A rubbing of the Song Dynasty stone star chart at Suzhou – credit, public domain

Using modern digital rendering of ancient depictions, scientists have presented evidence that a Chinese astronomer created the first star catalogue more than 100 years before the Greeks accomplished the same.

Called the Star Manual of Master Shi, and complied by Shi Shen, it was likely compiled around 335 BCE, making it far older than that composed by Hipparchus of Nicaea.

Consisting of the names and coordinates of 120 stars, Shi’s star manual didn’t include a date of when the record was made. Master Shi used spherical coordinates similar to modern star charts to deign the positions of each star in the firmament, reports South China Morning Post. 

But there’s always been a problem with this method, and it’s called precession. Precession is the phenomenon of the Earth wobbling on its tilted axis, changing the positions of the stars relative to an earthborn viewer. These wobbles are extremely subtle, and precession takes 26,000 years to complete one cycle.

To use precession as a scale in measurements, one needs multiple data points spread out over millennia. Fortunately, the Chinese are a long-lived society, and a team of scientists from the National Astronomical Observatories under the Chinese Academy of Sciences leveraged algorithmic image-rendering techniques to compare Shi’s star manual with star charts made in the subsequent Tang and Yuan dynasties.

This gave a time period of around 1,300 years for the scientists to study. Authors of the analysis, Zhao Yongheng and He Boliang, created an algorithm based on the Hough transform—an image-processing technique primarily used for detecting geometric shapes in images.

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Placing the positions of the 120 stars in Master Shi’s catalogue and comparing them at 10,000 different moments with added references of the Tang and Yuan star catalogues, the algorithm used precession to place the stars on Master Shi’s manual at around 335 BCE—exactly during the years when Shi Shen lived.

A section of the Dunhuang Star Atlas from the Tang Dynasty – public domain

Further enhancing the success of their experiments, the algorithm technique demonstrated that Shi Shen’s star calculations were updated in the year 125 BCE by the Han Dynasty Grand Astronomer Zhang Heng, who calculated the positions of 2,500 stars and 124 constellations with the help of an armillary sphere, the first in the world, which he invented.

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Zhang led an effort to update Shi’s original work, Zhao and He propose, as positions from 59 of the stars from Zhang’s map could have been recorded during the time of Shi. The hypothesis connects two of ancient China’s most brilliant astronomers and settles any arguments about whether the Greeks or the Chinese first mapped the stars using coordinates.

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