Technology may have revealed a piece of the long-lost works of Greek astronomer Hipparchus, one of the greatest astronomers of antiquity.
Sometime between 162 and 127 BC, the great Greek astronomer Hipparchus drew up the first star catalog of the western world, containing descriptions and coordinates of some 850 stars in the northern sky.
At least, that’s what later sources say — copies of Hipparchus’s list have never been found. Until now, that is. Writing in the Journal of the History of Astronomy, three European scientists claim to have uncovered a small part of the long-lost catalog. “I felt nothing short of awe when I first heard about it,” says astronomy historian and writer William Sheehan.
In the 2nd century AD, Claudius Ptolemy compiled a catalog of 1,025 stars in 48 constellations to include in his magnum opus, Almagest. Ptolemy also hinted at the existence of an earlier listing, some scholars have suggested that Ptolemy’s work was based not on his own observations but on Hipparchus’s catalog.
Hipparchus was born around 190 BC in Nicaea, in what is now northwestern Turkey, but he carried out most of his astronomical work on the island of Rhodes. He made the first estimates of the distances and sizes of the Moon and the Sun, and was the first to discover precession: the slow wobble in the orientation of the Earth’s spin axis. According to astronomy historian Bradley Schaefer (Louisiana State University), Hipparchus was “arguably the best and greatest astronomer in the world before Copernicus.”
“Only one of his many books, The Commentaries, has survived,” says Schaefer, “with the most important loss being his influential star catalog.”
That’s why scientists are elated about the new find. “It […] illuminates a crucial moment in the birth of science, when astronomers shifted from simply describing the patterns they saw in the sky to measuring and predicting them,” astronomy historian James Evans (University of Puget Sound) told Nature.
Peter Williams (Tyndale House, UK) and Victor Gysembergh and Emmanuel Zingg (Sorbonne University, France) studied the Codex Climaci Rescriptus, a 9th- or 10th-century manuscript from the Saint Catherine’s Monastery on the Sinai peninsula in Egypt. The codex, which is now kept at the Museum of the Bible in Washington, D.C., contains Christian texts in Syriac.
Beneath the visible text on one of the folios is a so-called palimpsest — the almost indiscernible imprint of an older text that medieval copyists had scraped off in order to re-use the costly parchment. By meticulously studying dozens of images made at various ultraviolet wavelengths, the scientists revealed the older text, which probably dates from the 5th or 6th century A.D.
The Greek palimpsest text describes the constellation Corona Borealis, the Northern Crown, providing information on its extent on the sky and on the coordinates of four of its stars (now labeled α, β, δ, and ι). These coordinates neatly agree with the stellar positions around the time Hipparchus drew up his catalog.
“The ‘style’ and coordinate system of the brief text in the palimpsest is similar to that used by Hipparchus in The Commentaries,” says Schaefer, “so the palimpsest might plausibly have a Hipparchan heritage.” However, he warns that this is “far from proven.” Still, most scientists appear to be convinced that part of the oldest star catalog in history has finally come to light. “It is one of the most thrilling things to happen in astronomy history for a while,” says Sheehan. “What else remains out there to be found out from one of the ancient palimpsests?”
The discovery shows that Hipparchus used equatorial coordinates, and that his measurements were accurate to within one degree. In contrast, Ptolemy’s catalog, compiled almost three centuries later, used ecliptic coordinates and was significantly less accurate. According to Williams, Gysembergh and Zingg, this implies that Ptolemy’s work was not based solely on data from Hipparchus.
“It’s a remarkable discovery,” says Sheehan. “It will force us to completely rethink our views about the ancient world, and in particular it shows that the cycles of discovery and science do not, as we like to think, follow a progressive line forward, and that even Ptolemy’s work represents something of a dark age compared to some of his predecessors.”