Hipparchus, << hih PAHR kuhs >> (180 B.C.?-125 B.C.?), an ancient Greek astronomer, is credited with discovering the precession (movement) of the equinoxes. The ancient Roman writer Pliny the Elder wrote that Hipparchus, excited by the appearance of a new star, studied earlier observations and noticed that the stars shifted eastward. He explained this shift by a slow forward motion of the equinoxes. See Equinox . 
Hipparchus’ study of precession was probably based on a more general study of the stars. He may have drawn up the first catalog of the stars, showing their brightness and position (see Magnitude ). He was also concerned with plotting the relative motions of the sun and moon as accurately as possible. His work led to some important improvements in both solar and lunar theory. For example, he distinguished between two different lengths (tropical and sidereal) of year. From his observations of the unequal length of the year’s four seasons, he derived a better account of the apparent movement of the sun. This account and Hipparchus’ improved theory of lunar motion enabled him to predict eclipses more accurately. Hipparchus was born in Nicaea, near what is now Istanbul, Turkey.
See also Ptolemy .
