Based on Babylonian observations, he improved the accuracy of the length of the lunar, solar and sidereal years. He estimated the solar year with an accuracy which is only 6 1/2 minutes different from the current value. For the lunar month he obtained a value of 29 days, 12 hours, 44 minutes and 2 ½ seconds, only less than 1 second from the current value!
The tropical (solar) year is the period of the Sun's apparent revolution from an equinox to the same equinox again (The time reference that we use in everyday life), and the sidereal year is the period of the Sun's apparent revolution from a fixed star to the same fixed star.
Hipparchus discovered the precession of the equinoxes and was influential in the development of trigonometry, redefined and formalized the projection as a method for solving complex astronomical problems without spherical trigonometry and probably proved its main characteristics.
Hipparchus produced until 129 BC a catalogue of 850 stars after observing 134 BC a new star in the sky. This catalogue was of high precision and used even by the astronomer Edmund Halley. Hipparchus compared his star positions with those of Timocharis and Aristillus. He could not find any stars that had appeared or disappeared in the last 150 years but all the stars seemed to have changed their places with reference to that point in the heavens where the ecliptic is 90° from the poles of the earth i.e. the equinox. He found that this could be explained by a motion of the equinox in the direction of the apparent diurnal motion of the stars. He found the precession of the equinoxes, which takes place at the rate of 52",1 every year due to a steady revolution of the earth's pole round the pole of the ecliptic once in 26000 years in the opposite direction to the planetary revolutions.
Sources:http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Hipparchus.html
http://www.mlahanas.de/Greeks/Hipparchus.htm
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