"[The record] states that two court astrologers were beheaded because they didn't anticipate the total eclipse of the sun, and the emperor was mighty annoyed," said Sten Odenwald, an astronomer at NASA's Goddard Space Flight Center and Catholic University.
Some scholars suggest the scapegoat astrologers were not actual individuals but references to minor sun gods and that the tale of their demise is merely an allegorical reference to the eclipse.
Reading the Bones
Xi and He's tangled tale shows the sometimes confusing nature of the earliest Chinese eclipse records.
Beginning as far back as 2400 B.C., and especially during the Shang dynasty (1600 B.C. to 1046 B.C.)—a thousand years before the Chinese began to use paper—oracle bones were commonly used for divinations.
Questions were posed, and animal bones or shells were then heated until they cracked into patterns that expert diviners were believed to "read." The bones or shells were then inscribed with the interpretations and predictions.
Many astronomy-related inscriptions survive in these objects, but they are often cryptic and difficult to comprehend, sometimes lacking even the dates of the eclipses to which they refer.
"Because of the nature of the subject matter, oracle bones are not necessarily meant to be literal descriptions," Odenwald explained.
Accuracy aside, the bones are the earliest known evidence of an interest in tracking eclipses.
Ancient China's eclipse record keeping steadily improved over the centuries thanks to continued refinements in the calendar system driven by a search for signs that might tell the emperor's future.
Systematic, dated eclipse records began in China in 719 B.C.
Odenwald explains that in order to predict a total solar eclipse one has to understand the moon's orbit and be able to measure its path across the sky with great accuracy.
"There was not even a sense that the moon was an agent in causing this," Odenwald noted.
"They tended to think more mythologically, that there was a dragon taking a chomp out of the sun and that sort of thing," he said, referring to the ancients.
"It really wasn't until the first or second century A.D. that this kind of capability came into play."
Today astronomers can predict eclipses with great accuracy, and the ancient Chinese records—imperfect as they are—helped to make it possible.
Prediction models are created by entering data from known eclipses. Reliably dated and located events from the distant past help make the models much more accurate.
"There is a whole feedback between archaeology and modern solar eclipse prediction that enormously helps us in improving the precision of our current models," Odenwald explained.
"Accurately predicting future eclipses in the far future hinges on how well we can [pinpoint] past eclipses thousands of years ago."
Historic eclipse data also helps scientists get a handle on Earth's ever-lengthening days, caused mainly by ocean tides slowing the planet's spin rate.
"Over the last 2,500 years the length of the day has increased by [about] .0018 seconds each century," said F. Richard Stephenson, an astronomer at the University of Durham, in the U.K.
That might not sound like much, Stephenson says, but it adds up.
"About a million days have gone by since then," he said.
"If your watch is losing a second a day, put it in a drawer and pull it out after a year and it's lost about six minutes," he said. "It's the same sort of thing. In a million days, all those fractions add up."
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