Ancient Greek mathematician and astronomer who determined the distance from the Earth to the Moon and produced the first-known star catalogues.
Hipparchus was born in Nicaea, Bithynia (now Iznik, Turkey), circa 190 B.C. He spent much of his time in Rhodes, Greece. Very little is known about his life, but he was one of the greatest astronomers and mathematicians of antiquity. Most of our knowledge of Hipparchus is contained in the writings of Strabo of Amaseia (c. 64 B.C.- A.D. 19) and Ptolemy (A.D. 85-165), especially the Almagest of Ptolemy.
Using a thin tube called a dioptra to scan the heavens, Hipparchus made extensive observations of star positions, and produced the first-known catalogue of stars. He is also credited with the first use of geometric models to account for astronomical motions. Geometric calculations also helped him to determine the distance between the Earth and the Moon. He discovered the precessions of the equinox and calculating the length of the year to within six and one-half minutes, remarkable achievements for his time. He also made the earliest known calculations employing trigonometry, a branch of mathematics he may have created, leading some to call him the "father of trigonometry."
The only extant work of Hipparchus is his "Commentary on the Phainomena of Eudoxus and Aratus," a discussion of an astronomical poem. Other writings by Hipparchus, since lost, included an astronomical calendar, books on optics and arithmetics, geographical and astrological works, and a catalogue of his own work.
Hipparchus recorded astronomical observations from 147 to 127 BC, all apparently from the island of Rhodes. During this period he may have invented the planispheric astrolabe, a device on which the celestial sphere is projected onto the plane of the equator. Not only did he make extensive observations of star positions, Hipparchus computed lunar and solar eclipses, primarily by using trigonometry.
Hipparchus discovered the precessions of equinoxes by comparing his notes with earlier observers; his realization that the points of solstice and equinox moved slowly from east to west against the fixed background of the stars was one of his most significant achievements. Another was his catalog of at least 850 stars, which may have been triggered by the appearance of what he called a "New Star" (a nova) in the constellation Scorpius in 134 BC.
Hipparchus was also the first to divide the stars into classes, dependant on their brightness. The twenty brightest stars he said were of the "first magnitude." Then, in order of decreasing brightness, were second, third, fourth, and fifth magnitudes, those of the sixth magnitude were just visible to the naked eye. Though revised, his system is still in use.
Hipparchus apparently made extensive use of much earlier Babylonian astronomical material, especially the writings of Kiddinu, an astronomer who lived about two hundred years before him. Hipparchus probably played a major role in the direct transmission of both Babylonian observations and procedures, resulting in the successful synthesis of Babylonian and Greek astronomy.
Hipparchus is often referred to as the "father of scientific astronomy," and is usually regarded as the greatest astronomer of ancient times. Working 1500 years before the invention of the telescope, he was able to create various scientific systems to classify numerous celestial objects basing his work on systematic observations. Hiatorians believe he died in Rhodes, sometime around 120 B.C.
The lunar crater Hipparchus, the Martian crater Hipparchus, and asteroid 4000 Hipparchus are all named in his honor, as was the European Space Agency's satellite Hipparcos, launched in August 1989. The first space experiment dedicated to the highly accurate measurements of star positions, distances, and space motions, it was terminated on August 15, 1993 after accomplishing all of its goals.