An international research team has reconstructed a total solar eclipse from 709 BCE using ancient records from China’s Lu Duchy to refine measurements of Earth’s historical rotation. Published in Astrophysical Journal Letters, the findings indicate the Sun was transitioning out of a period of low activity known as the Homer Grand Minimum.
Ancient Chinese Annals Pinpoint the 709 BCE Eclipse
On July 17, 709 BCE, court observers in the Lu Duchy witnessed a total solar eclipse. This event, documented in the Spring and Autumn Annals,
serves as one of the oldest precisely dated records of a total solar eclipse. The original chronicle notes only that the Sun was totally eclipsed.
A later account in the Hanshu (Book of Han) includes a description that researchers believe may refer to the solar corona.
By combining historical data with modern astronomical calculations, the research team corrected the geographic positioning of the ancient capital, Qufu, by eight kilometers.
Calculating Changes in Earth’s Rotation
Earth does not maintain a constant rotation rate. Its spin has gradually slowed over millennia due to friction from ocean tides caused by the Moon’s gravitational pull.
Because Earth rotated faster 2,700 years ago than it does today, precise dating of historical eclipses is essential for mapping these variations. The researchers determined that the value for delta T (ΔT)—a measure of Earth’s rotation relative to a uniform time scale—fell between 20,264 and 21,204 seconds during the 709 BCE event.
Coauthor Mitsuru Sôma of the National Astronomical Observatory of Japan stated, This new dataset fixes coordinate errors in previous Earth rotation studies. Additionally, it improves the accuracy of dating and reconstructing historical astronomical events.
Reconstructing Solar Activity After the Homer Grand Minimum
The possible description of the corona provides a rare window into solar behavior during the 8th century BCE. The Sun had recently emerged from the Homer Grand Minimum,
a period of unusually low activity lasting from approximately 808 to 717 BCE, during which sunspots were rare.
The researchers suggest that the coronal structure observed in 709 BCE indicates the Sun had resumed regular magnetic activity. It may have been near the peak of an 11-year cycle.
The team emphasizes that the corona account requires caution. This is due to the centuries-long gap between the event and the later Hanshu description.
Still, the findings align with radiocarbon data preserved in tree rings. As coauthor Dr. Meng Jin 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.
Jin works at the Lockheed Martin Solar and Astrophysics Laboratory.