Hipparchus was born in Nicaea in Bithynia, in modern day Turkey. The exact dates of his life are unknown, but he is believed to have observed from 162 to 126 B.C. His only preserved work is the Commentary on Aratus, a commentary on a poem by Aratus which describes the constellations and the stars which comprise them. This work contains many measurements of stellar positions. Most of what is known about Hipparchus is from Ptolemy's Almagest.
It is thought that Hipparchus compiled the first catalog of stars, and also compiled the first trigonometry tables. He tabulated values for the chord function, which gave the length of the chord for each angle. In modern terms, the chord of an angle equals twice the sine of half of the angle.
Hipparchus is perhaps most famous for having been the first to measure the precession of the equinoxes (There is some suggestion that the Babylonians may have known about precession but it appears that Hipparchus was to first to really understand it and measure it). By comparing his own measurements of the position of the equinoxes to the star Spica with those of Timocharis 150 years earlier, he observed that the equinox had moved 2° relative to Spica. He also noticed this motion in other stars. He obtained a value of not less than 1° in a century. The modern value is 1° in 72 years. Later, Edmund Halley would use his star catalog to discover proper motions as well.
Hipparchus had measurements of the times of solstices from Aristarchus dating from 279 B.C. and from the school of Meton and Euctemon dating from 431 B.C. This was a long enough period of time to allow him to calculate the difference between the length of the sidereal year and the tropical year, and led him to the discovery of precession mentioned above.
Hipparchus described the motion of the sun and obtained a value for the eccentricity. It was known that the seasons were of unequal length, not something that would be expected if the sun moved around the earth in a circle at uniform speed (Of course today we know that the planets move in ellipses, but this was not discovered until Johannes Kepler published his first two laws of planetary motion in 1609). His solution was to place the earth not at the center of the sun's motion, but at distance from the center. This model of the sun's motion described the actual motion of the sun fairly well.
Hipparchus also studied the motion of the moon and obtained more accurate measurements of some periods of the motion than existed previously, and undertook to find the distances and sizes of the sun and moon.