A catalog of lost Greek stars decoded by a particle accelerator

Synchrotron radiation has revealed a map of the stars made by the ancient astronomer Hipparchus, which was thought lost to time.

By KR Callaway edited by Clara Moskowitz

Before telescopes, ancient Greek astronomers relied on naked-eye observations of the night sky to understand the universe around them. It was long thought that the careful catalog of stars belonging to one of the best of these observers, Hipparchus, was lost to time, but a hidden copy survived centuries. Erased and buried under layers of other texts in a medieval codex, the catalog was almost illegible – until now.

Researchers say they were finally able to decode some of the lost text using a type of particle accelerator called a synchrotron. They hope their analysis will shed light on the methods of early astronomers and how Hipparchus’ work influenced later scientists.

“As this catalog of stars is so important for understanding the birth of science, it made us want to do everything possible,” explains Victor Gysembergh, researcher at the National Center for Scientific Research (CNRS), who led the experiment. “What we saw was astonishing compared to previous images.”

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The researchers’ journey with the paper began in 2021, when they discovered names and measurements of constellations attributable to Hipparchus hidden under layers of other texts in the Codex Climaci Rescriptus, a palimpsest with parts dating from the 5th century CE to the 9th or 10th century.

The term “palimpsest” comes from the ancient Greek meaning “scratched again” and refers to a manuscript whose words have been erased and overwritten. Such erasure was a common practice throughout history to reuse expensive parchments, but it poses a unique challenge to researchers hoping to recover lost texts. For centuries, scientists have tried different lights and chemicals to revive erased texts. Modern imaging techniques using particle accelerators provide the best view yet.

The synchrotron used in the new experiment operates at the SLAC National Accelerator Laboratory in Menlo Park, California. accelerate charged particles at almost the speed of light and circulate them around a curved track. When the particles constantly change direction, they emit extremely bright X-ray beams. This light can penetrate deep into materials and create an in-depth x-ray image of an object.

This month, scientists shined the beams on the Codex Climaci Rescriptus. Light reacted differently to different inks used over the centuries: in some cases it was scattered and in others it was diffracted or absorbed. Newer inks on the upper layers of the palimpsest contained more iron, while those used to transcribe Hipparchus’ catalog a few hundred years earlier left a calcium-rich residue that researchers focused on using X-ray imaging.

“Fortunately, these documents were very well preserved and we saw some beautiful images and beautiful texts,” says Samuel Webb, senior scientist at the SLAC National Accelerator Laboratory’s Stanford Synchrotron Radiation Light Source.

Some analysis will have to wait until the new images can be processed, but researchers are already able to decode text from many raw data. “This is one of the rare examples of research where you know very quickly that you have obtained good results,” says Uwe Bergmann, a physics professor at the University of Wisconsin-Madison, who is overseeing the X-ray analysis of the experiment.

Once their analysis is complete, the researchers expect that Codex Climaci Rescriptus to be the most complete repository to date of Hipparchus’ observations. However, this is not our only vision of the astronomer’s work.

Although Hipparchus’s star catalog was lost, his comments associated with the work have been passed down through the ages, says Bradley Schaefer, an astronomy historian at Louisiana State University, who was not involved in the experiment. This commentary, along with the works of other authors who mention Hipparchus’s data and a Hipparchus star map accurately reproduced on a statue called the Farnese Atlas, have given scholars of classical astronomy a good idea of ​​Hipparchus’s astronomical information.

“The great promise of this SLAC idea is that, according to another page of this palimpsest, you may be able to recover substantial amounts of [previously unknown] text,” Schaefer says. He adds that the newly discovered pages could lead to valuable information that will tell us more about Hipparchus and his discoveries or dispel age-old questions about whether later renowned astronomers, such as Ptolemy, were making original observations or, in part, compiling the work of those who came before them.

With image processing and analysis by more researchers on the horizon, researchers involved in the synchrotron experiment hope their work will do more than shed light on ancient science hidden around the world. Codex Climaci Rescriptus. “The manuscript is exceptionally interesting,” says Gysembergh. “But it’s also an opportunity to launch more studies like this on more manuscripts.”