, and find the distance from one point to another. When once the size and figure of the earth are known, it is simpler to find these positions and distances by astronomical observation than it is to measure them by triaugulation. Latitude. — To determine the latitude of a place, the astron- omer determines the exact point in the celestial sphere which mmmmmmmsmummmmsmm-.-- 96 ASTRONOMY corresponds to his zenith. He might do this in a rough way by sighting upward on a phunb line, and noticing what stars were near liis zenith, but he could not get any exact result by such a process as this. The principle of the method now com- monly employed is this:. — Imagine a telescope pointed nearly at the zenith, and a spirit level like that used by masons and architects, only much more sensitive, to be attached to it. Fancy the telescope to be fastened to a vertical axis which turns on a pivot at the bot- tom. Adjust this axis so that as Ave turn the telescope round, the level shall always read the same. Then we know that the line of sight of this telescope will describe a circle on the celestial sphere with the exact zenith in its center. The astronomer finds a pair of stars at the north and south points of this circle, of which he knows the declinations. Half the sum of these declinations is the declination of the zenith. This is equal to the latitude of the place, as will be seen by §§ 11 and 12 of Chapter I. Yet other methods may be used. We have explained that the latitude of a place is equal to the altitude of the pole above the horizon. It is also equal to the angle between the zenith of the place and the celestial equator. The astronom- ical observer can determine these angles with great precision, by specially constructed instruments, and can thus obtain his latitude without knowing the declinations of any stars. Longitude. — We have already shown that the difference of longitude corresponds to the difference in the local time at two places, 15° of longitude always corresponding to one hour's difference of time. We may also define the differ- ence of time as equal to the time which it takes noon to travel from one place to the other. To show how these principles are applied, suppose that an observer at New York telegraphs to San Francisco the exact moment at which the sun crosses his meridian. Then when the sun gets to San Francisco, an observer there telegraphs to New York the moment the svm is passing the meridian of San Francisco. The elapsed time , rough way by ng what stars jxact result by thocl now com- ±h, and a spirit ily much more jlescope to be vot at the bot- glescope round, know that the , circle on the center. The id south points ons. Half the of the zenith, irill be seen by explained that le of the pole le between the The astronom- freat precision, bhus obtain his y stars. the difference the local time ponding to one ine the differ- 1 noon to travel hese principles rork telegraphs the sun crosses n Francisco, an lonient the svm le elajised time THE EARTH between the two signals would be the time required by noon to travel from one city to the other. Multiplying the hours, minutes, and seconds by 15 would then give us the degrees, minutes, and seconds of difference of longitude between the two cities. The same result is found by each observer telegraphing the other at a given moment the exact sidereal time at his place. He finds the time by noting the time of transit of stars over his me- ridian with a transit instrument and sidereal clock. To make the determination with the greatest exactness, the clock is so arranged that its pendulum shall make a telegraphic signal which is heard at the distant station as well as recorded at the station where the clock is. Then the other observer sends a signal back from his clock, so that there are really two records of the same difference of time at the two stations. Of these differences one will be a little too great and the other a little too small in consequence of the time it takes electricity to travel from one station to the other. The mean of the two will be the correct difference of longitude in time. A longitude thus determined is called a telegraphic longitude. Difference of longitude between places can be determined by skillful observers in this way with an error of only a few hun- dredths of a second of time, or a few yards of distance -on the earth. If you should place two transit instruments three or four hundred yards east or west of each other, skillful observers would have no trouble in determining their distance apart within a few yards, by astronomical observations on the stars, combined with electric signals in the way described. 6. Density of the Earth, Gravity, etc. — By the density of the earth is meant the average specific gravity of the material composing it, or the average weight of a cubic foot of the earth's matter compared with that of a cubic foot of water. As the earth is composed of many different materials, the specific gravity of various portions of it is very different. What we want is the mean density of the whole earth. NEWCOMB'8 ASTIION, — 7 98 AflTRONOMr We can find out what materials compose the interior of the earth only by digging mines so as to get at tliem. But we cannot dig to any great depth ; only in the rarest cases can we go three or four thousand feet below the surface; hence we know nothing of the materials that compose the great bulk of the interior of the earth, liut we can determine the mean density of these materials by measuring the attraction of bodies whose mass is known. Attraction of a Sphere. — Imagine a sphere of lead a yard in diameter. Since, by the law of gravitation, every particle of matter attracts every other i)article, it follows that this sphere of lead nnist attract small bodies near it. The attraction is indeed very minute; it can be made sensible only by exceed- ingly delicate instruments. But in recent times methods have been contrived by whicih this very small force can be measured with great exactness. We have to see how, from the attrac- tion of the sphere of lead, we can determine the mean density of the earth. Fio. 64. — Attraction of two splieres of different sizes. Consider two spheres of matter, A and B, figure 54, of the same density, each attracting a particle P at its surface. Let A be twice the diameter of li. It is found by mathematical processes that each sphere attracts an external body as if the ivhole matter of the sphere were concentrated in its center. Sphere A being twice the diameter of B, has eight times its mass. Attraction varying directly as the mass, it will exert eight times the attraction of B at the same distance. 