Hipparchus discovered that Earth wobbles, almost by accident
The Greek astronomer credited with founding trigonometry is best known for a side discovery: the slow drift of the equinoxes caused by Earth's precession. He also compiled the West's first comprehensive star catalogue. Yet of his fourteen or so books, only one critical commentary on a poem survives intact.
Hipparchus was born in Nicaea in Bithynia around 190 BC, a date Jean-Baptiste Delambre worked out from clues in his writing, and probably died on Rhodes. Ptolemy attributes observations to him between 147 and 127 BC, some explicitly from Rhodes, and he may have been observing since 162 BC. Centuries later, Bithynian coins showed him with a globe. Delambre gave him the title father of astronomy in 1817.
He was the first whose accurate, quantitative models of the Sun's and Moon's motion survive, and many rank him the finest observer of antiquity. He drew on centuries of Babylonian records and on Greek predecessors such as Meton, Aristarchus and Eratosthenes, and he seems to have been the first to exploit Babylonian methods systematically. Historian Gerald Toomer suggests Ptolemy's Babylonian eclipse records came from a list Hipparchus compiled. His long lunar period of 5,458 months also appears on Babylonian tablets, though the only dated one is later, so who borrowed from whom is unsettled.
To compute the eccentricity of solar and lunar orbits he needed trigonometry, and he is the first mathematician known to have had a trigonometric table, listing chords of a circle. That let Greek astronomers solve any triangle and turn geometry into predictions. He must have used a closer value of pi than Archimedes' bounds. Some claim his table survived in Indian works such as the Surya Siddhanta, and he is sometimes credited with stereographic projection, though scholars dispute the evidence. He may also have invented the astrolabe and used an armillary sphere for his star catalogue.
Most of what we know comes secondhand, through Strabo, Pliny, Ptolemy's Almagest and later commentators. His catalogue was absorbed into Ptolemy's and can be nearly reconstructed by subtracting two and two-thirds degrees from Ptolemy's star longitudes. His lone surviving work is a two-book critique of Aratus's popular astronomical poem, based on Eudoxus.
Source: Hipparchus
