SLAC scientists use X-rays to search for the world’s oldest star map

At SLAC National Accelerator Laboratory in Menlo Park on Wednesday morning, researchers focused a powerful X-ray machine on pages of parchment from a medieval desert monastery, looking to reveal hidden traces of the world’s oldest star map.

The catalogue of stars was created more than 2,100 years ago by the Greek astronomer Hipparchus. Known only by mentions from other scientists of antiquity such as Pliny the Elder in the first century, scholars say it described more than 800 celestial bodies by brightness and their positions in the sky, and included illustrations of constellations. The original document has never been found.

But in recent years, sophisticated photo-based imaging of a parchment manuscript found at a monastery in Egypt’s Sinai Desert revealed portions of what scientists say is a high-quality copy of the map. They believe it was inscribed around the 6th Century but later overwritten with Christian text.

“I’m at the peak of my excitement right now,” lead scholar Victor Gysembergh of the French National Centre for Scientific Research said Wednesday morning as he watched imagery appear on a computer screen beside the lead-walled room where the accelerator’s X-ray beam probed one of the 22 pages.

“We just have line after line of text showing up, in ancient Greek from the astronomical manuscript. I’m hoping to see the first star coordinates pop up. We expect this to be a real game-changer. It will answer some big questions about how astronomy was born.”

The copied star map is believed to have been erased around the 10th Century, and the goat- or sheep-skin parchment pages — a valuable commodity in the medieval world — ended up at St. Catherine’s Monastery at the base of Mount Sinai, where monks overwrote them with Christian teachings.

In 2012, an undergraduate student working on a summer project at the University of Cambridge found among the 200 or so pages of the Christian manuscript — called the Codex Climaci Rescriptus — a faint passage in ancient Greek concerning astronomy.

“There did seem to be astronomical figures on those pages, but we didn’t know what they were,” said Keith Knox, an Early Manuscripts Electronic Library imaging scientist working on the Hipparchus project.

Five years later, sophisticated analysis of close-up camera imagery revealed more of the writings. Researchers compared the star coordinates on the pages to where those stars appeared earlier in human history — changes resulting from shifts in Earth’s rotational axis — and pinpointed the period of the map’s creation to Hipparchus’s time in the second century B.C.

References to Hipparchus’s work by later astronomers including Ptolemy reinforced the map’s connection to the pioneering Greek.

Now, a team of scientists is using the X-ray beams from SLAC’s synchrotron particle accelerator — the premier high-energy X-ray facility in the western U.S. — to bring back as much as possible of what they believe to be Hipparchus’s vanished map, made before telescopes existed.

Each of the 22 pages, about the size of those in a typical hardcover book, is inscribed on both sides with easily visible black and faded-brown words written by the monks. Also on both sides, and in a few places barely visible as smudges, is the star map.

“The coordinates we are finding are incredibly accurate for something that’s done with the naked eye,” Knox said.

The manuscript pages were transported to SLAC in a special, moisture-controlled case from the Museum of the Bible in Washington, D.C.

At SLAC, the researchers are mounting one page at a time in front of a steel nozzle that blasts them with X-rays from the accelerator. The beam is magnitudes stronger than a medical X-ray, and can rapidly identify minute traces of chemicals from ink that has been erased, said University of Wisconsin physics researcher Minhal Gardezi, part of the Hipparchus team.

Some of the pages were erased, overwritten by the monks and then erased and overwritten on one side again, creating up to six layers of ink, said Hipparchus team member Uwe Bergmann, a University of Wisconsin physics professor and former SLAC scientist. The apparent copy of Hipparchus’ catalog was written using ink made from tannin-rich oak galls, the common, often-round growths on oak trees triggered by larval wasps. By distinguishing between the chemical signatures of the monks’ ink and the star map’s, Hipparchus’s copied words can be brought into view through image processing, Gardezi said.

SLAC, one of the world’s leading facilities for uncovering the universe’s mysteries, has also become a leading center for analyzing historical documents using X-rays. In 2022, researchers fired particle accelerator beams at fragile pages from Germany’s Gutenberg Bible and Confucian texts from Korea, both from the mid-15th Century, with the Korean pages older. They were seeking to solve the mystery of whether the printing presses that created them were invented independently, or whether Johannes Gutenberg obtained one from Korea via traders plying the Silk Road from the East.

A paper describing the results of the printing press research is soon to be published, said Bergmann, who worked on the project. The findings may not solve the mystery completely, he said, but “the paper will make a strong suggestion.”

On Wednesday, as the SLAC X-ray machine continued to probe the ancient pages, Gysembergh watched with anticipation as Hipparchus’ description of the constellation Aquarius emerged.

“We have the word for Aquarius, we have the word for bright, so he’s talking about a bright star within Aquarius,” Gysembergh said. “It could be a section where the manuscript is counting how many stars there are in a constellation, which would be great. It could be a section where the manuscript is giving us star coordinates within the constellation, which would be exactly what we’re here for.”