7^ THE KAHTII 99 interior of the hem. Kut we rest cases can nrface; hence the great bulk nine the mean ction of bodies lead a yard in jry particle of lat this sphere ? attraction is nly by exceed- methods have n be measured im the attrac- ! mean density it sizes. jure 54, of the surface. Let ' mathematical body as if the I its center, eight times its s, it will exert •nee. Attraction also varies inversely iis the square of the dis- tance, and 7* is twice as far from the center of A as from that of B. Hence the same matter at the center of A will attract P one fourth as much as if it were at the center of B. There being eight times as much matter in ^1, its actual attraction on P will be double that of B. That is : — - Spheres of equal density attract bodies at their surfaces with a fot which yarles directly as their diameter. Hence, if we find the attracticm of a sphere of lead, and mul- tiply it by the numter of tinu;s the diameter of the earth exceeds that of the sphere, we shall have the attraction of a ball of lead the size of the earth. Comparing this with the attraction of the earth, we shall have the ratio between the density of the earth and that of leful. It is thus found that : — Mean density of the earth = 5^ times that of water. This is much greater than the density of the materials com- posing the earth's crust, and results from the enormous force with which the interior of our globe is compressed by the weight of the matter around it. 7. Condition of the Earth's Interior. — It is a very curious fact that when a mine is sunk in the earth the temperature is found to increaae with the depth. The rate of increase is different in different places, but is commonly not far from 1° Fahr. in 50 feet. At the depth of 3000 feet the temperature would therefore, generally, be 60° above the mean at the sur- face. This temperature is so high that miners cannot live and work at the bottom of a deep mine except by having cool air pumped down to them. There is every reason to believe that the increase continues to a great depth at the same rate, so that a few miles under- ground the whole earth must be red-hot. At a still greater depth, the temperature is probably sufficient to melt all the materials of which the earth is composed. This fact, taken in i ii 9i^e^-'^^s«;s«fe«.s»^s;?i . <" ' -5 ^ '.' 100 ASTRONOMY connection with the phenomena of volcanoes, has led to the view tliat the earth is really a mass of melted matter, with a hard crust a few miles thick, on which wc live. But it is found that the earth does not yield to the attractive forces of the sun and moon as it would if it were liquid. Hence, the view now generally accepted is that the materials in the inte- rior are kept .solid by the enormons pressure to which the whole interior of the earth is subjected by the mutual gravitji- tion of its parts. How great this pressure is can be con- ceived when we reflect that every square foot a hundred miles below the surface will be pressed by the weight of a column of earth a foot square and a hundred miles high. This weight would be not far from 40,000 tons. Such a pressure would crush any substance at the earth's surface. The reason the substance inside the earth is not crushed is that it is pressed equally on all sides so that it is merely condensed into a smaller space and made solid. 8. The Atmosphere. — The atmosphere is densest at the sur- face of the earth, and grows rarer as we ascend in it. This is due to the fact that every part of it is pressed by the weight of the whole mass of air above it. At the height of three miles the air becomes so rare that most people find a difficulty in breathing at such a height, and the difficulty, of course, increases with the height. The temperature of the air continually diminishes as we ascend. Even on a summer day it is generally freezing cold at the height of a few miles. Hail is due to the freezing of rain- drops in the cooler region of the air. Air is not perfectly transparent, although it seems to be so when we look through only short distances. We all know that when we look at objects at a great distance they have a blurred, hazy aspect, due to the imperfect transparency of the air through which the light comes. The light from the heavenly bodies, as it passes through the air, is diminished before it reaches our eyes through part of it being absorbed 18 led to the atter, with a Q. But it is active forces Hence, the in the inte- o which the itual gravita- can be con- indred miles of a column This weight essure would e reason the it is pressed insed into a t at the sur- it. This is y the weight ght of three i a difficulty y, of course, lishes as we ezing cold at zing of rain- ms to be so le all know they have a .rency of the it from the ) diminished ug absorbed TIIK KAtlTll m by the air as it paasPM. The loss is smallest at tho zt'nitii, and increases near the hori/on, because the rays of liglit have to pass through a greater distance in the air when the body from which they come is near tlie liorizon. The blue rays are more absorl)ed than the red rays ; hence a larger prop«)rtion of red light than of bliu" ligiit reaches our eyes from the heavenly bodies, and the latter look more or less red when near the horizon. This is why the sun and moon have a reddish tinge when rising or setting. The air also reflects a small part of the light which passes through it ; were it not for this, the sky would be iis dark by day as by night, and we should see the stars all day. There would be no twilight, because darkness would come on as soon as the sun had set. Twilight is caused by the reflection of the sunlight from the upper part of the air after the sun has set to us. Twilight ends when the sun is about 18° below the horizon. This shows that the air reflects no sunlight at a height greater than 45 miles. This is, therefore, commonly taken as the limit of the earth's atmosphere. But the phenomena of shoot- ing stars, of which we shall speak in a subsequent chapter, show that there is really