300

ASTRONOMY.

will be no night for the space of six months. Here likewise of the Iparallels between the equator and the north pole, the illuminatedportions are much less than the portions which remain in darkness,the contrary of which happened in [lie former position ; so like-wise the Sun at mid-day will appear vertical to all the inhabitantsthat live in the tropic MX ; so that it will appear to have descendedtowards the south from the parallel TC, to the parallel MX,through the arc CQX, which is forty-seven degrees. This willbe the season called winter. Lastly, as the Earth journeys onfrom Cancer through SC and Tljj, to the Sun appears to passthrough jar and X to t y > ., and the northern climes begin to return,and receive more directly the enlivening beams of (he Sun , whosemeridian height does now each day' increase; the days nowlengthen, and the tedious nights contract their respective arcs ;and every thing conspires to advance the delightful season of thespring, with the equality of days and nights, as was shewn whenthe Earth Wits in Libra, from which point we began to trace its motion.

(13S.) All these phamomena may be thus represented: Take asmall globe that has the equinoctial and parallels drawn on it; andplacing a candle upon a table move the globe round the candlein a circle parallel to the table, so that the axis of the equator maybe oblique to that circle, and he kept always in a parallel positionwhilst it moves about. The candle will illuminate the globe as itis curried round, just as the Sun does the Earth in its orbit ; andthe poles anti parallels will be the same way affected with light anddarkness as the globe. The orlnt of the Earth being elliptical,ami the Sim constantly keeping in its lower focus, which is1,604,744 miles from the middle point of the longer axis, theEarth approaches twice as near, or 3,‘209,488 miles nearer the Sunat one time ef the year than at another; for the Sun appearingunder a larger angle in our winter than summer, proves that theEarth is nearer the Sun in Winter . But here this question natu-rally arises, Why have we not the hottest weather when the Earth is nearest tlie Sun ? In answer itmust.be observed, that the eccen-tricity of the Earth ’s orbit, or 1,604,744 miles, bears no greaterproportion to the Earth’s mean distance from the Sun , Iliandoes to 1000; and therefore this small difference of distance can-not occasion any great differenee of heat or cold. But the prin-cipal cause of this difference is, that in w inter the Sun ’s rays fallso obliquely upon us, that any given number of them is spreadover a much greater portion of the Earth ’s surface where we live ;and each point must then have fewer rays than in summer. Therecomes also a greater degree of cold in the long winter nights thanthere can return of heat in so short days; and on both these ac-counts the cold must increase. In summer the rays fall more per-pendicularly upon us; come with greater force, and in greaternumbers, on the same place; and by their long continuance, amuch greater degree of heat is imparted by day, than can fly offby night. Besides, those parts which are once heated, retain theheat tor some time; which, with the additional heat daily im-parted, makes it continue to increase, though the Sun declines to-wards the south. This is the reason why July is hotter than June,and often, in our cold climate, August hotter than both, althoughthe Sun lias withdraw n from the summer tropic ; as we find it isgenerally hotter at 3 in the afternoon, when the Sun has gonetowards the west, than at neon when he is in the meridian. Thoseplaces too which have been well cooled require time to be heatedagain ; for the Sun ’s rays do not heat even the surface of any body,till they have been some time upon it. Ileucc we lind Januaryfor the most part colder than December, although the Sun haswithdrawn from the winter tropic, and begins to dart his beamsmore perpendicularly upon us. An iron bar is not heated imme-diately upon being put into the fire, nor grows cold till some timeafter it has been taken out.

(139.) The Sun completes what is called a tropical year, whenhe arrives at the same equinoctial or solstitial point. 1 his he doesin 365 da vs, :> hours, 43 minutes, 4S seconds. When lie arrivesat the same fixed star again, as seen from the Earth , he completesthe sidete d year, which contains 365 days, 6 hours, 9 minutes,11 1 seconds. The sidereal year is therefore'30 minutes 231 secondslonger than the solar or tropical year, and 9 minutes 114 secondslonger than the Julian or the civil year, which we slate at 365 days(i hours; so that the civil year is almost a mean between the side-real and tropical. As the Sun describes the whole ecliptic, or360°, hi a tropical year, he moves 59' 8",330064 of a degreeevery day at a mean rate ; aud consequently 50"-} ol a degree in

30 minutes 231 seconds of time: therefore he will arrive at (hesame equinox or solstice, when he is 50"} of a degree short of thesame star or lixed point in the heavens, from which he set out theyear before. So that, with respect to the fixed stars, the Sun andequinoctial points fall back (as it were) 30° in 2145 T 7 3 years, whichwill make the stars appear to have gone 30° forward with respectto the signs of the ecliptic in that time ; for the same signs alwayskeep in the same points of the ecliptic, w ithout regard to the con-stellations. Thus, the fixed stars wine’.) were 2143-/ 3 years hack inthe zodiacal sign <Y> are now in y , those that were in B now in n, &e.

