HIPPARCHUS

Precession of the equinoxes (146 BC-130 BC)

Hipparchus is perhaps most famous for having been the first to measure the precession of the equinoxes. There is some suggestion that the Babylonians may have known about precession, but it appears that Hipparchus was the first to really understand it and measure it. According to al-Battani, Chaldean astronomers had distinguished the tropical and sidereal year. He stated that they had, around 330 BC, an estimation for the length of the sidereal year to be SK = 365 days 6 hours 11 min (= 365.258 days) with an error of (about) 2 min. This phenomenon was probably also known to Kidinnu around 314 BC. Yu Xi (fourth century) was the first Chinese astronomer to mention precession.

Hipparchus and his predecessors mostly used simple instruments for astronomical calculations, such as the gnomon, astrolabe, armillary sphere, etc.

.Additionally, as the first in history to correctly explain this with retrogradical movement of vernal point „ over the ecliptic for about 45", 46" or 47" (36" or 3/4' according to Ptolemy) per annum (today's value is 50.29"), he showed the Earth's axis is not fixed in space. By comparing his own measurements of the position of the equinoxes to the star Spica during a lunar eclipse at the time of equinox with those of Euclid's contemporaries (Timocharis of Alexandria (ca. 320 BC - 260 BC), Aristyllus 150 years earlier, the records of Chaldean astronomers (especially Kidinnu's records), and observations of a temple in Thebes, Egypt, that was built around 2000 BC) he still later observed that the equinox had moved 2ƒ relative to Spica. He also noticed this motion in other stars. He obtained a value of not less than 1ƒ in a century. The modern value is 1ƒ in 72 years.

After him many Greek and Arab astronomers had confirmed this phenomenon. Ptolemy compared his catalogue with those of Aristyllus, Timocharis, Hipparchus and the observations of Agrippa and Menelaus of Alexandria from the early 1st century and he finally confirmed Hipparchus' empirical fact that the poles of the celestial equator in one Platonic year (approximately 25,777 sidereal years) encircle the ecliptical pole. The diameter of this circle is equal to the inclination of ecliptic relative to the celestial equator. The equinoctial points in this time traverse the whole ecliptic and they move 1ƒ in a century. This velocity is equal to that calculated by Hipparchus. Because of these accordances, Delambre, P. Tannery and other French historians of astronomy had wrongly jumped to conclusions that Ptolemy recorded his star catalogue from Hipparchus' with an ordinary extrapolation. It was not known until 1898 when Marcel Boll and others had found that Ptolemy's catalogue differs from Hipparchus' not only in the number of stars but in other respects.

This phenomenon was named by Ptolemy just because the vernal point „ leads the Sun. In Latin praecesse means "to overtake" or "to outpass", and today also means to twist or to turn. Its own name shows this phenomenon was discovered before its theoretical explanation, otherwise it would have been given a better term. Many later astronomers, physicists and mathematicians had occupied themselves with this problem, practically and theoretically. The phenomenon itself had opened many new promising solutions in several branches of celestial mechanics: Thabit ibn Qurra's theory of trepidation and oscillation of equinoctial points, Isaac Newton's general gravitational law (which had explained it in full), Leonhard Euler's kinematic equations and Joseph Lagrange's equations of motion, Jean d'Alembert's dynamical theory of the movement of a rigid body, some algebraic solutions for special cases of precession, John Flamsteed's and James Bradley's difficulties in the making of precise telescopic star catalogues, Friedrich Bessel's and Simon Newcomb's measurements of precession, and finally the precession of perihelion in Albert Einstein's general theory of relativity.

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