some kind of an atmosphere at a height of nearly 100 miles. But we do not certainly know what this atmosphere is. 9. The Zodiacal Light. — If we look at the western sky on a clear evening of winter or spring just after the end of twilight, we shall see a very faint, soft colunm of light extending along the region of the ecliptic, and gradually fading away as we look farther from the horizon. The same appearance may be seen in the eastern horizon before daybreak, in the summer and autumn. This appearance is called the zodiacal lUjht, because it extends along the region of the zodiac. In our latitudes we cannot see it in the evenings of summer and autumn, because then the ecliptic is too near the horizon, and the light is absorbed by the thickness of the air through which it has to 'ijBiMMiaiifji^ ^^^^i^W/}'-^ 102 ASTRONOMV puHH. Witliin the tropirs it iiui)' lie seen on every clear pvt'iiinj,'. TImtc is soiiiptliiiifj inyHtorious alwmt the 7.o<1iacal lifjht, but it in pr()l)alily i-aimod l>y iiiasseH of very teimouH matter, like fine particlos of diiHt, wiiich circulate around the sun in the whole region inside the orbit of Mars. What wh see is the Hunlight reflected from these very minute particles. Connected with the zodiacal light is a phenomenon called the ffegen- achein, a (icrman word signifying counter-ijluw. It is an extremely faint light near the z(v diac, exactly o[)- posite the direc- tion of the sun. Its faintness is such that an ordinary observer would never notice it, nor can it be seen except under the most favorable cir- cumstances. The sky must be very clear ; there must be no moon visible ; the observer must be away from the lights of a city ; the point where the phenomenon appears must not be in or near the Milky Way. For the latter reason the Gegenschein is not visible in June or July, nor in December or Jai uary. Even under the most favorable circumstances, the observer must have some practice in seeing a faint light in order to dis- tinguish it. Its cause is still involved in mystery. Fio. 65. — The zodiacal light an seen mi a clear spring evening. I every clear ical lifjht, btit H matter, like lie 8UI1 in tlio we see is the light reflected II tliese very »te particles, onnected with zodiacal light b phenomeuuii ed the f/egen- in, a (ierinan d signifying Uer-ijhtw. It is xtreniely faint t near the zo- I, exactly o[)- te the direc- of the sun. Its tness is such ; an ordinary e r V e r would Br notice it, can it be seen ipt under the t favorable cir- e must be no the lights of a iiust not be in e Gegenschein >r or Jai uary. the observer n order to dis- y- "^n CHAI'TKR Vll THK SUN 1. Partlculara about the Sun. —The sun is a globe whose diameter is nu>re than a hundred times the diameter of the earth. Hence the distance round it is more than a hundred times that round the earth. Because the volumes of globes vary as the cubes of their diameters, it fol- lows that the volume of the sun is more than a million times that of the earth. More exactly, it is 1,297,000 times that of the earth. You understand, without further explanation, that the sun looks small because of its great distance of 93,- 000,000 of miles, a distance which the swiftest train would not run in a hundred F.o. 66. -Showing how an image of tl.e «un years. may be thrown on a screen with a spyglass. The Sun's Density. Mass, and Gravity. — The density or spe- cific gravity of the matter composing the sun is less than that of the matter composing the earth. The mass of the sun is 108 104 ASTRONOMY about 333,000 times the mass of the earth, ; istead of a million times and more, as it would be if it had the same density. In consequence of its great mass, the attraction of gravita- tion at tlie surface of the sun is about 27 times the attraction of the earth on bodies at its surface. A pound of matter on the earth would weigh 27 pounds on the sun. Under an attraction so great, a man of ordinary size would weigh two or three tons, and would therefore be crushed to death by his own weight. The Photosphere. — When astronomers speak of the sun they mean the whole body of the sun, inside as well as outside. But we cannot see the inside of the sun ; we can see only its X'lG. 67. — Mottling of the stm as photographed by Janssen. surface. This visible surface is called the photosphere or light- sphere, because it is the part of tbr sun which sends us light and heat. itead of a million une density, iction of gravita- les the attraction nd of matter on sun. Under an vould weigh two d to death by his I of the sun they well as outside, can see only its I by Janssen. ttoaphere or light- ik sends us light THE SUN 105 When we look at the sun with a good telescope, we see that the photosphere presents a mottled appearance, like a plate of rice soup. The grains which produce this mottling are hun- dreds of miles in extent. They are probably caused by the matter of the photosjjhere, as it cools off, continually falling back into the still hotter interior of the sun, and its place being taken by gaseous matter arising from inside, as we shall next describe. 2. Heat of the Sun. — The sun shines in consequence of its very high temperature, as iron shines when we make it red- hot. But the temperature of the photosphere is much higher than that of red-hot iron ; higher than the burning coal in the hottest furnace. Possibly the temperature in the most power- ful electric furnace is nearly equal to that of the photosphere. The inside of the sun is far hotter than the photosphere, and there is reason to believe that it gets hotter and hotter toward the center. This heat is so intense that, subjected to it, all substances known to us would boil away like water over a fire and thus be transformed into vapor. Hence it is believed that matter cannot exist in a solid state in the sun. The vapor into which the substances composing the sun are changed by the fervent heat is so compressed by the enormous gravitation of the mass of the matter around it that it is forced into something between a gas and a liquid. The intense elastit; force of this gaseous matter causes portions of it to be continually thrown up to the sun's surface, or to the region of the photosphere. There it speedily gets colder by radiating heat into space, and portions of it perhaps condense into solids, much as a red-hot crust will form