Hipparchus

As a young man in Bithynia, Hipparchus compiled records of local weather patterns throughout the year. Such weather calendars (parapēgmata), which synchronized the onset of winds, rains, and storms with the astronomical seasons and the risings and settings of the constellations, were produced by many Greek astronomers from at least as early as the 4th century bc.

Most of Hipparchus’s adult life, however, seems to have been spent carrying out a program of astronomical observation and research on the island of Rhodes. Ptolemy cites more than 20 observations made there by Hipparchus on specific dates from 147 to 127, as well as three earlier observations from 162 to 158 that may be attributed to him. These must have been only a tiny fraction of Hipparchus’s recorded observations. In fact, his astronomical writings were numerous enough that he published an annotated list of them.

Hipparchus also wrote critical commentaries on some of his predecessors and contemporaries. In Tōn Aratou kai Eudoxou Phainomenōn exēgēseōs biblia tria (“Commentary on the Phaenomena of Aratus and Eudoxus”), his only surviving book, he ruthlessly exposed errors in Phaenomena, a popular poem written by Aratus and based on a now-lost treatise of Eudoxus of Cnidus that named and described the constellations. Apparently his commentary Against the Geography of Eratosthenes was similarly unforgiving of loose and inconsistent reasoning. Ptolemy characterized him as a “lover of truth” (philalēthēs)—a trait that was more amiably manifested in Hipparchus’s readiness to revise his own beliefs in the light of new evidence. He communicated with observers at Alexandria in Egypt, who provided him with some times of equinoxes, and probably also with astronomers at Babylon.