709 BCE Chinese Eclipse Record Sharpens Earth's Rotation Estimates

Key facts

• A total solar eclipse observed on 17 July 709 BCE at the court of the Lu Duchy, recorded in the Spring and Autumn Annals, is the earliest securely dated written account of a total solar eclipse and helped scientists refine estimates of Earth's historical rotation.
• Researchers corrected an approximately eight-kilometre error in the mapped location of ancient Qufu, improving reconstruction of the eclipse and establishing more accurate limits for Earth's rotation calculations.
• The revised reconstruction places Delta T (ΔT), the difference between Earth's rotational time and a uniform time scale, between 20,264 and 21,204 seconds for the 709 BCE eclipse.
• Earth's rotation has gradually slowed mainly due to tidal friction from the Moon's gravitational pull, meaning a day was slightly shorter around 2,700 years ago than today.
• The Hanshu (Book of Han) describes the eclipsed Sun as 'completely yellow above and below', possibly referring to the solar corona, though this interpretation remains uncertain.
• The study, published in The Astrophysical Journal Letters, was conducted by Hisashi Hayakawa of Nagoya University and Mitsuru Sôma of the National Astronomical Observatory of Japan.
• Researchers have considered whether the Sun was entering a more active phase after the extended low-activity period known as the Neo-Assyrian (Homer) Grand Minimum.