on the surface of a pot of melted iron taken out of a furnace. It would not be correct to say that the matter of the sun is burning, because things are said to burn when they unite with the oxygen of the air, thus producing light and heat. The sun is so much hotter than an ordinary tire that its substance could 106 AsTitoNoiar not burn. In other words, it differs from an immense fire in being so much hotter. If the eartli should fall into the sun, everything on its surface would be melted in an instiint, as if a small ball of wax fell into the hottest furnace. All life on the surface of the earth is sustained by the heat of the sun, which is radiated to us as heat from a tire in an open fireplace is radiated to all parts of a room. If the sun should cease to give us heat, the air and the whole surface of the earth would slowly cool off. In a few days it would be freezing cold, even at the equator. In a few weeks the whole ocean would freeze over, and the soil would freeze to such a depth as to kill every plant. Men and animals might be able to keep alive for a while by artificial heat, but they would soon starve in consequence of not having anything to eat. 3. Spots and Rotation of the Sun. — When the sun is viewed through a telescope, dark looking spots are frequently seen on his surface. These spots are not really dark, but would seem of dazzling brightness against the sky if the rest of the sun were not there. They look dark only in contrast to the intense brightness of the photosphere. The spots are of various sizes and shapes. Occasionally one appears so large as to be visible to the naked eye. Commonly, however, they can be seen only with the telescope. Sometimes a number of small ones are clustered together, forming a group of spots. The spots are extremely irregular, as may be seen from the figures which we give. The central part of a spot is the darkest. It is called the umbra, or nucleus. Around this nucleus is a border, inter- mediate in brightness between the darkness of the spot and the brilliancy of the photosphere. This border is called the penumbra. When a spot is carefully examined with a good telescope in a steady atmosphere, it is found to be striated, looking much like the bottom of a thatched roof, the separate straws bending nmense fire in I into the sun, 1 instant, as if 3(1 by the heat m a fire in an n. If the sun lole surface of ys it would be eeks the whole eeze to such a might be able lut they would ng to eat. sun is viewed uently seen on »ut would seem est of the sun tto the intense ccasionally one e. Commonly, )e. Sometimes brming a group seen from the ;t is called the I border, inter- he spot and the is called the od telescope in , looking much straws bending THE SUN toward the interior of the spot. This appearance is shown in figures 59 and fiO. Astronomers are not agreed as to the nature or cause of these spots on the sun, though they have been studied for nearly three centuries. Fig. 68. —The sun, with its spots and prominences, the latter being shown by the spectroscope. The Sun'B Rotation. — When the spots are carefully watched they are seen to change their position from day to day by moving slowly across the photosphere from east toward west. In this way it is found that lue sun, like the earth, rotates on an axis. The time of rotation is about 26 days. The points where the sun's axis of rotation intersect its surface are called the poles of the sun. 108 ASTRONOMY A belt around the sun, 90° from each pole, is called the sun's equator. It is found by watching the spots that they make a revolution in a little less time when on the equator than when at a distance from it. Fig. 59. —A typical solar spot, after Langley, showing the forms which such a spot often presents. Periodicity of the Spots. — The spots are much more numer- ous in some years than in others. In years when spots are scarce, there will sometimes be none visible for several days, and dt other times only one or two will be seen. In years when spots are numerous, quite a number of them, and some- times very large ones, will be seen nearly all the time. It is found by the records of the years when spots were numerous I, is called the ipots that they on the equator g the forms which ch more mimer- when spots are )r several days, seen. In years ;hem, and some- the time. It is were numerous THE SUN 109 and when they were scarce, that there is a period of about eleven years, during one half of which there are few spots, while during the other half there are many. Fio. 00. — A solar spot, after SeccUi. 4. Corona and Prominences. — When the sun is totally eclipsed by the moon, very curious and beautiful phenomena are seen. One of these consist of red. patches or cloudlike forms around the body of the moo'i. These objects are called prominences or protuberances, and are found to belong to the sun. They cannot be seen with a telescope when there is no eclipse, because of the intense light of the sun dazzling our eyes. This is why we see them only when the light of the sun is cut off by the moon. But w;th a spectroscope they are visible on almost any clear day. This instrument shows that they are composed of masses of gas, mostly hydrogen, which are from time to time shot up from the photosphere. Sometimes they have the form of immense flames blazing up suddenly to a height of many thousand miles with a velocity of more than 100 miles a second. In such flames 110 ASTRONOMY everything on the surface of the earth wouhl be destroyed iu an instant. Another object surrounding the sun is a beautiful effulgence called the sun's corona. It cannot be seen, even with a spec- troscope, except during a total eclipse of the sun. It will, therefore, be described in the chapter on eclipses. 