the other heavenly bodies on the equatorial protuberance of the Earth. A similar conical motion can also be observed in a gyroscope that is subjected to lateral forces. The resultant motion of the Earth's axis is called general precession and the equinox points in the ecliptic move westward along the ecliptic at the rate of about 50.3 seconds of arc per year as a result. In 25,772 years, the points are once again at the same point in the sky where observations began. In addition the tilt, or obliquity, of the Earth's axis is not constant but changes in a cycle of its own. During a cycle that averages about 40,000 years, the tilt of the axis varies between 22.1 and 24.5 degrees. History of both definitions Plato (c. 360 BC) used the term "perfect year" to describe the return of the celestial bodies (planets) and the diurnal rotation of the fixed stars (circle of the Same) to their original positions; there is no evidence he had any knowledge of axial precession. The cycle which Plato describes is one of planetary and astral conjunction, which can be postulated without any awareness of axial precession. Hipparchus (c. 120 BC) is the first Greek credited with discovering axial precession roughly two hundred years after Plato's death (see below). Cicero (1st century BC) followed Plato in defining the Great Year as a combination of solar, lunar and planetary cycles. Plato's description of the perfect year is found in his dialogue Timaeus: In De Natura Deorum, Cicero wrote Macrobius (early fifth century AD) in his commentary on Cicero's Somnium Scipionis states that 'the philosophers' reckon the Great Year as 15,000 years. Censorinus (3rd century AD) wrote that Aristarchus of Samos reckoned a Great Year as 2484 years: but it has been argued that this is a miscopying of 2434, which represents 45 Exeligmos cycles. The origin of the Platonic Year would seem to have no connection with the precession of the equinoxes as this was unknown in Plato's time. Two centuries after Plato, Hipparchus is credited with discovering the period of equinox precession, and the term "Great Year" eventually came to be applied to the period of that precession caused by the slow gyration of the Earth's axis. It is argued that a confusion between the two originated with the astronomer Ptolemy (c. 170 AD), who "adopted the larger, erroneous, figure, with the result