Hipparchus (of Rhodes)

[hi pah® kuhs] ( c .170– c .125 BC ) Greek astronomer and geographer: discovered the precession of the equinoxes, constructed the first star catalogue and invented trigonometry.

Stimulated by the observation of a new star in 134 BC , Hipparchus constructed a catalogue of about 850 stars and was the first to assign a scale of ‘magnitudes’ to indicate their apparent luminosity, the brightest being first magnitude and the faintest visible to the naked eye being sixth magnitude. His scale, much refined, is still used. Comparison with earlier records of star positions led him to the realization that the equinoxes grew progressively earlier in relation to the sidereal year. (The equinoxes are the twice-yearly times when day and night are of equal length; they are the points when the ecliptic, the Sun’s path, crosses the celestial equator.) He evaluated the amount of precession as 45” of arc per year and determined the length of the sidereal and tropical years, the latter accurate to within 6 minutes. Hipparchus suggested improved methods of determining latitude and longitude on the Earth’s surface, following the work of . He constructed a table of chords, a precursor of the sine, and is therefore credited with the invention of trigonometry. All his major writing is lost, but his work was preserved and developed by .