5. Source and Period of the Sun's Heat. — The sun, as we have alreiuly explained, is merely an extremely hot body radiating heat to us, as a white-hot globe of iron would radiate heat if hung in the middle of a room. One of the most interesting and important cpiestions iu astronomy is why the sun does not cool off and thus gradually cease to give us light and heat, as the iron globe would do. If, as is generally supposed, the earth and sun are many millions of years old, then the sun must have been radiating heat during this immense period. We must, therefore, account not only for the heat the sun now gives us, but for the heat which it has radiated to the earth in past ages. One explanation that has been proposed is that of meteors. It is now believed that there are great numbers of small bodies moving round the sun in its immediate neighbor- hood, and it is quite likely that such bodies might, from time to time, fall into the sun. Each body thus falling would gen- erate heat. But this view is now generally given up, because it seems hardly possible that meteors in sufficient number to generate the sun's heat could be falling into the sun. The view now commonly held is that the heat of the sun is kept up by the constant contraction of its mass through the gravitation of its particles toward the center. The theory of energy teaches us that heat is produced when a body falls toward a center without having its velocity increased. For example, the temperature of the water of Niagara Falls must be about one-quarter of a degree higher after it strikes the bottom than it is before it goes over the falls. As the sun cools off it must grow smaller, so that its outer portions fall toward its center. In this falling so much heat is acquired I 1)6 destroyed tiful effulgence II with a spec- sun. It will, pses. sun, as we have body radiating radiate heat if lost interesting lie sun does not ht and heat, as supposed, the I, then the sun nmense period. !at the sun now to the earth in losed is that of ■eat numbers of idiate neighbor- ight, from time ling would gen- ven up, because •ient number to ? sun. it of the sun is iss thro\igh the The theory of en a body falls increased. For gara Falls must r it strikes the Is. As the sun er portions fall leat is acquired THE SUN -m that, if the sun remains gaseous, it Avill continually grow hotter. It may, therefore, continue to radiate the same amount of heat every year, so long as it does not become a solid. If tliis view be correct, a time must ((ome when the sun can contract no more. Then a solid (Must will form over its sur- face, this crust will gradually cool off by the heat which it radiates, and the sun will gradually grow dark and cold. But the period necessary for this is nuiny millions of years, so we need not trouble ourselves about it. A question of more immediate concern is whether the quan- tity of heat which the sun gives us is subject to variations. We know that at one time, probably not many thousand years ago, the whole of New England and the northern states was buried all the year round in snow and ice. The time when this was the case is called the UUuiul Epoch. Tt is possible that during the glacial epoch the sun gave less heat than it does now. If so, it may again give less heat at some future time. There is, however, no evidence of any change in the tempera- ture of the earth since the invention of the thermometer. The meteorological observations made two or three hundred years ago give about the same mean temperature that they do in our time. But the earlier observations of this kind are, perhaps, not very reliable, so that their evidence cannot be conclusive against a very small change of one or two degrees. But the observa- tions made at the Greenwich Observatory from 1840 to 1890 show that there was no perceptible change during those fifty years. This disproves a view which has sometimes been main- tained, that the variations in the solar spots produce corre- sponding variations in the temperature of the earth. It also leads us to believe that there will be no change in the amount of the sun's heat for many years to come. nil m i nMpff CHAPTER VIII THE MOON AND ECLIPSES 1. Distance, Size, and Aspect of the Moon. — The moon is a globe like the earth. It looks flat to the eye because we can- not see its roundness without a telescope. In a telescope we can see it to be round like a glol)e. Distance. — The moon is much nearer to us than any other of the heavenly bodies. Its average distance is a little less o Fio. 61. —Showing the relative size of the eartli and the moon. The diameter of the moon is a little more than one fourth that of the eartli. than 240,000 miles. The diameter of the earth being nearly 8000 miles, the distance of the moon is about 30 times the diameter of the earth, and therefore GO times its radius. A railway train rinining 60 miles an hour would reach the moon in five or six months. An idea of the relation between the 112 ■ ii m, j. > -" ' "W -The moon is a because we can- i a telescope we than any other is a little less id the moon. The fourth that of the •th being nearly iut 30 times the s its radius. A reach the moon ;ion between the THE MOON AND KCI.IPfiEH m distances of tlio sun and moon may ho giined by romcmbering that the sun is nearly 4(M) times us I'ar as liu^ moon. Size and Density. — The diamctci- oi the moon is about 2100 miles. Tliis is a little more than \ of the diameter of the earth. In bulk it is al)out ,'o that of the earth. Hut the materials which ccmipose it are not so delists as tliose of the earth. They liave about i^ (jr 4 times the density of water. Thus the mass of the moon is about -^ that of the earth. Moon a— 240,000 mHet Earth 'O Fkj. 02. — Showing the nlze and diHtance of the earth and moon nearly in their true jtroportionH. 'I'lieir diKtance a] art is about 3U diam- eters of the earth and more tlian 110 that of tl e moon. The Moon's Surface. — If we look carefully at the moon near the time of fust quarter we shall see little irregularities near the left-hand edge of the bright surfa(!P. Throui,'li a telescope this edge looks very jagged. This is because the svirface of the moon has mountains and valleys upon it. Sixty or seventy of these mountains are more than a mile high, and a few are four miles or upwaid. They are therefore nearly as high as the highest mountains on the earth. But the shape of the mountains on the moon is very differ- ent from that of our mountains (see figures 6tl and G4). Their tops are frequently rounded like the rim of a saucer or shal- low plate, the inside being hollow, and black like the bottom of the plate. In the center of this fiat region there is very frequently a little sharp conical peak. These appearances make it probable that long ages ago these mountains were volcanoes. There is, in fact, a remarkable resemblance between these lunar hollows and the craters of volcanoes like Vesuvius. A hiuulred