Hipparchus (c. 190 - c. 120 BCE) was a Greek astronomer, mathematician and geographer, regarded as one of the greatest observational astronomers of antiquity. He discovered the precession of the equinoxes, compiled one of the earliest known star catalogues, and developed the mathematical methods that underlie trigonometry [1]. Much of his work survives only through its later use by the astronomer Ptolemy [2].
Life
Hipparchus was born around 190 BCE in Nicaea, a city in the region of Bithynia in northwestern Anatolia [1]. Little is known of his personal life, and most information about him derives from later writers, above all the astronomer Claudius Ptolemy, who drew extensively on his findings in the second century CE [1]. Hipparchus is thought to have carried out much of his observational work on the island of Rhodes, where ancient tradition places his death around 120 BCE [2].
Astronomical work
Hipparchus is best remembered for his discovery of the precession of the equinoxes, the slow drift of the equinoctial points along the ecliptic [1]. By comparing his own measurements of stellar positions with earlier Babylonian and Greek records, he estimated the rate of this shift and recognized that it gradually altered the celestial coordinates of the stars [2]. He also compiled one of the earliest known star catalogues, recording the positions and relative brightnesses of several hundred stars, and he is credited with introducing the system of apparent magnitudes still used, in refined form, to describe stellar brightness [1].
Working from both his own observations and Babylonian astronomical data, Hipparchus constructed quantitative models of the motions of the Sun and Moon [3]. He improved earlier estimates of the length of the year and the lunar month and devised methods for predicting eclipses. From observations of a solar eclipse he produced one of antiquity's most accurate estimates of the distance to the Moon, expressed in terms of Earth radii [2].
Mathematics and geography
To carry out his astronomical calculations, Hipparchus developed the mathematical tools that underpin trigonometry, and he is often described as its founder [3]. He is credited with compiling the first known table of chords, relating the lengths of chords in a circle to the angles they subtend, which gave later astronomers a systematic means of solving problems in spherical geometry [3]. In geography he criticized earlier mapmakers and argued that positions on Earth should be fixed by astronomically determined coordinates of latitude and longitude [2].
Legacy
Almost none of Hipparchus's own writings survive; his sole extant work is a commentary on an astronomical poem by Aratus [1]. His results were preserved and extended by Ptolemy, whose Almagest became the dominant astronomical text for more than a thousand years and transmitted much of Hipparchus's data and methods to medieval and Renaissance astronomers [1]. In recognition of his influence, later ages attached his name to many projects, including the European Space Agency's Hipparcos astrometry satellite, and he was inducted into the International Space Hall of Fame [2].
