The incredible precision of ancient astronomers

“The dioptra was a kind of surveying instrument,” says James Evans, physicist and historian of science at the University of Puget Sound in Washington State. It could be used “to measure angles in surveying operations, but it can very well be used to measure angles in the sky. The armillary sphere, whose name comes from the Latin armilla meaning circle or bracelet, is a sphere composed of concentric rings which could have been equipped with viewfinders. “We could very well modify it to measure angles. »

Hipparchus was probably influenced by the earlier work of Babylonian astronomers who measured the distances of certain constellations from the ecliptic. By following the movements of the constellations of the zodiac, i.e. the constellations located in the region of the celestial sphere traversed by the Sun during a year, the Babylonians could measure the seasons and predict astronomical events, in particular the eclipses.

As many consider him today, Hipparchus’ genius lay in combining Babylonian techniques for measuring and predicting stellar motions with Greek concepts of mathematics and geometry. “Modern astronomy was born from the fusion of these two approaches,” says Evans. “The Greek approach based on geometry and the philosophy of nature. The Babylonian approach based on regular observation and calculations. »

The newly discovered coordinates represent a tiny fraction of the 800 stars whose position would have been cataloged by Hipparchus. In total, only about ten coordinates attributable to Hipparchus have passed through the ages, but his work seems more accurate than Ptolemy’s later catalog. “You have to admit that the sample is thin, maybe there were errors elsewhere,” says Gysembergh. “But as it stands, it is more accurate than Ptolemy’s. »

In a new study, a team of researchers compared newly discovered coordinates to values ​​provided by other sources and the results agree, although there are some discrepancies, possibly due to differences in measurements or changes since the numbers were transcribed over the centuries.


The text mentioning the coordinates calculated by Hipparchus is a palimpsest, a parchment reused several times, with traces of erased writing still detectable. He is part of the Codex Climaci Rescriptusa set of parchments dating from the 10e or 11e century found in Saint Catherine’s Monastery in Egypt, showing scriptures in Syriac, an ancient West Asian language.

The astronomical content was detected in 2012, when the biblical Peter Williams and his students at Cambridge University were analyzing images of the Codex Climaci Rescriptus. One of them, Jamie Klair, noticed the Greek writing visible under the Syriac text. In 2017, the scrolls were photographed using the latest spectral imaging instruments to reveal the underlying text in more detail.

Some of these documents revealed fragments of Aratus’s poem, transcribed onto parchment that was later cleaned and reused for the Codex Climaci Rescriptus. The poem is accompanied by illustrations and mythological accounts of the constellations.

In 2021, while poring over the poem’s multispectral images during lockdown, our bible expert noticed numbers that he immediately interpreted as stellar coordinates. It turns out that these numbers are the dimensions of Corona Borealis and the coordinates of its outer stars, probably from the work of the first astronomer to attempt to map the sky.


Using a phenomenon called precession, the oscillation of the Earth on its axis of rotation, the researchers were able to determine that the coordinates corresponded to the positions of the stars of Corona Borealis seen from the island of Rhodes around 130 before our era, i.e. the place and time of most of the observations made by Hipparchus.

This is a detail that would have greatly amused Hipparchus, because he was also the first scientist to describe the movement of precession.

On the other hand, it’s a safe bet that the astronomer would gladly have done without appearing alongside the poet Aratus. The Phenomenons “was a best-seller of antiquity, a schoolhouse classic,” says Gysembergh, and the poem remained popular well into Roman times. For his part, Hipparchus was not crazy about it, and that is an understatement: the only text of the astronomer brought to our knowledge is a criticism of the poem by Aratus for its lack of precision in the description of the constellations.

“We have lost all of Hipparchus’ work, we only have fragments, except for a commentary on Aratus, because Aratus is still popular today,” says Francesca Schironia scholar of classicism at the University of Michigan who studied commentary on Phenomena.

This text, attributed to Hipparchus after the creation of his catalog of stars, also includes coordinates of certain constellations. The Latin version of the poem, titled Latin plowed and dating from the 8e century, also contains celestial coordinates, again from the work of Hipparchus according to the new study.

The study of other pages of the Codex Climaci Rescriptus and other palimpsests could lift the veil on new hidden passages. According to Gysembergh, the new celestial coordinates would have been part of a book containing Phenomenons and other texts, brought to Saint Catherine before being disassembled to reuse the parchment. While the medieval palimpsest only hints at the coordinates of a single constellation, the original book could have contained the coordinates of all the constellations in Aratus’s poem.

“The technique of multispectral imaging has only been applied to a tiny fraction of the palimpsests in our possession. There are still literally thousands and thousands of them to study. For many of them, the content is completely unknown,” says Gysembergh. “We can reasonably expect more discoveries of this kind in the years to come. »

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.