years ago it was thought that there Avas a volcano in eruption on the moon ; but we now know that this was a mistake. What was seen was only a spot of iniusual brightness. NEWCOMU'S ASTKON. — 8 •iivtm^issn 114 ASTRONOMY Some parts of the moon are much darker than the general surface. It is said that Galileo and others who tirst used a Fio. 03. — The moon, photographed by Dr. Henry Draper. telescope supposed these dark portions to be seas, because they looked smoother than the others. Thus Milton, in allusion to J w iii m i|iiiii»iiii i > «M ii i i » i ij ban th<< general rhu tii-st used a ../■-^yv- 'M lenry Draper. seas, because they ;oD, in allusion to THE MOON AND EC LI PS KS 'mitiiimm 116 Galileo, who was a native of Tuscany, says of Satan's shield that it " Hung on lilnKlioulderH like the mnnn whoso orb Through optic glass the Tuscan artist views At evening, from the top of FeHol(^ , . Or in Valdiirno, to dewry new lands, * Rivers or mountains in lier spotty globe." Fi«. 64. — Telescopic view of a region on the moon. But when more powerful telescopes were made, these sup- posed seas were found to have mountains and valleys like the rest of the surface. The darkness was merely the result of a difference of shade in the matter forming different parts of the moon. Absence of Air and Water. — It is now certain that the moon has neither water nor air in any quantity sufficient for us to detect its existence. Consequently there is no weather on the 116 Asi iioyoMV moon an«l, h<» far as \vh hiiv«f y«'t disoovorfd, nothing over happens tliorc, <'xcoj>t, that the siirfare nets wariii when tho sua sliiiH'H oil it aiul cold \vh»'u it, (htfs not, 2. The Moon's Revolution. —We inuHt think of th«' earth anil moon as two companions, revolving round the sun toKi'tlicr, wliiit), at the same time, tiiey revolve roimd eiu^h other. Tiie exaet truth is that they hoth revolve round their common center of ^nivity, while tiie earth k'<)«« round the sun in the orbit we have described. Let /•; !>«• the center of the earth, M that of the iiioon, and C their common center of gravity. E4- e*' Fio. fl5. —As the moon moves irom M to N, the center of the eerth describes tho small arc Ee In the opposite direction, both moving round the common center of gravity at C Then EM will be the radius vector of the moon, which means the line from the center of the earth to that of the moon. We must now conceive that this radius vector turns round on C as on a pivot, so that, while the moon is moving from M to N, the earth nn)ves from JJ to c Thus the center of the earth describes the small dotted circle, wliile at the same time the tuoon describes the larger circle MN, of whi(di only an arc is shown in the diagram. This condnned motion arises from the fact that the moon attracts the eartli as much as the earth docs the moon. , uothiiiK over mill wht!ii tho : of tlu' t'iirtli 10 sun t.DHi'tln'r, Ai otluT. Tl.<^ tht'ir ('(iiiinion llii' sun ill tlm !• of tlie earth, tcr of gravity. niter of the eprth ion, both inovint; ) moon, which to that of the us vector turns (lOon is moving rhiis the center e, while at tlie i MX, of whi(^li >mbinecl motion B eartli as much 77/ A' MOO.y A.\n ECLIl'SlSS 117 Tho j'ommon renter of pruvity C is re.'illy inside the earth, altiMit, one ttdirtli til' the way f i '>in its cin umferfnce to its center. Its tlistanee linm the eiirtii's center is llurefnre so Hinall that we coiiniiniilv sjieaU 'if the niiinti as revolviiii^c roiiiii! tlie earth, without reterciice to the niotioii of the earth itself round <'. Sidereal and Synodic Revolution. — liCt AUC he an arc of the eartli's orhit louiid the sun. l-et us start with tiie earth at .1, and aroiinil it thu orhit of the iiioon, 5^,, Qj with the moon at 7"; 3/, between the earth ,' ', and the sun. In this / \ position the iiionii is / said to Ihmii (''(/(//ofc- / lion with tiie sun. While the earth is movin;^' from .1 to J{, tli(! moon makes one revolu- tion around it, and reaches the point X such that the line BN is parallel to tho lino J3A. These linos being parallel, the moon has made a com pie* ^ revolu- tion, ai i iS seen in ■^"- the same real dire(^- ''k'' *"'• — Siiowing the (liffrrenco between tlio .. . ,r 1 sideri^al and synodic i)cri(«ls of tlie moon, tion at A as she was ■' at M. This revolution of tho moon around the earth is called a milcri'dl ri'roliitioii because, when it is completed, the moon has returned to the same apparent point a'liong tho stars. It takes place in about 27 d. 8 h. Although the moon has actually juade one revolution round i^r i m n B »'wmwffliti' * ri>t> i j t i t^ ^i AiM ^ - ^ 118 ASTRONOMY the earth Avhen it comes to N, yet slie will not be in conjunction with the sun at ^V, but will have to move through an arc NP to catch np to where the sun appears to be. This takes it more than two clays lUin-e. Thus the time between the moon's con- junctions with the sun is on the average 29 d. 13 h. This period between two conjunctions with the sun is called a synodic revo- lution. 3. The Moon's Phaseo and Rotation. — The moon is an opaque body which shines only by reflecting the light of the sun. That hemisphere which is toward the sun is always brightly illumi- nated by the sun's rays ; the other is in darkness so that we do not plainly see it. When the moon is in conjunction with the sun, her dark side is turned toward us, and we cannot see her at all. The almanacs then call it neiv moon, though we cannot see the moon. Two or three days later she has moved away from the sun so far that a small portion of her illuminated hemisphere is visible. The form which .she then shows, and with which we are so familiar, is called a crescent, because the moon is then increasing. At this time, if we look carefully, we shall see the entire round disk of the moon, the dark part having a very faint gray illumination. This is caused by the light from the earth being reflected upon the moon. The earth being several times larger shines much more brightly upon the moon than the naoon does upon the earth. The appearance is familiarly called " the old moon in the new moon's arms." In three or four days more the moon has got to the posi- tion of first quarter. One half the illuminated hemisphere is now visible to us and her visible disk has the form of a semi- circle. During the next few days we see more