Ancient Chinese astronomer Shi Shen’s star catalog, known as The Star Manual of Master Shi, is now considered the oldest star chart in the world.
Later observations written into the catalog showed inconsistencies with Shi’s findings. This was because of Earth’s shift on its axis, or precession, which was corrected by an AI algorithm.
This catalog predates the second oldest chart—compiled by ancient Greek astronomer Hipparchus—by several hundred years.
When astronomer and astrologer Shi Shen gazed skyward thousands of years ago, he beheld what must have looked like another realm glittering in the heavens. Constellations were seen by the ancient Chinese as palaces or celestial mansions. The imperial court of advisers, princes, and concubines were mirrored in individual stars. In the event of a solar eclipse, a dragon swallowed the Sun.
Shi—who lived during the tumultuous Zhanguo or Warring States period—mapped the coordinates of 120 stars ranging from the Supreme Palace Enclosure of the Virgo, Coma Berenices, and Leo constellations to Polaris (the North Star), which represented the emperor himself. Eventually, the Star Manual of Master Shi, or Shi Shi Xing Jing, became a meticulous record of his observations.
For centuries, despite this document being the oldest surviving catalog of stars in China, the star catalog of ancient Greek astronomer Hipparchus (dated to 130 B.C.) was thought to be older. But ancient and modern worlds collided when a team of researchers at the Chinese National Astronomical Observatories (NAO)—led by Boliang He and Yongheng Zhao—used AI to date Shi’s catalog to the 4th century B.C. That makes it 200 years older than the Hipparchus catalog, and the earliest known surviving catalog of stars.
“Based on the records of precession and retrograde precession, it is inferred that the Shi’s Star Catalog approximately corresponds to the Zhanguo period, around 360 B.C.,” the researchers said in their study, which is awaiting peer review and has been uploaded to the preprint server arXiv.
Earth rotates on an axis that slowly shifts position—the gravity of the Sun and Moon pulling at the planet and causing tidal forces that create a bulge at the equator results in changes to the Earth’s rotation over time. This phenomenon is known as axial precession, and it affects how stars are observed from terra firma. Ancient celestial coordinates, as a result, eventually become outdated. When updates were made to Shi Shi Xing Jing by Han Dynasty astronomer Zhang Heng sometime around 125 A.D., axial precession meant that star positions were inconsistent with Shi’s original catalog.
To prove that precession was the reason for these inconsistencies, He, Zhao, and their team used an AI algorithm based on the Hough Transform, which can isolate features of shapes in an image, so long as those shapes are specified. Their version of the algorithm was designed to measure the difference in stellar positions due to precession. It ruled out errors made by later astronomers who copied coordinates (since the manuscript changed hands many times), and used statistics to estimate where the North Star—and therefore the stars surrounding it—must have been in relation to Earth at the time Shi mapped them.
The results dated more recent observations to the 2nd Century A.D., but the oldest traced back to Shi’s epoch. Still, there is some controversy over the findings, with skeptics such as historian Bosun Yang (from the University of Science and Technology of China) and Ziaochun Sun (from the Chinese Academy of Sciences) arguing that the spherical measurements suggests the use of an armillary sphere—an ancient Chinese invention used to measure the positions of stars. The armillary sphere did not yet exist when Shi was mapping celestial objects.
If Shi used another instrument that had been misaligned, Yang and others found that would push the catalog to about 103 B.C.—still older than Hipparchus. Zhao and He still insist that their findings are accurate, and plan to investigate more of Shi’s observations.
“[Our method] effectively mitigates the error issues encountered in previous methodologies, thus reliably resolving the challenge of determining the observation epochs of ancient Chinese stars,” they said. “The Shi’s Star Catalog predates even the oldest Western star catalogs, affirming its status as the oldest star catalog in the world.”
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