Catalog of Greek stars from early astronomy revealed

Surrounded by metal pipes and tangles of cables, two researchers show bright orange squiggles on a computer screen. The Scribbles is a poem written in ancient Greek about celestial phenomena, seen for the first time by human eyes in almost a millennium and a half.

“There is an appendix that includes the coordinates of the stars mentioned in the poem, as well as small sketches of the star maps,” says Minhal Allezi, a physicist at the University of Wisconsin-Madison.

Gardei is part of a team working at the SLAC National Accelerator Laboratory in Menlo Park, California, to discover these star maps. The maps come from a catalog created by the Greek astronomer Hipparchus of Nicaea around 150 BC and were copied during the 6th century AD. Transcribed on animal skin, the poem and the cards were then erased and overwritten by a new text. By exposing the skin to powerful X-rays from SLAC’s particle accelerator, the invisible writing is revealed again.

Direct knowledge of the ancient world is rare. Most Greek scholars wrote on papyrus, a material that rarely survives the centuries. Almost no writings of Hipparchus have been found, although second-hand sources indicate that he created one of the first star catalogs and helped invent trigonometry. The copy kept at SLAC represents a treasure for researchers hoping to better understand the birth of science more than 2,000 years ago.

The document measures approximately 18 by 21 centimeters, about the size of a paperback book, and is known as a palimpsest, a piece of parchment made from goat or sheep skin from which the original text has been scraped out and then written. This one in particular, called the Codex Climaci Rescriptuscomes from the monastery of Saint Catherine in the Egyptian desert of Sinai. In the 9th or 10th century, a scribe used the blank palimpsest – erased either by monks or someone before them – to record monastic treatises.

Even though the erased text is no longer visible to the naked eye, advanced imaging techniques had already partially revealed hidden writing. This is possible because chemical residue from the ink used in the original document penetrates the parchment, subtly changing the way the material absorbs light. By exposing these faint marks to different wavelengths of light – some within our visible range and others slightly beyond – parts of the erased text can be recovered.

To get the complete picture, the researchers projected concentrated, intense SLAC X-rays onto the manuscript, far beyond visible light and which can be a million times more powerful than those used in a dentist’s office, taking precautions to avoid damaging the material. X-rays excite the chemical elements in the ink, making them fluorescent. “You don’t see them, but they’re still there,” says Uwe Bergmann, also a physicist at UW-Madison. X-rays detected calcium signals in the old, hidden writings, which were more visible than in the new ones.

The first text of the palimpsest was the poem “Phenomena” by the Greek poet Aratus de Soli. Originally composed around 275 BC, it describes the rising and setting of different constellations. Whoever copied the poem onto the palimpsest – an unknown 6th-century scribe – also included appendix-like sections describing the positions of the stars in the constellations. Researchers know that these sections come from Hipparchus because their precision and distinct coordinate system match later descriptions of his work.

Gardei says it’s like an editor adding footnotes to a copy of Shakespeare’s “Hamlet” that “gives us fun facts, like a recipe for food that was eaten in the play.”

Having recovered some extracts, the team now plans to scan the remaining palimpsests of the codex. Computer algorithms will help further improve the writing and maps so the team can glean more data from these rare scribbles. The advanced imaging has so far helped settle a long-standing debate over whether the Roman-Egyptian astronomer Ptolemy, who lived in the 2nd century AD, plagiarized Hipparchus’ work. It turns out that Ptolemy’s star catalogs used Hipparchus’s as a reference, but also incorporated material from other scholars.

“It’s not plagiarism, it’s science,” says Victor Gysembergh, co-author of the study and historian of science at the CNRS in Paris. “We still do it today, combining sources to get the best possible data.”

Other researchers are eagerly waiting to see what additional secrets the palimpsests might hold. Previous team experiences revealed descriptions of the foundations of calculation — generally considered to have been invented in the late 1600s — in a copy of Archimedes’ writings from the 3rd century BC, says Graham George, a chemist at the University of Saskatchewan in Saskatoon, Canada, who was not involved in the work.

“Who knows what the study of star maps will show? he asks. “I can’t wait to find out.”