Hipparchus of Nicaea (c. 190 - c. 120 BCE) was a Greek astronomer, geographer, and mathematician active primarily in Rhodes. He is generally regarded as one of the most significant figures in ancient astronomy, credited with developing systematic trigonometry for astronomical use, compiling one of the earliest known star catalogues, and identifying the Precession of the Equinoxes. Almost none of his original writings survive; knowledge of his work derives chiefly from later authors, most importantly Claudius Ptolemy, whose Almagest cites Hipparchus extensively. Because so much depends on secondhand transmission, some specifics of his methods and originality are subjects of ongoing scholarly discussion.

Hipparchus's surviving body of direct work is limited to a commentary on the poem of Eudoxus and Aratus and a treatise on the sizes and distances of the sun and moon, known mainly through citation. Scholars credit him with the discovery of the precession of the equinoxes, derived by comparing his own stellar observations with earlier data, and with constructing a catalogue of several hundred to roughly a thousand fixed stars using a system of celestial coordinates. He is also credited with introducing or significantly developing the chord-table method underlying trigonometric calculation, with producing improved estimates of the lunar and solar distances and the length of the synodic month, and with contributing the geometric device of the eccentric and epicyclic models later elaborated by Ptolemy in connection with the Epicycle system of planetary motion.

Because nearly all detailed knowledge of Hipparchus's procedures comes through Ptolemy's later citations and summaries rather than his own surviving texts, modern historians differ on how precisely Ptolemy's reports reflect Hipparchus's original methods, and on how much of the mature Ptolemaic system should be attributed to Hipparchus as opposed to Ptolemy's own development. For a fuller account of his biography and the trajectory of ancient astronomy in which he worked, see hipparchus-history.

There is broad consensus among historians of science that Hipparchus discovered the precession of the equinoxes and that he made foundational contributions to mathematical astronomy, including early trigonometric methods and stellar cataloguing, largely as transmitted through Ptolemy. This consensus concerns the existence and approximate content of these achievements rather than the fine-grained reconstruction of his original techniques, which remains a matter of scholarly interpretation. See Almagest - History of Science Consensus and Claudius Ptolemy - History of Science Consensus.

• Reliable transmitter view - Ptolemy's reports of Hipparchus's data and methods are generally accurate and can be used with confidence to reconstruct Hipparchus's astronomy. See Claudius Ptolemy - Synthesizer Viewpoint.
• Skeptical-of-attribution view - Because no primary texts survive for most of Hipparchus's astronomical work, claims about his precise methods, originality, and the extent of his contribution relative to Ptolemy's later synthesis should be treated with caution.
• Babylonian-influence view - Hipparchus's astronomical parameters and methods show substantial dependence on earlier Babylonian observational records and arithmetic techniques, which he is held to have had access to and adapted within a Greek geometric framework.
• Originator-of-trigonometry view - Hipparchus is credited as the founder of trigonometry as a systematic mathematical discipline applied to astronomy, based on his construction of a table of chords, in contrast to views that see his work as a more limited or transitional step.

• hipparchus-history
• Claudius Ptolemy
• Claudius Ptolemy - History of Science Consensus
• Claudius Ptolemy - Synthesizer Viewpoint
• Almagest - History of Science Consensus
• Aristarchus of Samos - Isolated Speculation Viewpoint
• Epicycle
• Precession of the Equinoxes

1. Toomer, G. J. “Hipparchus.” In Dictionary of Scientific Biography, edited by Charles Coulston Gillispie, vol. 15, 207-224. New York: Charles Scribner's Sons, 1978.
2. Neugebauer, Otto. A History of Ancient Mathematical Astronomy. Berlin: Springer-Verlag, 1975.
3. Toomer, G. J., trans. Ptolemy's Almagest. Princeton: Princeton University Press, 1998.
4. Jones, Alexander. “Ptolemy's Mathematical Models and Their Meaning.” In Mathematical Methods in Ancient Astronomy and Astrology. Princeton: Institute for Advanced Study, 2005.
5. Evans, James. The History and Practice of Ancient Astronomy. New York: Oxford University Press, 1998.