Hipparchus, born circa 190 B.C. in Nicea in Turkey, is foremost amongst Greek astronomers. He lived on the Dodecanese island of Rhodes and there became the greatest observer of antiquity. He correctly estimated the distance from the Earth to the Moon by parallax. By the same reasoning, he concluded that the sun and stars were immeasurably farther away. He understood the geometry of  lunar eclipses and predicted them accurately. Hipparchus noted that the seasons were not equal in length, and he offset the Earth from the center of the solar system to better account for the observation. He ranked stars by their brightness, initiating the system of magnitudes still used today. 

Although largely lost to history, Hipparchus created a revolutionary star catalog employing equatorial celestial coordinates. His observations were accurate enough that he discovered the precession of equinoxes, the slow westward shift of stars along the ecliptic. Although he could not know that this effect was due to the 27,000-year wobble of the Earth’s axis, Hipparchus realized that any catalog would become outdated with time, and estimated the rate of precession at about 1° per century.

No celestial charts survive from the classical Greek era. But Hipparchus’ catalog may have been rediscovered recently in Naples, Italy on the Farnese Atlas, a Roman copy of a Greek statue with an accurately carved celestial globe. Noted astro-historian Bradley Schaefer has just published his analysis of the engraved coordinates of constellations and figures. Accounting for precession, he has determined that the source of the atlas dates from Hipparchus’ time and the latitude of Rhodes and excluded all other possiblities. This remarkable discovery will no doubt shed light on long-standing questions surround this pioneering astronomer and his work.

Unfortunately, aside from this recent detective work and a single, surviving commentary on an astronomical poem, none of Hipparchus' work has survived. What we do know of him is recounted in the Syntaxis or as it is more commonly known in Arabic, the Almagest. This text, possibly the most important in all of astronomy, was written some three centuries later by Claudius Ptolemy (ca. 85 to ca. 165 A.D.), a descendent of a Greek family, living in Alexandria, Egypt and a Roman citizen. Although there continues rancorous debate (see Sky & Telescope, May 1984, p. 421 and February 2002, p. 39) about the degree to which Ptolemy appropriated Hipparchus’s work as his own, it is Ptolemy’s catalog, organized by the 48 “ancient” constellations that has come down to us. Of the treatise's 15 books, the last five focus on the geocentric theory of the solar system most closely associated with the author. Ptolemy also published on celestial cartography, sundials, optics and geography.

These two Ancient Greek astronomers, Hipparchus and Ptolemy dominated astronomy for the nearly fifteen centuries and were not surpassed until Copernicus moved the Sun to the center of the solar system. While our understanding of the scale and physics of the universe has grown unimaginably since these times, perhaps the best of the Greek Astronomer’s legacy, ancient constellations, still pass above us essentially unchanged, on each clear night.

Moondark is written by Doug Miller, published online, and printed in the Delmarva Star Gazers' Star Gazer News. Last revised on 24 April 2005. Text and images are copyright © 2005 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.