Museo Galileo
Virtual Museum
Hipparchus of Nicaea
2nd C. B.C.E.

Hipparchus apparently began observing the heavens in c. 146 B.C.E. in his native town of Nicaea, in Bithynia, and continued to do so in Rhodes until 127 B.C.E. Nothing else is known about his life except the following anecdote: one day, to the amusement of passers-by, he sat in the square wearing a cloak because he had predicted a storm. Hipparchus revitalized Hellenistic astronomy. He invented and used observation instruments and essential conceptual aids such as mathematical tables of the angles subtended by the chords of a circumference and stereographic projection. Of his writings - some of which are mentioned in Claudius Ptolemy's Almagest - we have only the Comment against the phenomena of Aratus and Eudoxus (c. 140 B.C.E.), in which he criticized the imprecise stellar surveys conducted before his time. In c. 129 B.C.E., he pursued the topic by compiling one of the first star catalogues of antiquity, whose data were amalgamated into the similar catalogue of the Almagest. Comparing his stellar positions with those recorded by some of his predecessors, Hipparchus identified the phenomenon of the precession of the equinoxes. His analysis of planetary motion represented the encounter between Babylonian numerical astronomy and Greek geometrical astronomy - i.e., the attempt to combine observational data with the theory of epicycles, already outlined by Apollonius of Perga, in order to establish a rigorous predictive science. Hipparchus was only partly successful in achieving his goal. He explained the unequal lengths of the seasons by assuming that the Sun revolved around the Earth on a fixed eccentric. At the same time, he used an epicyclical model to describe the Moon's zodiacal motion. After estimating the distances of the Sun and Moon from the Earth, he was able, thanks to the new models, to forecast lunar and solar eclipses.