Today is seven hundredths of a second longer than a day in the year 1876 B.C., say scientists who studied ancient Chinese records of solar eclipses to learn how much Earth's rotation is slowing.
Just as a spinning ice skater slows down by extending her arms, Earth's rotation on its axis slows as tidal interactions make the moon orbit Earth more quickly and become more distant from the planet, astronomer Kevin Pang, of NASA's Jet Propulsion Laboratory, said this week."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," said Pang, whose study will be published soon in the British journal Vistas in Astronomy.
Pang and his co-authors 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.
Other studies have shown Earth's rotation varies slightly over time because the oceans and atmosphere produce drag on the planet's topography, and because molten rock within the Earth sloshes against solid rock to produce a similar drag.
Knowing how Earth's rotation rate changes "helps you understand things like the interaction between the ocean, solid Earth and atmosphere, and tells you something about the interior of the Earth," Pang said.
While ancient Chinese annals report thousands of eclipses, the researchers limited themselves to those that occurred at sunrise or sunset, allowing them to compute the time of the eclipses.
Knowing the times, along with the fact the eclipses were visible from China, the scientists were able to calculate how much Earth's rotation has slowed since the dates the eclipses occurred.
For example, had the day always been 24 hours long, the 899 B.C. eclipse would have been seen in the Middle East instead of China, Pang explained.
Pang's calculations of the rate at which Earth's rotation is slowing are consistent with previous studies, but extend further back in time. The oldest Arabian and Babylonian records of solar eclipses date to about 700 B.C.
Pang's findings on the 899 B.C. eclipse first were reported in early 1987. In that study, he and his colleagues studied ancient Chinese chronicles, called the Bamboo Annals, which mentioned a time when "the day dawned twice."
By performing a computer simulation of the history of the Earth's rotation around the sun, they determined the phrase meant that the moon eclipsed the sun just after dawn on April 21, 899 B.C. That let them figure out the Earth's rotation rate at the time and also place a date on the reign of King Yi of the Western Zhou dynasty.
In the new study, funded by the National Aeronautics and Space Administration, they were able to compute the time of the Nov. 13, 532 A.D. eclipse by studying records of the Wei dynasty, and of the Oct. 16, 1876 B.C. eclipse by analyzing the Xiang Shu, a book of records edited in about 500 B.C. by Confucius.
Co-authors of the study include Kevin Yau, a physicist at the University of Durham, England; Hung-hsiang Chou, an Asian languages professor at the University of California, Los Angeles; and Robert Wolff, a computer expert at Apple Computer Inc. in Cupertino, Calif.
Some people incorrectly assume Earth's gravity pulls the moon closer to Earth, Pang said. But the moon's gravity creates a tidal bulge in Earth's oceans. Because the Earth rotates on its axis, the bulge moves ahead of the moon as the moon orbits Earth. So gravity from the bulge exerts pull on the moon, speeding the moon in its orbit and transferring momentum that gradually pushes the moon further from Earth.
As for the length of a day, he said, "it just keeps getting longer and longer."