Researchers who studied ancient Chinese chronicles of solar eclipses found that a day is now seven hundredths of a second longer than it was 4,000 years ago because the Earth is spinning more slowly. The day “just keeps getting longer and longer,” said Kevin Pang, an astronomer at NASA’s Jet Propulsion Laboratory. Just as a spinning skater slows down by extending the arms, Earth’s rotation slows as tidal interactions make the moon orbit Earth more quickly, he said.
“Four billion years ago, the moon was only one-third as far away as it is now, and the day was only eight hours long at the time.”
Pang determined that, compared to today, the length of a day was 22 thousandths of second shorter in A.D. 532, 42 thousandths of a second shorter in 899 B.C., and 70 thousandths of a second shorter in 1876 B.C. In the study, they were able to compute the time of the Nov. 13, A.D. 532, eclipse by studying records of the Wei dynasty, and of the Oct. 16, 1876 B.C., eclipse by analyzing records edited in 500 B.C. by Confucius.