and more of the illuminated hemisphere, and the moon is said to be (jibbous. When the jnoon gets opposite the sun she presents the same iWs>w « i i Mi i i miiiti'i'i>f B i i «¥ri»«» i i>^'^' ' " t be in conjunction ■oiigli an arc NPto This takes it more en tlie moon's con- , 13 h. This period lied a synodic revo- ! moon IS an opaque it of the sun. That lys brightly illumi- irkness so that we the sun, her dark e her at all. The we cannot see the away from the sun ited hemisphere is and with which we ! the moon is then hall see the entire ig a very faint gray om the earth being leveral times larger ;han the moon does rly called " the old as got to the posi- ated hemisphere is bhe form of a semi- ) and more of the id to be (jihhouii. i presents the same mi^ia&mm^^^minmmM THE MOON AND ECLIPSES 119 face to the earth and to the sun. We see lier whole illumi- nated hemisphere and call xtfull moon. During the second half of the revolution the phases recur in the reverse order, and a week after full moon she has got round through another quarter of her journey. We then say that she is in her third quarter. We can then again see one half the illuminated hemisphere. -^ Direction ofSurK Fig. 67. — The moon's phases. In 7 or 8 days more she is again in conjunction with the sun and we lose sight of her. The age of the moon is the time elapsed since new moon. When we first see her as a thin crescent after sunset, she is commonly 2 or 3 days old. At first quarter she is 7 or 8; at full moon about 15 ; at last quarter 22 days old. The best time to see the moon through a telescope is not when she is full, as people commonly suppose, but when she is between 4 and 8 days old, ir t> 1 i VL i i » W l i'1W'- ' 'l i ^'*'J ''l' >!P**!|Sf'iW j j i. 120 ASTRONOMY Form of the Moon's Orbit. — We shall expluin in unother chapter that the planets move roinul the sun in ellipses having very nearly tlie form of a cirele. If the earth and moon were attracted by no body hut the sun, they would move around each other in ellijjses, as the planets move round the sun. But the sun attracts both, and thus prevents the orbit being an exact ellipse, and also makes it change its form slightly, but continually. The result is that the orbit is much like a mov- ing ellipse. Tlie point of this ellipse where the moon comes nearest the earth is called the pemjce ; that where she is far- thest is called the aprnjee. The positions of the apogee and perigee are continually changing, and they make a complete revolution round the earth in about nine years. The Moon's Effect on the Weather. — It used to be supposed that the moon had souie effect on the weather, and that changes of weather were more likely to occur at new or full moon, or at one of the (luarters. It is now known that this is not the case. The most careful observations show that the moon has no effect at all on the weather. Rotation of the Moon. — As the moon revolves around the earth, she always presents nearly the same face toward us. This shows that she turns on her axis in the same time that she revolves around the earth. It should be noticed that if the moon did not turn on her axis at all, then as she went r jund the earth we should see her from various directions, and so should get a view of all parts of her surface. As she always turns the sauie face toward us, it follows that -we can never see the other side of the moon. But there are small changes in the speed with which she performs her revolu- tion round the earth, while her rotation on her axis is uniform. Hence we can sometimes see a little farther on one side or the other of her body. Such an appearance is called lihration. This word means a balancing, and is applied because, to our eyes, the moon seems to have a slight swing back and forth on her axis, as a balance has when the weights in the pans are equal. l| 8HI»yj I gff l tillB f at : uin in another ellipses having and moon were tl move around I the sun. But orbit being an m slightly, but uch like a mov- he moon comes rhere she is far- are continually tion round the I to be supposed nd that changes jr full moon, or this is not the at the moon has Ives around the face toward us. same time that I noticed that if len as she went s directions, and 3, it follows that Hut there are orms her revohi- axis is uniform. 1 one side or the called libration. because, to our [ick and forth on ic pans are equal. THE MOON AND ECLIPSES 121 4, The Tides. — In consequence of its gravitation, the earth attracts the moon and thus keeps her iu her orbit. If it were not for this attraction the moon would gradually leave the earth altogether, as has already been explained. Hut, by the third law of motion, the moon attracts the earth fia well as the earth the moon. Hence the eartii is being continually drawn toward the moon. Hut it can never move far in con- sequence of this drawing, because of the constantly changing direction in which the moon acts : at one time of the month the attraction is in one direction, and at the opposite time in the other direction. S»i>- ->-Moon Fio. 68. — Showlii'' how the moon causes the tides. We have already said that gravitation is less the greater the distance. Hence the portion of the earth near the moon is attracted more strongly than the portion most distant from it. The result is that the attraction of the moon tends to draw the earth out into an ellipsoidal form. The earth itself, however, being a solid body, cannot be stretched out by this increased attraction. But the water of the ocean, being movable, is stretched out a little. Thus a wave, very broad, but only a few feet deep, is made in the ocean, and follows the moon around every day. There is also a similar wave on the oppo- site side of the earth. This is because at that point the water is attracted less than the average of the solid earth, so that the moon pulls the earth away from the water. Thus there are two waves a day moving round the earth. These waves are called tklal waven. The rise and fall of the water of the ocean which they produce are called tides. They ■ i Htl i ri ( H lK l Mijif iinfilw iiw »n!