Hipparchus (second century B.C.), a Greek astronomer and mathematician. He was born in Nicaea in Asia Minor (modern Turkey). Hipparchus created one of the earliest star charts, classifying about 1,000 stars in orders of magnitude (brightness). He recorded the position of each star by using a globe as a model of the heavens and dividing it into degrees of longitude and latitude. (He used the same method for locating points on the earth.) Hipparchus is often regarded as the founder of plane and spherical trigonometry.

Hipparchus' outstanding discovery was that the length of time between two successive spring equinoxes is not the same as the sidereal year. This phenomenon, called the precession of equinoxes, is caused by irregularities in the earth's rotation. Hipparchus believed the heavenly bodies all moved around the earth in epicycles; that is, each body revolved around a point that, in turn, revolved around the earth. This concept, probably not original with Hipparchus, was later adopted by Ptolemy and became a central idea of astronomy for more than a thousand years.