Hipparchus is considered by many to be the greatest astronomical observer from antiquity. He also made many advances in geography and mathematics, specifically trigonometry.
Born around 190 BC in Nicaea, Bithynia (now known as now Iznik,Turkey), Hipparchus's life is mostly a mystery. Much of what we do know about Hipparchus comes from Ptolemy's Almagest, however, he is mentioned in other writings as well. Strabo, a Greek geographer and historian who lived around 64BC to 24AD called Hipparchus one of the famous men of Bithynia. His image, usually depicted sitting and looking at a globe, has been found on many coins minted between 138AD and 253AD.
Hipparchus apparently traveled extensively. There are records of observations he made in his native Bithynia as well as the island of Rhodes and Alexandria. The only example of his writing which still exists is his Commentary on Aratus and Eudoxus, which while it is not one of his major writings is still noteworthy because it does give us an insight into his work.
As a mathematician, Hipparchus made a number of contributions, including the division of a circle into 360 degrees and the creation of one of the first trigonometric tables for solving triangles. In fact, many say that he was the inventor of trigonometry.
As an astronomer, Hipparchus calculated the length of the year to within 6 1/2 minutes. He also discovered the precession of equinoxes, with a value of 46", which is fairly close to our modern number of 50.26". Three hundred years later, Ptolemy only came up with a figure of 36". While his star catalogue does not survive today, it is believed that he included around 850 stars. He also made a careful study of the motions of the moon.
It is unfortunate that more of his writings do not survive. It seems clear that the work of many who followed was developed using the groundwork laid by Hipparchus.
Although little else is known about him, it is probable that he died around 120 BC most likely in Rhodes, Greece.