Jan. 19, 2005 — An ancient star catalogue lost for centuries has emerged from a Titan's shoulder, according to a study into a Roman statue of Atlas presented at the recent American Astronomical Society meeting in San Diego, Calif.  Known as the Farnese Atlas, the seven-foot-tall marble statue depicting the mythical Titan carrying the Earth on his shoulder is a Roman copy from the 2nd century A.D. of a Greek original dating to before the birth of Christ.  It is part of the Farnese Collection in the National Archeological Museum in Naples, Italy. The statue is peculiar as the two-foot-wide globe carried by a bearded Atlas crouched down on one knee is covered with 41 star constellations placed against a grid of circles, including the celestial equator, the tropics, the colures, the ecliptic, the Arctic Circle and the Antarctic Circle. The location of these constellations carved in marble are an accurate rendition of the lost star catalogue of Hipparchus, according to astronomer Bradley Schaefer of Louisiana State University in Baton Rouge. Dating to 129 B.C., Hipparchus' catalogue listed about 1,000 stars classified not only by location, but by magnitude, or brightness. The work was probably lost in the fire at the great library in Alexandria, but survived partly as a description in Ptolemy's "Almagest." Perhaps the greatest astronomer of antiquity, Hipparcus, who flourished around 140-125 B.C., is also credited with the discovery of precession, the slow motion of the stars and constellations across the sky with respect to the celestial equator, tropics and meridian lines. Indeed, following Hipparcus' idea of precession, Schaefer realized that the Atlas' marble globe referred to Hipparchus' star catalog. "Constellations positions shift slowly with the centuries due to precession, so the depicted positions (in the statue) provide a 'clock'," Schaefer said. The researcher measured the positions of 70 points in the constellation figures and dated the star positions on the statue to 125 B.C., plus or minus 55 years. The resulting date points directly to Hipparchus and strongly excludes all previously proposed candidates, ranging from an Assyrian observer around 1130 B.C., too early, and astronomer Ptolemy around 128 A.D., too late. Schaefer's astronomical analysis also compared the Farnese Atlas with all surviving ancient sources. "The comparison shows a virtually perfect match with the constellation descriptions of Hipparchus. In contrast, all other ancient sources differ profoundly from the Atlas," Schaefer wrote in a paper to be published by the Journal for the History of Astronomy. Moreover, the positioning of the constellation figures on the Farnese Atlas has a accuracy of about 2 degrees, impossible to be achieved by simple verbal descriptions. "This implies that the source was a star catalogue, and the only known star catalogues are those of Hipparchus and Ptolemy," said Schaefer. According to Schaefer, it is likely that the original Greek sculptor was not knowledgeable in astronomy, and needed some visual aid. "The obvious scenario is that Hipparchus constructed a small working globe based on his star catalogue. This globe was then used by the original Greek sculptor as a model for the constellation placement on a statue.

Finally, the later Roman sculptor used the now lost Greek statue to create the globe that is now in Naples," said Schaefer. The discovery of the lost star catalog will allow scholars to study Hipparchus' work deeper, making exhaustive correlations between all constellation figures on the Farnese Atlas and Ptolemy's Almagest. "There has long been a debate whether Hipparchus actually had a star catalog or at least what its nature was. Schaefer's work shows that evidence was there all the time, but it required the proper expertise to decipher it.," stated Owen Gingerich, professor of astronomy and history of science.