PRECESSION of the equinoxes,is a very slow motion of them, by which they change their place, going from east to west, or backward, in antecedentia, as astronomers call it, or contrary to the order of the signs.
From the late improvements in astrono my it appears, that the pole, the solstices, the equinoxes, and all the other points of the ecliptic, have a retrogacle motion, and are constantly moving from east to west, or from Aries towards Pisces, &c. by means of which, the equinoctial points are carried further and further back among the preceding signs of stars, at the rate of about 50"f each year; which retro grade motion is called the precession, re cession, or retrocession of the equinoxes.
Hence, as the stars remain immoveable, and the equinoxes go backward, the stars will seem to move more and more east. ward with respect to them ; for which reason the longitudes of all the stars, being reckoned from the first point of Aries, or the vernal equinox, are continu ally increasing.
From this cause it is, that the constel lations seem all to have changed the pla ces assigned to them by the ancient astro nomers. In the time of Hipparchus, and the oldest astronomers, the equinoctial points were fixed to the first stars of Aries and Libra: but the signs do not now answer to the same points ; and the stars which were then in conjunction with the sun, when he was in the equinox, are now.a whole sign, or :30 degrees, to the eastward of it : so, the first star of Aries is now in the portion of the ecliptic, called Taurus ; and the stars of Taurus are now in Gemini; and those of Gemini in Cancer ; and so on.
This seeming change of place in the stars was first observed by Hipparchus of Rhodes, who 128 years before Christ, found that the longitudes of the stars in his time were greater than they had been before observed by Tymochares, and than they were in the sphere of Eudoxus, who wrote 380 years before Christ. Ptolemy also perceived the gradual change in the longitudes of the stars ; but he stated the quantity at too little, making it but 1° in 100 years, which is at the rate of only 36" per year. V-hang, a Chinese, in the year 721, stated the quan. city of this change at 1° in 83 years, which is at the rate of 431 per year. Other more modern astronomers have made this precession still more, but with some small differences from each other ; and it is now usually taken at 50"/ per year. All these rates are deduced from a COM. parison of the longitude of certain stars, as observed by more ancient astronomers, with the later observations of the same stars ; viz, by subtracting the former from the latter, and dividing the remainder by the number of years in the interval be tween the dates of the observations. Thus, by a medium of a great number of comparisons, the quantity of the an nual change has been fixed at 50•1, ac cording to which rate it will require 25,791 years fur the equinoxes to make their revolutions westward quite around the circle, and return to the same point again.
The phenomena of this retrograde mo tion of the equinoxes, or intersections of the equinoctial with the ecliptic, and con sequently of the conical motion of the earth's axis, by which the pole of the equator describes a small circle in the same period of time, may be understood an illustrated as follows ; Let NZSVL be the earth. (See Plate Perspective, &c. fig. 6.) SOMA its axis produced to the starry heavens, and terminating in A, the present north pole of the heavens, which is vertical to N, the north pole of the earth. Let EOQ be the equator, Taz the tropic of cancer, and the tro pic of capricorn ; VOZ the ecliptic, and BO its axis, both of which are immovea ble among the stars. But as the equi
noctial points recede in the ecliptic, the earth's axis SON is in motion upon the earth's centre 0, in such a manner as to describe the double cone NOn and SOS, round the axis of the ecliptic BO, in the time that the equinoctial points move round the ecliptic, which is 25,791 years, and in that length of time, the north pole of the earth's axis produced, describes the circle ABCDA in the starry hea vens, round the pole of the ecliptic, which keeps immoveable in the centre of that circle. The earth's axis being now 23° 28' inclined to the axis of the eclip tic, the circle ABCDA, described by the north pole of the earth's axis produced to A, is 46° 56' in diameter, or double the inclination of the earth's axis. In consequence of this, the point A, which is at present the north pole of the hea vens, and near to a star of the 2d mag nitude in the end of the Little Bear's tail, must be deserted by the earth's axis ; which, moving backwards one degree every 711 years, will be directed towards the star or point B in 64474 years hence ; and in double of that time, or 12,895i years, it will be directed towards the star or point C ; which will then be the north pole of the heavens, although it is at present 8i degrees south of the zenith of London L. The present position of the equator EOQ will then be changed into eOq, the tropic of cancer TS.r1Z into V/276, and the tropic of Capricorn VTV5 into tlej Z ; as is evident by the figure. And the sun, in the same part of the hea vens where he is now over the earthly tropic of capricorn, and makes the short est days and longest nights in the north ern hemisphere, will then be over the earthly tropic of cancer, and make the days longest and nights shortest. So that it will require 12,895 years yet more, or from that time, to bring the north pole N quite round, so as to be di. rected towards that point of the heavens which is vertical to it at present. And then, and not till then, the same stars which at present describe the equator, tropics, and polar cricles, &c. by the earth's diurnal motion, will describe them over again.
From this shifting of the equinoctial points, and with them all the signs of the ecliptic, it follows, that those stars, which in the infancy of astronomy were in Aries, are now found in Taurus ; those of Tau rus in Gemini, &c. Hence likewise it is, that the stars which rose or set at any particular season of the year, in the times of Hesiod, Eudoxus, Virgil, Pliny, &c. by no means answer at this time their de scriptions.
As to the physical cause of the preces sion of the equinoxes, Sir Isaac Newton demonstrates, that it arises from the broad or flat spheroidal figure of the earth ; which itself arises from the earth's rotation about its axis : for as more mat tor has thus been accumulated all round the equatorial parts than any where else on the earth, the sun and moon, when on either side of the equator, by attracting this redundant matter, bring the equator sooner under them, in every return to• wards it, than if there was no such ac cumulation.
Sir Isaac Newton in determining the quantity of the annual precession from the theory of gravity, on supposition that the equatorial diameter of the earth is to the polar diameter, as 230 to 229, finds the sun's action sufficient to produce a precession of 91. only ; and collecting from the tides the proportion between the sun's force and the moon's, to be as 1 to 41, he settles the mean precession result ing from their joint actions at 50" ; which is nearly the same as it has since been found by the best observations.