Precession Of The Equinoxes, a slow regression of the equinoctial points upon the plane of the ecliptic. It is so called from its causing the sun to arrive in either equinox a little earlier than he otherwise would. The effect is to increase the longitudes of the fixed stars at the rate of about 50 1/4" annually. The discovery of the movement is due to Hippar-chus, about 150 B. C. Copernicus was the first to give a true explanation of the phenomenon. Newton discovered its physical cause. This cause is the attraction of the sun, moon, and planets upon the spheroidal figure of the earth, giving to the axis a gyratory or conical motion well represented by the waving or nodding of a top in spinning. The pole of the equator is thus made to shift its place, performing a complete revolution around the pole of the ecliptic in 25,868 years. Ptolemy's assumption of the value of precession led him to assign incorrect positions to stars which he catalogued as if observed by him. Delambre compared the positions of 312 stars catalogued by Ptolemy with the positions observed by Flamsteed, and found that the deduced precession amounted to 52.4". But by treating Ptolemy's longitudes as simply deduced from Hipparchus's by adding 2° 40' for the interval of 267 years between Hipparchus and Ptolemy, Delambre deduced 50.12", which is very nearly correct.

Ptolemy ought to have added 3° 37' instead of 2° 40'. But the most serious error was his publishing as his own a catalogue derived from that of his illustrious predecessor, instead of indicating the manner in which the catalogue was obtained. The result is the same as though he had handed down Hipparchus's catalogue, otherwise unknown; but grave doubts have been thrown on all Ptolemy's observations since his detection, 1,700 years after the deed, in this serious offence. The Arabian astronomers reached a result much truer than that assumed by Ptolemy.