Hipparchus

(c. 170 bc–c. 120 bc) Greek astronomer and geographer

Born at Nicaea, which is now in Turkey, Hipparchus (hi-par -kus) worked in Rhodes, where he built an observatory, and in Alexandria. None of his works has survived but many of them were recorded by Ptolemy. In 134 bc he observed a new star in the constellation of Scorpio. This led him to construct a catalog of about 850 stars. By comparing the position of the stars of his day with those given 150 years earlier he found that Spica, which was then 6° from the autumn equinox, had previously been 8°. He used this observation to deduce not the movement of Spica but the east to west precession (motion) of the equinoctial point. He calculated the rate of the precession as about 45 seconds of arc a year – a value close to the 50.27 seconds now accepted. He also introduced the practice of dividing the stars into different classes of magnitude based on their apparent brightness. The brightest stars he classed as first magnitude and those just visible to the naked eye he classed as sixth magnitude.

As a theorist Hipparchus worked on the orbits of the Sun and Moon. He established more accurate lengths of both the year and the month and was able to produce more accurate eclipse predictions. One of his lasting achievements was the construction of a table of chords, which virtually began the discipline of trigonometry. The concept of a sine had not yet been developed. Instead, Hipparchus calculated the ratio of the chord to the diameter of its own circle, which was divided into 120 parts. Thus if a chord produced by an angle of 60° is half the length of the radius, it would have, for Hipparchus, 60 parts. He much improved the geography of Eratosthenes, fixing the parallels astronomically.