ABSTRACT
The slow westward motion of the equinoctial points along the ecliptic, whose effect is to make the tropical year (the Sun’s passage from one vernal equinox to the next) slightly shorter than the sidereal year (one circuit of the Sun with respect to the fixed stars). Precession was first noted by Hipparchus (fl. 127 B.C.E.). Ptolemy (ca. 100-ca. 170), employing earlier observations, accepted an annual rate of 36 arc-seconds. During the tenth century, Arabic astronomers believed that the rate was variable and that precession reversed itself periodically. This so-called trepidation (trepidatio) became a standard feature of later Islamic astronomy. A ninth sphere was required to account for the motion. In the thirteenth century, Nasr Eddin, who doubted trepidation, calculated the precessional rate at 51 arc-seconds, close to the modern 50.3 arc-seconds.
