PRECESSION OF THE EQUINOXES.] 



ASTRONOMY. 



931 



angular motion at the plane of the equator, in a contrary 

 direction to the earth's rotation. 



PRECESSION OF THE EQUINOXES. This motion, though 

 slow, being always in the same direction, and therefore 

 continually accumulating, was, as we have seen, early 

 remarked, and was one of the celestial appearances that 

 suggested the idea of the Annus Mar/nits, or great astro- 

 nomical period, by which so many days and years are 

 circumscribed. As it affects the whole heavens, and as 

 the changes produced are spread over a vast extent of 

 years, it has proved a valuable guide amid the darkness 

 of antiquity, and has enabled the astronomer to steer his 

 course with tolerable certainty, and here and there to 

 discover a truth in the midst of the traditions and fables 

 of the heroic ages. The accurate analysis of the compli- 

 cated effect thus produced, was a work that surpassed the 

 power either of geometry or mechanics at the time when 

 Newton wrote. His investigation accordingly was 

 founded on the assumption that, though not destitute of 

 probability, it could not be shown to be perfectly con- 

 formable to truth ; and it even involved a mechanical 

 principle, which was taken up without due consideration. 

 The first who solved this difficult problem was D'Alem- 

 bert. He employed the principle of the equilibrium 

 among the forces destroyed, when any change of motion 

 is produced ; and by means of the equation this propor- 

 tion furnished, this great mathematician was enabled to 

 proceed with a solution that has never been surpassed 

 for accuracy or depth of reasoning. Laplace proceeded 

 on a more general principle, and with broader conclusions. 

 He has shown that the phenomena of the precession and 

 nutation must be the same ; that, whatever may be 

 the irregularity of the depth or currents of the sea, 

 nothing can effect an alteration in the earth's rotation on 

 it-t axi.s. 



Wherever the sun, in his apparent annual course, 

 crosses the equinoctial in spring, there is the vernal 

 equinox ; and wherever he crosses it in autumn, there 

 is the autumnal equinox. The two points of intersection 

 are not, however, the same year after year, but are sub- 

 ject to a slow annual displacement westward, so that the 

 sun does not cross the equinoctial, spring and autumn, 

 exactly in the same points, but every year a little behind 

 those of the preceding year. This etfect is termed the 

 precession of the equinoxes, because it accelerates their 

 time, though it is really their retrocession. It amounts 

 to about 50 y in a year, or to 1 in 70J years, to 30, or 

 a whole sign, in 2,140 years ; so that in somewhat more 

 than 25,000 years, the equinoctial points will complete a 

 revolution westward along the ecliptic, and return to 

 the same position. 



One obvious effect of the falling back of the equinoctial 

 points, is a progressive increase of longitude in all the 

 heavenly bodies. Hence those stars which, in the time 

 of Hipparchus, were in conjunction with the sun when 

 he was in the equinox, are now 30, or a whole sign, 

 eastward of it ; and the constellations and signs of the 

 zodiac no longer correspond, as may be seen by reference 

 to a celestial globe. 



The annual precession of the equinoxes, apparently a 



Fig. 32. 



its two hemispheres are equally exposed to the sun. 

 The cause of this remained unknown till the age of 

 Newton, who showed that it resulted from the form of 

 the earth, and the unequal attraction of the sun and 

 moon on the unequal masses of matter at the equator 

 and the poles, producing a slow reeling motion of the 

 earth's axis from east to west, and the recession west- 

 ward of the equinoctial points. Fig. 32 illustrates its 

 etfect. 



Let S be the sun, T K the pole of the ecliptic, T P 

 the pole of the earth revolving around T K, in the direc- 

 tion of the arrow. In the space of one year, the revo- 

 lution will change from TP, to Tp ; in the second year 

 to 1p' ; and so on. Let E E be the earth's equator, 

 then, when the pole of the earth moves from T P, to 

 T p, the line of the equinoxes, or the intersection of the 

 ecliptic and equator, will move from T A, to T a, and so 

 on, turning slowly around the centre of the earth. It is 

 evident that this progressive change of the direction of 

 the poles of the earth will have a corresponding influence 

 on the planes of the fixed stars, and on the commence- 

 ments of the seasons. Thus, spring commences when 

 the line of equinoxes is in T'A. The succeeding 

 year would have the same commencement ; but, in the 

 meantime, the line of nodes has moved to T T , A, ; in 

 the following year it will have moved to T 2 ', A 2 ' all pass- 

 ing through the sun S. But as the direction of the arrow 

 shows direct motions, it is easily to be seen that the in- 

 tersections precede at every year : for this reason this 

 phenomenon is termed, as we have already seen, the 

 precession of the equinoxes. 



In order to exhibit the effect of this phenomenon on 

 the planes of the stars, let E E (Fig. 33) be the earth's 

 equator, A B C D the 

 ecliptic, and let the in- 

 tersections of the ecliptic 

 and equator occur suc- 

 cessively at A', A", &c. , 

 in an opposite direction 

 to the sun's path. These 

 successive changes of the 

 equinoxes will cause cor- 

 responding changes in the 

 pole P to P' and P", the 

 right ascensions. Decli- 

 nations of all bodies will 

 be changed, as also their 

 longitudes ; but it will not have any effect on the position 

 of the stars, the longitudes of which will be constantly 

 the same. 



ON THB ABERRATION OF LIGHT. In ancient times it 

 was supposed that the velocity of light was infinite and 

 immeasurable. Koe'mer, a Danish astronomer, pointed 

 out, in the year 1675, that in comparing Cassini's tables 



Kg. M. 



change in the sun's passage across the equinoctial, is I of Jupiter's first satellite with observation, he discovered 

 really a change in the point of the earth's orbit at which [ the following fact : When Jupiter was near opposition, 



