New Light on Eclipses : Scientists and Linguists Team Up to Interpret Early Chinese Accounts of Astronomical Events
“The sun rose at night,” Chinese philosopher Motze wrote in the 4th century BC in an account of an epic battle that had occurred about 1,500 years earlier.
Pasadena geologist Kevin Pang, paging through Chinese texts at the UCLA library, immediately realized that those five words were not poetic allusion to fiery combat, but rather a description of a total eclipse. For the ancient observers, the moon passing in front of the sun marked passage into night. The sun reemerging from the eclipse was thus a sunrise at night.
That discovery, coupled with computer simulations by Jet Propulsion Laboratory astronomer Kevin Yau, pinpointed the exact date of the battle--Sept. 24, 1912 BC--and how much the Earth’s spin has slowed down in the succeeding 39 centuries. The day is now 0.07 seconds longer.
Knowing the deceleration rate tells scientists how quickly the Earth is bouncing back into a rounder shape after having been squashed by giant polar ice caps during the last Ice Age.
“It’s a very interesting geophysical detective story,” said W. Richard Peltier, a University of Toronto physicist examining the post-Ice Age rebound.
Pang, a private consultant on satellite sensing issues, reported the latest results at a recent meeting of the American Geophysical Union in San Francisco.
The problem attracted Pang and Yau, along with UCLA East Asian languages professor Hung-Hsiang Chou, eight years ago. It is only within the last few decades that scientists have been able to directly measure the Earth’s rotation, so clues about what happened thousands of years ago must be deduced from imprecise and incomplete descriptions by people who did not understand what they were seeing.
“It’s one of the few fields that requires not only knowledge of geophysics, but also the ability to read ancient texts,” Pang said.
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Pang found his first clue in the “Bamboo Annals,” a book from the 4th century BC: “In the first year of Emperor Yi of the Western Zhou dynasty, the day dawned twice at Zheng,” the text said.
The find was “like you won the Lotto jackpot,” Pang said. The “double dawn,” he said, was “nothing more than a primitive person’s description of a sunrise followed by an eclipse.”
The motions of the Earth and the moon around the sun are well understood, so a computer program can easily trace back through history the dates of each solar eclipse--the passing of the moon between the sun and the Earth. The deceleration of the spin, however, shifted the location of the shadow on the Earth’s surface.
With the knowledge of where the central China city of Zheng once stood and the approximate years of Yi’s rule, Yau’s computer programs twiddled with the Earth’s rotation--sliding the eclipse path across the Earth until the moon’s shadow passed directly over Zheng. The eclipse, the computer concluded, occurred April 21, 899 BC.
In another ancient book, Pang found reference to a second eclipse at Zheng: “In the second year of Sheng Ping reign period of Emperor Shang, the day began twice at Zheng.” That sentence, however, was nonsensical. There is only one Emperor Shang in Chinese history, and he ruled only one year, AD 106.
“I beat my brain for a few days,” Pang said. Then he realized the scribe who copied the text had introduced two calligraphic errors, miswriting half of one character and omitting another. The passage should have said the 12th, not second, year of the reign of Emperor Xi, not Shang. Those corrections enabled the researchers to nail down the date of the second eclipse: April 4, AD 368.
These eclipses gave the researchers their first fix on how quickly the Earth’s rotation is slowing down. Friction from ocean tides sloshing around the Earth gradually brakes the planet’s spin and transfers the angular momentum to the moon which, in turn, is moving farther away. “It’s like a slingshot,” Pang said.
But the deceleration is not as pronounced as scientists once thought. The end of the last Ice Age triggered a phenomenon that offsets about one-third of the tidal forces.
When the two-mile-thick glacial sheets that blanketed much of North America and Europe melted away about 11,000 years ago, much of the pressure pushing down at the poles was released. Even today, areas such as Canada’s Hudson Bay are springing back more than half an inch a year, while those farther south that had bulged outward are sinking.
Just as bringing the arms closer to the body speeds up a spinning skater, the Earth’s shift in shape from slightly squashed to spherical pulls the mass closer to the center and accelerates the spin.
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According to Peltier, the deceleration rate “is extremely important,” providing scientists with a measure of the planet’s “squishability,” which in turn gives them a better idea of how the planet’s interior behaves.
For geologists, plugging the deceleration rate into the equations reveals that the Earth’s interior is “a thousand billion billion times the viscosity of water,” Peltier said.
To refine their results, Pang and his co-workers needed to find more eclipses. “If there are double dawns, there must be double sunsets,” Pang realized. Sure enough, Pang found what he was looking for in Motze’s “Condemnation of Offensive War”: “Anciently, the three Miao tribes were in great confusion. Heaven ordered their destruction. The sun rose at night. . . . Emperor Yu, founder of Xia, first dynasty, vanquished them.”
Pang found confirmation in the “Bamboo Annals”: “When the Miao perished . . . the sun disappeared and reappeared at night.”
Historians now know which autumn evening Emperor Yu’s armies closed in on the Prince of Miao near the Yangtze River. The exact dates of the eclipses should allow researchers to build a more accurate picture of China’s long history, which has been largely distilled from the ancient texts.
“The dating of those materials is very controversial,” said Chou, the East Asian languages professor. “That is the reason why Kevin’s work is so interesting and important--using modern science to prove the accuracy of ancient history and chronology.”
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Forces Affecting the Earth’s Rotation
1. The sloshing of the oceans caused by the moon’s tidal pull is gradually slowing down the Earth’s spin.
2. During the Ice Age, huge polar ice caps squashed the Earth. The melting of the ice caps 11,000 years ago released the pressure that had squeezed down on the planet. Like an ice skater who spins faster when the arms are brought in, the Earth tends to spin faster as it returns to a rounder shape.
3. The net effect of (1) and (2) is that the Earth’s rotation is still slowing, but not as much as scientists once thought. The day is now 0.07 seconds longer than it was 3,800 years ago- a fact that researchers have confirmed with the help of ancient Chinese accounts.
Researched by KENNETH CHANG / Los Angeles Times
