A 2,700-Year-Old Eclipse Record Helps Measure Earth's Ancient Spin
A total eclipse seen in China in 709 BCE, once placed eight kilometres off on the map, is now helping scientists measure how Earth turned and what the Sun was doing.
On 17 July 709 BCE, people at the court of the Lu Duchy in ancient China watched the Sun vanish completely. Their account, kept in a chronicle called the Spring and Autumn Annals, is the earliest written record of a total solar eclipse that can be securely dated. More than 2,700 years later it is doing something unexpected: helping scientists work out how fast Earth was turning back then, and what the Sun was doing.
An eight-kilometre mistake
When researchers used modern astronomy to calculate where the eclipse should have been visible, the answer did not fit. Their sums said a total eclipse should not have been seen from Qufu, the city long assumed to be the site of the Lu court.
The team traced the problem to the city's coordinates. By going through archaeological excavation reports and historical geographic evidence, they found that earlier calculations had put ancient Qufu about eight kilometres from where it actually stood. Eight kilometres is a short drive. In eclipse terms it was enough to move the court out of the shadow.
What the correction made possible
Once the city sat in the right place, the shadow lined up. The team, led by Hisashi Hayakawa of Nagoya University, could then measure Earth's rotation at the moment of the eclipse, work out the orientation of the Sun's rotation axis, and simulate what the corona would have looked like. The corona is the Sun's faint outer atmosphere, which normally stays hidden and becomes visible to the naked eye only during a total eclipse. The findings were published in Astrophysical Journal Letters.
Earth does not spin at a perfectly constant rate. Its rotation has changed over long periods, so a careful record of where a shadow fell thousands of years ago is a useful check on the sums.
A Sun described as yellow above and below
The eclipse itself is only part of the story. A later book, the Hanshu, or Book of Han, preserves a passage written about seven centuries after the event. It describes the eclipsed Sun as "completely yellow above and below."
That odd phrase has traditionally been linked to the solar corona. If the link is right, it is one of the earliest surviving written descriptions of the corona. The shape it hints at suggests the Sun was returning to a more active phase after a long stretch of unusually low activity, and it offers possible independent support for earlier reconstructions of ancient solar activity based on radiocarbon measurements.
The researchers are careful here. The eclipse of 709 BCE is securely dated; the corona description is not, because it comes from a source written centuries later. So that reading remains uncertain.
The record survived because ancient Chinese courts watched the sky closely. Dynasties employed specialists to track celestial events, since unusual sights were often read as omens, sometimes as warnings that an emperor had behaved badly. Eclipses and auroras were written down for that reason, and those notes are now among the world's most valuable sources for studying eclipses of the distant past.