Note 127 — Chapter XII · Globes and Globe Makers of the First Half of the Eighteenth Century—from Delisle to Ferguson

The term “Precession of the Equinoxes,” as used in astronomy, refers to the slow retrograde motion of the equinoctial point to the west, or contrary to the order of the signs of the zodiac, this precession being estimated by Hipparchus to be one degree in one hundred years; in sixty-nine years by Ptolemy; in sixty-six years by Albategnius; in seventy years by Cassini, but it is now estimated to be one degree in about seventy and one half years. For one complete revolution of this equinoctial point through the twelve signs of the zodiac Hipparchus estimated a period of 36,000 years would be required; according to Ptolemy a period of 24,840 years; according to Albategnius 23,760 years; according to Cassini 25,200 years; whereas the period is now estimated to be a little more than 25,800 years. An important consequence of the precession of the equinoxes lies in the fact that the zodiacal constellations do not agree with the signs with which they coincided in ancient times, i.e., in the beginnings of astronomical science. The first star of Aries, which at the time of Eudoxus was at the intersection of the equator and the ecliptic, or at the equinoctial colure, has continued to increase its position in longitude. At the time of Ptolemy this was 6 degrees 40 minutes. Its longitude is now about 31 degrees, which places it entirely out of its original sign.