Historical geography helps researchers solve 2,700-year old eclipse mystery

(Press-News.org) An international team of researchers has used knowledge of historical geography to reexamine the earliest datable total solar eclipse record known to the scientific community, enabling accurate measurements of Earth's variable rotation speed from 709 BCE. The researchers calculated how the Sun would have appeared from Qufu, the ancient Chinese capital of the Lu Duchy, during the total solar eclipse. Using this information, they analyzed the ancient description of what has been considered the solar corona—the dim outer atmosphere of the Sun visible to the naked eye only during total eclipses—and found that its morphology supports recent solar cycle reconstructions for the 8th century BCE. 

Their findings, published in Astrophysical Journal Letters, provide reliable new data about Earth's rotation speed during this period and suggest the Sun was becoming more active after a long quiet period, independently confirming what other scientists have found using radiocarbon analysis. 
 

Finding the true location of an ancient capital 

The total solar eclipse occurred on 17 July 709 BCE and was reported from Lu Duchy Court. Its description was found in a chronicle titled the “Spring and Autumn Annals” that was compiled roughly 2-3 centuries after the eclipse. The event was recorded as “the Sun was totally eclipsed.”  

“What makes this record special isn’t just its age, but also a later addendum in the ‘Hanshu’ (Book of Han) based on a quote written seven centuries after the eclipse. It describes the eclipsed Sun as ‘completely yellow above and below.’ This addendum has been traditionally associated with a record of a solar corona. If this is truly the case, it represents one of the earliest surviving written descriptions of the solar corona,” lead author Hisashi Hayakawa, Assistant Professor from the Institute for Space-Earth Environmental Research and Institute for Advanced Research at Nagoya University explained. 

When the researchers tried to verify the solar eclipse record using modern astronomical calculations and reconstructions of Earth’s rotation speed, they found that a total eclipse would not have been visible from the Lu Court at Qufu. They realized earlier studies had missed the exact location of the ancient city. 

To correct the coordinates of ancient Qufu the researchers used knowledge of historical geography, consulting archaeological excavation reports of the ancient city. They found that previous studies used coordinates that were some eight kilometers away from the true location.  

"This correction allowed us to accurately measure the Earth's rotation during the total eclipse, calculate the orientation of the Sun’s rotation axis, and simulate the corona’s appearance," explained Hayakawa, who holds PhDs in both solar physics and oriental history. 

China developed exceptional traditions for astronomical records because ancient dynasties hired experts to monitor celestial events for omenological reasons—the practice of interpreting celestial events as omens or signs. They believed strange sky phenomena indicated political wrongdoing by emperors, which motivated careful tracking of eclipses, auroras, and other astronomical events. Consequently, this systematic record-keeping across multiple dynasties has given China some of the world’s best ancient eclipse records. 

While the 709 BCE event represents the earliest explicit written mention of a total solar eclipse and possibly the earliest surviving description that refers to a solar corona, Hayakawa and his colleagues raise caveats on the reliability of the corona description because it appears only in the Hanshu as a quote written some seven centuries after the event. Although questions remain about the reliability of the later corona description, the eclipse timing itself is based on scientific consensus and provides reliable new information on Earth’s rotation and potential independent support for recent solar cycle studies. 

 

When Earth spun faster and the Sun was quieter 

Our planet spins slightly slower now than it did 2,700 years ago because of several factors including friction from ocean tides caused by the Moon’s gravity. Using the corrected coordinates, the team derived new accurate measurements of Earth’s rotation speed in the 8th to 6th centuries BCE.  

The study revealed that delta T (ΔT), a parameter for the Earth’s rotation speed variability, during this eclipse was between 20,264 and 21,204 seconds. “This new dataset fixes coordinate errors in previous Earth rotation studies. Additionally, it improves the accuracy of dating and reconstructing historical astronomical events,” Mitsuru Sôma, coauthor from the National Astronomical Observatory of Japan said. 

The research also supports recent solar cycle studies based on radiocarbon data from tree rings. “This unique historical addendum for the possible solar coronal structure is critical for providing a spot reference on solar activity reconstructions from tree rings and ice cores, as well as providing independent validation of solar activity models,” Mathew Owens, coauthor and professor of Space Physics at the University of Reading explained.  

During photosynthesis, trees absorb carbon, including radiocarbon, which is stored in their annual growth rings. Because radiocarbon concentrations reflect past cosmic ray levels and cosmic rays decrease when solar activity increases, scientists measure these concentrations to track solar activity over time and reconstruct past solar cycles.  

Approximately every 11 years the Sun cycles between more-active and less-active phases. Sometimes this pattern is interrupted by longer quiet periods called “grand minima” when the Sun produces only stray sunspots. The eclipse occurred just after the end of a period of decreased solar activity known as the “Neo Assyrian Grand Minimum” or “Homer Grand Minimum,” lasting from 808 to 717 BCE.  

Morphologically, the ancient observers' descriptions of the probable coronal structure suggest the Sun had come back to regular solar cycles with substantial magnetic activity by 709 BCE and reached the peak of its 11-year cycle. This result supports what other scientists have reconstructed using tree ring data. 

This interdisciplinary study demonstrates how ancient human observations continue to provide invaluable scientific data. “Some of our ancestors were very skilled observers,” Dr. Meng Jin, coauthor from the Lockheed Martin Solar and Astrophysics Laboratory, noted. “When we combine their careful records with modern computational methods and historical evidence, we can potentially find new information about our planet and our star from thousands of years ago.”  

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