« 122 ASTRONOMY Strike our coast and make the water rise for G hours, until the top of the wave reaches us. It is then called high tide. During the next 6 hours the tide recedes. At its lowest it is called loic tide. .Six hours later there is another high tide, and so on. Thus there is a regidar rise and fall of the water twice every day, with which all who live on the seacoast are familiar. In conseciuence of the continual motion of the moon on the celestial sphere, from west toward esist, she passes the meridian on the average about 50 minutes later every day than she did the day before. Hence, the tides arrive later every day by this average amount. The amount of the rise and fall is very different in different regions. Gat in the ocean it is generally less than on the coast, commonly only 2 or 3 feet. As the tidal wave ap- proaches a coast the resistance of the latter causes the water to pile itself up against the coast, and thus rise to a height of 6, 10, or 20 feet, or, in rare cases, much more. Owing to the islands and continents, the tidal wave is not merely one wave going along uniformly, but sometimes there are several waves in different parts of the same ocean. When two of these waves happen to meet, they make one big wave. If there happens to be a deep, wide-mouthed bay where they meet, the water may rise to a very great height in consequence of the force with which it enters the bay. This is the case in the Bay of Fundy, on the coast of Nova Scotia and New Brunswick. At the head of this bay the tides rise 70 or 80 feet. The effect is here most extraordinary. The Basin of Minas is quite a large lake, at high tide being 12 miles across and 40 miles long. But at low tide it is almost empty. Spring and Neap Tides. — The attraction of the sun on the earth produces a tide as well as that of the moon. But this tide is smaller than that of the moon. At the times of new and full moon, the sim and moon unite their attraction to pro- duce tides. Consequently the tides are higher at those times than at others. These are called srpriurf tides. At first and last quarter the sun and moon pull against each () liovirs, until lied high tide. its lowest it is high tide, and lie water twice it are fainiliar. moon on the IS the meridian ' than she did every day by nt in different i than on the idal wave aj)- Lises the water to a height of il wave is not metimes there ocean. When one big wave, ly where they in consequence is the case in atia and New rise 70 or 80 The Basin of 12 miles across empty. he sun on the oon. But this times of new raction to pro- at those times 11 against each TH THE MOON AND KCLIPSKS other on the tides. Thus the sun diniinishcs the effect of the moon, and tlie tides arc not so high. They are then called neap tides. Another effect of this combined action of the sun and moon is that the actual intervals between the high tides on su«'ces- sive days sometimes vary considerably from the average inter- val. Sometimes high or low tide occurs at nearly tlu; same time on two successive days. At other times the diiference of time may be more than jin hour. 5. Eclipses of the Moon. — All opa(iue bodies cast shadows when the sun shines on them. Hence the moon and the earth cast shadows. Night is caused by our being in the shadow of the earth when our hemisjihere is turned away from the sun. Fiu. 69. Let S, figure 69, be the sun, E the earth, and M the moon. Draw the lines ABH and CDII meeting at H, and touching the sun and earth. You will then see that between these two lines, in the region between the earth and //, the light of the sun will be cut off. This region is that of the shadow of the earth. The shadow has the shape of a cone, with its point at //. This is called the shadow cone. Outside the shadow is a region PI> in which the light of the sun is partly but not wholly cut oif. An observer in this region, if he could fly up to a great distance from the earth, would see the latter hide a greater or less part of the sun, according to his nearness to the surface of the shadow cone. The region PP in which the sunlight is partly, but not wholly, cut off, is called the penumbra. 124 ASTRONOJUr Wlien the moon is entirely in the shadow of the earth, the direct light of the sun can no longer rejush hor, so she looks dark. We then say that there is an edqise of the moon. That is, an eclipse of the moon is caused by the moon passing through the shadow of the earth. It is very interesting to watch such .an eclipse. As the moon enters the shadow we see a small part of one edge of her disk grow dark and finally disappear. The darkness si)reads over the disk little by little until it covers tlie whole surface of the moon. During the first part of the eclipse we cannot see the eclipsed i)ortion of the moon because of the dazzling effect of Fio. 70. — Refraction of the light of the sun into tlie eartli's shadow. the bright part. But when the bright part has nearly or quite disappeared, we see the whole disk shining with a dim, reddish light. This is because the light of the sun is refracted by the earth's atmosphere as we have explained in C.'liapter IV, and shown by the above figure. Hence the rays of the sun, which pass very near the surface of the earth, are so refracted by the air that they enter the shadow, and keep it from being perfectly dark. To an observer on the moon, looking at the earth during an eclipse, the sun would be entirely hidden by the earth, but the latter would be surrounded by a thin ring of this refracted light, of a reddish tint. This tint is due to the absorption of It » f«»( > i :TVi < f the earth, the \vr, so she looks (he moon. That passing through le. As the moon (Ige of her disk Hs siJi-eads over e surface of the cannot see the izzling effect of earth's shadow. nearly or quite 1 a dim, reddish refracted by the '-liapter IV, and the sun, which refracted by the I being perfectly the earth during y the earth, but )f this refracted lie absorption of TUE MOON AND ECLIPSES m the blue rays by the atmosphere, and hence arises from the same cause that makes the sun look red when on the horizon. Sometimes only a part of the moon dips into the shadow. The eclipse is then called a partial vcUpHfl of the moon. When the moon is altogether immersed in the earth's shadow, the eclipse is saitl to be total. 6. The Moon's Orbit and Nodes. — You may now ask why it is that then! is not an eclipse of the moon at every full moon, because the moon is then always oi)posite the sun. The reason is that the shadow of the earth is always in the ecliptic, while the orbit of the moon aroun