Often described as the greatest astronomer of antiquity, Hipparchus (also known as Hipparchos, Hipparchus of Nicaea, or of Rhodes) revealed the precession of the equinoxes, determined the lengths of the four seasons on Earth, studied annual solar movements, and considered the distances of the Sun and Moon from Earth. He was also the first person to use longitude and latitude in an attempt to pinpoint locations on Earth. In addition, he indexed the latitude, longitude, and brightness of approximately 850 stars, creating the most complete astral catalog ever before assembled.
Born around 180 B.C. in Nicaea, Bithynia (in Anatolia), Hipparchus spent much of his life as an astronomer in Rhodes (one of the Greek islands), although he may also have spent some time at Alexandria, Egypt. Although only one of his 14 books remains, a review of Aratus's Phaenomena, his contributions to mathematics, astronomy, and geography were described in the famed work by Ptolemy known as the Algamest.
Hipparchus's principal scientific contribution was his detection of the precession of the equinoxes. By contrasting his observations of the Sun's path through Earth's sky against similar observations made in 281 and 432 B.C., he found that the path shifts from year to year, intersecting the celestial equator in slightly different positions. This shift is known as the precession of the equinoxes. From this data, Hipparchus was also able to infer the length of the year and was correct to within 6.5 minutes.
For much of his work, Hipparchus relied on a combination of traditional mathematical calculations and a "table of chords" that he developed. This table, a forerunner of the sine used in modern trigonometry, was critical in his estimations of stellar and planetary positions. His observational skills also played an important part in his studies. Using his own and historical observations of lunar eclipses, Hipparchus mathematically explained the apparent movement of the Moon as viewed from Earth. To resolve the Sun's apparent movements, he measured the span from vernal equinox to summer solstice, and from summer solstice to autumnal equinox. This work was particularly useful during the period when astronomers viewed Earth as the center of a spherical universe. In addition, Hipparchus employed his mathematical skills to determine the approximate size of the Sun and Moon, along with their relative distances from Earth. While he greatly underestimated the Sun's actual size, his assessment of the Moon's diameter was off by fewer than 500 miles (800 km).
In his other work, Hipparchus wished to extend the application of latitude and longitude so that both stars and terrestrial positions could be mapped. He put together a catalog charting the locations of some 850 stars and indicating their brightness using a scale of magnitude similar to the scale used today. The catalog became the standard reference. Back on Earth, however, Hipparchus was less successful. Although he calculated a value for the approximate size of one longitudinal or latitudinal degree, he was unable to develop a practical terrestrial system.
To future scientists, the work of Hipparchus became a testament to the importance of careful astronomical observation, a stepping stone in the development trigonometry, and a demonstration of how mathematics and observation could combine to give rise to important discoveries.