Hipparchus

Hipparchus was a Greek astronomer and mathematician of the second century BCE. His work on star positions, celestial coordinates, and the precession of the equinoxes helped establish positional astronomy.

Celestial coordinate system: A system for specifying positions of objects on the celestial sphere. Hipparchus used angular coordinates to describe and compare star positions.
Precession of the equinoxes: The slow westward shift of the equinoxes along the ecliptic caused by the changing orientation of Earth's axis. Hipparchus inferred this long-term shift by comparing stellar observations with earlier records.
Ancient Greek astronomy: The study of celestial phenomena by Greek thinkers from antiquity through late antiquity. Hipparchus's mathematical astronomy grew within this intellectual tradition.
Astrolabe: An instrument that represents the celestial sphere and supports astronomical measurement and calculation. Later positional astronomers used related geometric principles to measure the sky.
Ecliptic: The apparent annual path of the Sun across the celestial sphere. Hipparchus measured celestial positions relative to this fundamental circle.
Hipparchus's star catalogue: A catalogue of stellar positions traditionally attributed to Hipparchus, known chiefly through later accounts and surviving related material. Its positional records exemplify his effort to measure the sky systematically.
Babylonian astronomical diaries: Cuneiform records of celestial observations and related events made by Babylonian astronomers. Such long-running observations offered data Greek astronomers could compare with their own.
Al-Sufi: A tenth-century Persian astronomer known for his Book of Fixed Stars. His star descriptions belong to the catalogue tradition shaped by ancient Greek astronomy.
Equinox: A point where the celestial equator intersects the ecliptic, or the moment the Sun crosses it. The shifting equinoxes supplied the reference for his discovery of precession.
Eclipse prediction: The calculation of when and where solar or lunar eclipses will occur. Hipparchus used cycles and observations to improve predictions of lunar eclipses.

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