[126] “Gassendi, Pierre.” (In: Nouvelle biographie.) Gassendi achieved distinction for his works on astronomical subjects. In the year 1645 he was appointed Professor of Mathematics in the Collège Royal of Paris, a position he held with interruptions until his death. [127] 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. [128] Among the more important works of Cassini bearing upon this particular subject may be mentioned, Méthode pour trouver la différence des longitudes des lieux par les observations correspondantes des phases des éclipses de soleil 1670. (In: Histoire de l’Académie Royale des Sciences. Paris, 1733. Vol. I, p. 133.); La méthode de déterminer les longitudes des lieux de la terre par les observations des satellites de Jupiter. (In: Mémoires de l’Académie. Paris, 1743. Vol. X, p. 569.); De la méthode de déterminer les longitudes des lieux de la terre par les observations des satellites de Jupiter. (In: Observations physiques et mathématiques. Paris, 1688. pp. 232-278.); Les hypothèses et les tables des satellites de Jupiter, réformees sur de nouvelles observations. (In: Mémoires de l’Académie, 1693. Paris, 1730. Vol. VIII, p. 363.); Méthode de déterminer les longitudes des lieux de la terre par des étoiles fixes et des planètes par la Lune. (In: Mémoires de l’Académie. Paris, 1703.) [129] See p. 349 of Bion’s work referred to below, n. 138. [130] Histoire de l’Académie Royale des Sciences. Paris, 1727. [131] Fiorini. Sfere terrestri e celesti. pp. 401-402.