A medieval parchment from a monastery in Egypt yielded a surprising treasure. Hidden beneath Christian texts, researchers have discovered what appears to be part of astronomer Hipparchus’ long-lost catalog of stars – believed to be the first known attempt to map the entire sky.
Scholars have been searching for Hipparchus’ catalog for centuries. James Evans, a historian of astronomy at the University of Puget Sound in Tacoma, Washington, describes the discovery as “rare” and “remarkable”. The excerpt is published online this week in the astronomy history review1. Evans says it proves that Hipparchus, often considered Ancient Greece’s greatest astronomer, did map the heavens centuries before other known attempts. It also sheds light on a crucial moment in the birth of science, when astronomers moved from simply describing the patterns they saw in the sky to measuring and predicting them.
The manuscript originates from the Greek Orthodox St. Catherine’s Monastery in Egypt’s Sinai Peninsula, but most of its 146 leaves, or folios, now belong to the Museum of the Bible in Washington DC. The pages contain the Codex Climaci Rescriptus, a collection of Syriac texts written in the tenth or eleventh centuries. But the codex is a palimpsest: a scroll that has been scraped from an older text by the scribe so that it can be reused.
The older scripture was thought to contain other Christian texts, and in 2012 biblical scholar Peter Williams at the University of Cambridge, UK, asked his students to study the pages as part of a summer project. One of them, Jamie Klair, unexpectedly spotted a passage in Greek often attributed to the astronomer Eratosthenes. In 2017, the pages were reanalyzed using state-of-the-art multispectral imagery. Researchers at the Early Manuscripts Electronic Library in Rolling Hills Estates, California, and the University of Rochester in New York took 42 photographs of each page in different wavelengths of light and used computer algorithms to search for frequency combinations that enhanced hidden text.
Nine folios revealed astronomical material, which (according to radiocarbon dating and style of writing) was probably transcribed in the 5th or 6th century. It includes stellar origin myths from Eratosthenes and parts of a famous third century-BC titled poem Phenomena, which describes the constellations. Then, while reviewing footage during a coronavirus lockdown, Williams noticed something much more unusual. He alerted science historian Victor Gysembergh at the CNRS in Paris. “I was very excited from the start,” says Gysembergh. “It was immediately clear that we had star coordinates.”
The surviving passage, deciphered by Gysembergh and his colleague Emmanuel Zingg of the Sorbonne University in Paris, is about one page long. It shows the length and width in degrees of the constellation Corona Borealis, the northern corona, and gives the coordinates of the stars at its northern, southern, eastern and western extremes.
Several pieces of evidence point to Hipparchus as the source, starting with the idiosyncratic way in which some of the data is expressed. And, above all, the precision of the measurements of the former astronomer allowed the team to date the observations. The phenomenon of precession – in which the Earth slowly wobbles on its axis about one degree every 72 years – means that the position of fixed stars slowly shifts across the sky. The researchers were able to use it to check when the ancient astronomer must have made his observations and found that the coordinates corresponded to around 129 BC — at the time when Hipparchus was working.
Until now, says Evans, the only catalog of stars that had survived from antiquity was that compiled by astronomer Claudius Ptolemy in Alexandria, Egypt, in the second century. a d. His treatise Almagest, one of the most influential scientific texts in history, established a mathematical model of the cosmos – with the Earth at its center – that has been accepted for over 1,200 years. He also gave the coordinates and magnitudes of over 1,000 stars. However, it is repeatedly mentioned in ancient sources that the person who first measured the stars was Hipparchus, who worked on the Greek island of Rhodes three centuries before, approximately between 190 and 120 BC.
Location, location, location
Babylonian astronomers had previously measured the positions of certain stars around the zodiac, the constellations that lie along the ecliptic – the annual path of the Sun relative to “fixed” stars, seen from Earth. But Hipparchus was the first to define the locations of stars using two coordinates and to map stars throughout the sky. Among other things, it was Hipparchus himself who discovered the Earth’s precession, and he modeled the apparent motions of the Sun and Moon.
Gysembergh and his colleagues used the data they discovered to confirm that the coordinates of three other star constellations (Ursa Major, Ursa Minor, and Draco), in a medieval Latin manuscript known as the latin plowed, must also come directly from Hipparchus. “The new fragment makes that much, much clearer,” says Mathieu Ossendrijver, a historian of astronomy at the Free University of Berlin. “This catalog of stars that hovered in literature as an almost hypothetical thing has become very concrete.”
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Scholars believe that Hipparchus’ original list, like Ptolemy’s, would have included observations of nearly every star visible in the sky. Without a telescope, says Gysembergh, he had to use a sighting tube, called a dioptre, or a mechanism called an armillary sphere. “It represents countless hours of work.”
The relationship between Hipparchus and Ptolemy has always been troubled. Some scholars have suggested that Hipparchus’ catalog never existed. Others (starting with the 16th-century astronomer Tycho Brahe) have argued that Ptolemy stole Hipparchus’ data and claimed it as his own. “A lot of people think that Hipparchus was really the great discoverer,” says Gysembergh, while Ptolemy was “an incredible teacher” who compiled the work of his predecessors.
From the data in the fragments, the team concludes that Ptolemy did not simply copy Hipparchus’ ciphers. But perhaps he should have: Hipparchus’ observations seem to be much more precise, the coordinates read so far being correct to within a degree. And while Ptolemy based his coordinate system on the ecliptic, Hipparchus used the celestial equator, a system more common in modern star charts.
Birth of a domain
The discovery “enriches our image” of Hipparchus, says Evans. “It gives us a fascinating insight into what he actually did.” And in doing so, it sheds light on a key development in Western civilization, the “mathematization of nature”, in which researchers seeking to understand the Universe have moved from simply describing the patterns they have seen to objective to measure, calculate and predict.
Hipparchus was the central figure responsible for “transforming astronomy into a predictive science”, acknowledges Ossendrijver. In his only surviving work, Hipparchus criticized earlier astronomical writers for not caring about numerical precision in their views of celestial orbits and spheres.
It is believed that he was inspired by his contact with Babylonian astronomers and had access to centuries of precise observations. The Babylonians had no interest in modeling how the solar system was organized in three dimensions but, because of their belief in celestial omens, they made precise observations and developed mathematical methods to model and predict the timing of events such as lunar eclipses. With Hipparchus, this tradition merged with the Greek geometric approach, says Evans, and “modern astronomy really begins.”
The researchers hope that as imaging techniques improve, they will discover new coordinates of stars, giving them a larger data set to study. Several parts of the Codex Climaci Rescriptus have not yet been deciphered. It is also possible that additional pages of the star catalog survive in the St. Catherine’s Library, which contains over 160 palimpsests. Efforts to read them have already revealed previously unknown Greek medical texts, including medicine recipes, surgical instructions, and a guide to herbal medicines.
Beyond that, the multispectral imagery of palimpsests opens up a rich new vein of ancient texts in archives around the world. “In Europe alone, there are literally thousands of palimpsests in major libraries,” says Gysembergh. “This is just one very exciting case of a research possibility that can be applied to thousands of manuscripts with amazing discoveries every time.”
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