EQUINOX, the time when the sun en ters either of the equinoctial points, where the ecliptic intersects the equi noctial. It was evidently an important problem in practical astronomy, to de termine the exact moment of the sun's occupying these stations ; for it was na tural to compute the course of the year from that moment. Accordingly, this has been the leading problem in the as tronomy of all nations. It it susceptible of considerable precision, without any apparatus of instruments. It is only ne cessary to observe the sun's declination on the noon of two or three days before and after the equinoctial day. On two consecutive days of this number, his de clination must have changed from north to south, or from south to north. If his declination on one day was observed to be 21' north, and on the next 5' south, it follows that his declination was nothing, or that he was in the equinoctial point about 23 minutes after 7 in the morning of the second day. Knowing the pre cise moments, and knowing the rate of the sun's motion in the ecliptic, it is easy to ascertain the precise point of the ecliptic in which the equator intersected it. By a series of such observations made at Alexandria, between the years 161 and 127 before Christ, Hipparchus, the father of our astronomy, found that the point of the autumnal equinox was about six degrees to the eastward of the 'star called spica virginis. Eager to de termine every thing by multiplied obser vations, he ransacked all the Chaldean, .Egyptian, and other records to which his travels could procure him access, for +observations of the same kind ; hut he sloes not mention his .havine found any.
He found, however, some observations of Aristillus and Timochares, made about 150 years before. From these it appeared evident that the point of the autumnal equinox was then about eight degrees east of the same star. He these observations with great sagacity and rigour ; and, on their authority, he asserts that the equinoctial points are not fixed in the heavens, but move to the westward about a degree in 75 years, or somewhat less.
This motion is called the procession of the equinoxes, because by it the time and place of the sun's equinoctial station pre cedes the usual calculation : it is fully confirmed by all subsequent observations. In 1750, the autumnal equinctx was ob served to be 20° 21' westward of spica virginis. Supposing the motion to have been uniform during this period of ages, it follows that the annual precession is about 50" and one-third ; that is, if the celestial equator cuts the ecliptic in a particular point on any day of this !ear, it will on the same day of, the following year cut it in a point 50" and one-third to the west of it, and the sun will come to the equinox 20' 23" before he has com pleted his round of the heavens. Thus the equinoctial, or tropical year, or true year of seasons, is so much shorter than the revolution of the sun, or the sidereal year. It is this discovery that has chiefly immortalized the name of Hipparchus, though it most be acknowledged that all his astronomical researches have been conducted with the same sagacity and in telligence. It was natural, therefore, for him to value himself highly for the discovery. It must be acknowledged to be one of the most singular that has been made, that the revolution of the whole heavens should not be stable, but its axis continually changing. For it must be observed, that since the equator changes its position, and the equator is only an imaginary circle, equidistant from the two poles, or extremities of the axis, these pules, and this axis, must equally change their positions. The equinoctial points make a complete revolution in about 25,745 years, the equator being all the while inclined to the ecliptic in near ly the same angle. Therefore the poles of this diurnal revolution must describe a circle round the poles of the ecliptic, at the distance of about 23} degrees in 25,745 years ; and in the time of Timo chares, the north pole of the heavens must have been 30 degrees eastward of where it now is.