(140.) Since in the space of one year the stars advance 50"}, theywill in 25748} years perform an entire revolution, which is calh dannus magnus, the great year; after which, the stars being re-instated in their proper places, the ancients imagined there wouldbe a total renovation of all tilings. The cause of this motion of thestars was unknown to all the astronomers before Sir Isaac N ewton’stime, none of them being able to guess from whence it could pro-ceed ; blit this sublime geometrician investigated its cause, and de-monstrated that it results from the attraction of the Sun and Moonon the accumulated matter at the equator, by which the equinoc-tial points are moved backward, and that this produces the ap-parent motion of the stars in consequentia. This motion is calledthe precession of the equinoxes. LetACUD, (Fig. 10, PlateXXI11.) represent a ring composed of particles surrounding theEarth at the equator, and carried round with it in the diurnal mo-tion, and let the shaded part be in the plane of the ecliptic, thenA and B are the equinoctial points ; let the Bun he at T, then, aswas shewn respecting the lunar irregularities, each particle of thering between A and C will be accelerated by the disturbing forcemore or le=s according to its position. In the same maimer everyparticle between C anti B is retarded ; a like acceleration and re-tardation will affect the quadrants BD, DA; therefore, since thiswill happen in every situation of the Bun in respect of the equi-noctial points A and B, such a ring would, on the whole, neitherbe accelerated nor retarded. If we suppose the Sun , at T, in theequinox, then the disturbing force will be in the plane of the ring;hence its inclination, and the position of the equinoxes or nodes ofthe ring, will remain unaltered ; but if the bun be in any otherposition, then, as proved when treating on the motions of thesecondary planets, every particle of the ring, in consequence ofthe disturbing force, will on the whole be brought to the plane ofthe ecliptic, before it arrives at the point A or B, that is, the nodesof the ring will on the whole move backward ; (lie greatest effectwill be produced when the Sun is in the tropic as at T, for thenthe direction of the solar force makes the greatest angle with theplane of the ring. Because the Moon ’s path is also inclined tothe equator, the above-mentioned ring will be acted and its inter-sections moved backward in a similar manner. If we supposeseveral other rings parallel to the equator at a small distance fromit, on botli sides, then it is evident that each of them would beacted upon by the disturbing force nearly in the same way.

(141.) Xow the additional matter accumulated at the equator bythe centrifugal force arising from the Earth ’s revolution on its axis,may be conceived to consist of such parallel rings, and conse-quently their intersection with the ecliptic will move backward,and since these rings are attached to the Earth the whole globewill he acted upon, and while the equinoctial points move in au-tecedentia, the Earth ’s axis will be made to deviate a little from itsparallelism, and describe a conical motion round the axis of theecliptic, which may be illustrated by Eig. 14. Let Cl II be thecircle in which the plane of the ecliptic intersects the Earth , and letit be projected onthe plane of Ihe circle GO IJ; draw IT, the axis ofthe ecliptic, to meet the heavens in B ; about the pole P, at 23%}from it, describe the parallel circle A C D B, let I K be the equa-tor, then will its axis meet the heavens at B, the pole of the world:when the equinoctional point E lias moved backward to .y, the po-sition of the equator will be msn, and its axis E E will meet theheavens at IC, which will then be the pole, of the world ; when theequinoctial point comes to G, G LII will-be the po-ition of the*equator, aud c the pole of the world, and thus by the motion ofthe equinoctial point from F to G, the axis of the Earth product dto the heat ens has described the quadrant B E C, and thus it willcontinue till it has completed an entire revolution in the circleB C B A, ariving again at B in (he space of 25748} years, andthus the axis of the Earth will describe the surface of a cone whosevertex is the centre of the Earth .

(142.) The