Second half of the second century BCE. (His recorded observations are datable to the years 147-127.) See generally OCD(4) s.v. Hipparchus(3); also web address 1 and web address 2.
Although the frequency and respect with which Ptolemy refers to his work in the Almagest attest to his importance in the history of astronomy, his precise contributions are hard to determine. His careful observations supplemented his use of Babylonian sources, and made possible the discovery for which, perhaps, he is best known: the discovery of the precession of the equinoxes, that is, the recognition that the sun's position at the time of the equinoxes changes, over time, in relation to the background of the zodiacal constellations.
[1] In Bithynia. This reference to his birthplace is confirmed by later coins from Nicaea that bear his name and image. Because much of his professional life was apparently spent in Rhodes, he is often identified as Hipparchus of Rhodes.
[2] Textually corrupt, as Adler notes. (Such a dating would be unparalleled in the Suda.)
[3] (For Aratus see alpha 3745.) This is the only work that survives; Manitius' edition (below) includes a German translation.
[4] The last word translates katasthri/smou, rather than the text's katasthrigmou=, following Toomer. Hipparchus' star catalogue provided raw materials for Ptolemy's, despite their differences.
[5] (For Eratosthenes see epsilon 2898.) This title is known from Strabo (1.1.12; 2.1.41) to belong to a work on geography; it surely lies behind the garbled ei)s ta\s a)ri/stous of the text. [DW: other suggested possibilities, noted by Adler, are ei)s ta\s a)sterismou/s and ei)s ta\s a)steri/skous.]