Part of lost star catalog of Hipparchus found lurking under medieval codex

The Greek astronomer Hipparchus is often called the “father of astronomy.” He’s credited with discovering the Earth’s precession (how it wobbles on its axis), and calculating the motions of the Sun and Moon, among other achievements. Hipparchus was also believed to be compiling a star catalog—perhaps the earliest known attempt to map the night sky to date—sometime between 162 and 127 BCE, based on references in historical texts.

Scholars have been searching for that catalog for centuries. Now, thanks to a technique called multispectral imaging, they have found what seems to be the first known Greek remnants of Hipparchus’ star catalog. It was hidden beneath Christian texts on medieval parchment, according to a new paper published in the Journal for the History of Astronomy.

Multispectral imaging is a method that takes visible images in blue, green, and red and combines them with an infrared image and an X-ray image of an object. This can reveal minute hints of pigment, as well as hidden drawings or writings underneath various layers of paint or ink. For instance, researchers have previously used the technique to reveal hidden text on four Dead Sea Scroll fragments previously believed to be blank. And last year, Swiss scientists used multispectral imaging to reconstruct photographic plates created by French physicist Gabriel Lippmann, who pioneered color photography and snagged the 1908 Nobel Prize in Physics for his efforts. The method corrected for distortions of color that occurred as a result of Lippmann’s technique.

The current paper arose from research into the Codex Climaci Rescriptus, a palimpsest that originated at Saint Catherine’s Monastery on the Sinai Peninsula in Egypt. It consists of 11 individual manuscripts, with Aramaic texts of the Old and New Testament and Greek text of the New Testament, among other content. Those texts have been dated to the 6th, 7th, and 8th centuries, respectively. The codex was kept at Westminster College in Cambridge until 2010, when Steve Green, president of Hobby Lobby, purchased it from Sotheby’s. It’s now part of the Green Collection on display in the Museum of the Bible in Washington, DC, although a few folios are stored elsewhere.

It was common practice at the time to scrape clean old parchment for reuse, and that’s what was done with the codex. Initially, scholars assumed the older writing was more Christian texts. But when Peter Williams, a biblical scholar at Cambridge University, asked his summer students to study the pages as a special project back in 2012, one of them identified a Greek passage by the astronomer Eratosthenes.

That warranted further investigation, so Williams turned to scientists at the Early Manuscripts Electronic Library in California and the University of Rochester in New York to conduct multispectral imaging of the pages in the codex in 2017. The technique revealed a full nine folios pertaining to astronomy, dating to between the 5th and 6th centuries—not just the Eratosthenes passage about star origin myths, but also a famous poem (Phaenomena, circa 3rd century BCE)  describing constellations.

Williams spent a good portion of his time during the pandemic lockdown studying the resulting images, and one day he noted what seemed to be the coordinates of the constellation Corona Borealis. He promptly contacted science historian Victor Gysembergh of CNRS in Paris about his discovery. “I was very excited from the beginning,” Gysembergh told Nature. “It was immediately clear we had star coordinates.”

Gysembergh and his colleague Emanuel Zingg of Sorbonne University translated the one-page passage as follows:

Corona Borealis, lying in the northern hemisphere, in length spans 9°¼ from the first degree of Scorpius to 10°¼ in the same zodiacal sign (i.e. in Scorpius). In breadth it spans 6°¾ from 49° from the North Pole to 55°¾.

Within it, the star (β CrB) to the West next to the bright one (α CrB) leads (i.e. is the first to rise), being at Scorpius 0.5°. The fourth star (ι CrB) to the East of the bright one (α CrB) is the last (i.e. to rise) [. . .]10 49° from the North Pole. Southernmost (δ CrB) is the third counting from the bright one (α CrB) towards the East, which is 55°¾ from the North Pole.

But could this passage be attributed to Hipparchus? While they are cautious about making a definitive attribution, the authors cite several pieces of evidence that seem to link the text to the Greek astronomer. For instance, some of the data are recorded in an unusual manner consistent with the only other surviving work of Hipparchus. And the authors were able to use astronomical charts to determine that the observations recorded in the text were probably made around 129 BCE, when Hipparchus would have been working on his catalog.

So far, only the coordinates for Corona Borealis have been recovered, but the researchers believe it’s quite likely Hipparchus mapped the entire night sky at some point, including all the visible stars—just like Ptolemy did in his later Almagest treatise. Many scholars believe Hipparchus’ catalog was one of the sources Ptolemy used when compiling his treatise.

In fact, Williams et al. found that Hipparchus’ calculations of coordinates were actually much more accurate than Ptolemy’s—correct to within one degree. This was an astonishing feat, given that the telescope had not yet been invented. They surmise Hipparchus probably used a sighting tube called a dioptra or an armillary sphere to make his calculations. And they hope that other portions of the lost star catalog might yet be found lurking in the monastery’s library as imaging techniques continue to improve.

DOI: Journal for the History of Astronomy, 2022. 10.1038/d41586-022-03296-1  (About DOIs).

Listing image by Museum of the Bible, 2021/CC BY-SA 4.0