Emily Dickinson Lexicon

PRE-CES'SION, n. [Fr. precession; It. precessione; from the L. præcessus, præcedo, to go before.]

1. Literally, the act of going before.
2. In astronomy, the precession of the equinox, is an annual motion of the equinox, or point where the ecliptic intersects the equator, to the westward, amounting to 50 1/3”. This precession was discovered by Hipparchus, a century and a half before the Christian era, though it is alledged that the astronomers of India had discovered it long before. At that time, the point of the autumnal equinox was about six degrees to the eastward of the star called spica virginis. In 1750, that is, about nineteen hundred years after, this point was observed to be about 20° 21' westward of that star. Hence it appears that the equinoctial points will make an entire revolution in about 25,745 years.