Ancient Chinese eclipse records reveal something extraordinary about Earth’s rotation

An ancient Chinese chronicle of a solar eclipse in 709BC has revealed new details about how Earth's rotation has changed over thousands of years.

The eclipse, observed on 17 July 709BC, was recorded in the ancient Chinese “Spring and Autumn Annals”, a chronicle compiled about 200 to 300 years later.

During the eclipse, the Sun was “totally eclipsed”, according to observers at the court of the Lu Duchy, a vassal state during the Zhou dynasty.

Earth does not rotate at a constant rate. The planet’s rotation 2,700 years ago was faster than it is today.

Over hundreds and thousands of years, the planet’s spin has gradually slowed, partly due to ocean tides generated by the Moon's gravitational pull.

Initially, the historical account seemed to conflict with modern astronomy.

Previously, when astronomers calculated the eclipse's path based on estimates of Earth's rotation known at that time, their results indicated that a total eclipse should not have been visible from the ancient Chinese court.

Researchers thought this may have been due to a mistake in estimating the city's geographic coordinates.

But now, by correcting this geographical error using archaeological data, they could reconstruct the eclipse more accurately.

The findings reveal valuable information about Earth's rotation and the Sun's appearance, say scientists from Nagoya University in Japan.

"This correction allowed us to accurately measure the Earth's rotation during the total eclipse, calculate the orientation of the Sun's rotation axis, and simulate the corona's appearance," Dr Hayakawa explained.

"This new dataset fixes coordinate errors in previous Earth rotation studies. Additionally, it improves the accuracy of dating and reconstructing historical astronomical events," said Mitsuru Sôma, another author of the study from the National Astronomical Observatory of Japan.

The research also highlights the importance of astronomy in ancient China, where successive dynasties employed specialists to monitor celestial events since unusual phenomena in the sky were often interpreted as omens.

A later record of the eclipse in "Hanshu" (Book of Han), made seven centuries later, also has a passage with unusual details of the cosmic event that may even have referred to the Sun's corona.

“If this is truly the case, it represents one of the earliest surviving written descriptions of the solar corona,” said Hisashi Hayakawa, an author of the study published in The Astrophysical Journal Letters.

A historical account of the solar corona can help reconstruct the Sun’s activity at the time.

The Sun follows an activity cycle lasting approximately 11 years, during which the number of sunspots and the strength of solar magnetic activity rise and fall.

Occasionally, the Sun may enter long periods of reduced activity called "grand minima”.

For instance, the "Neo Assyrian Grand Minimum” lasted from approximately 808 to 717BCE.

The possible description of the solar corona in 709BC is consistent with a Sun that had resumed its regular activity cycles, researchers say.

Historical description of the corona’s structure suggests there was substantial magnetic activity on the Sun, indicating that it may have been near the peak of its approximately 11-year cycle at the time.

"When we combine their careful records with modern computational methods and historical evidence, we can potentially find new information about our planet and our star from thousands of years ago,” said study author Meng Jin from the Lockheed Martin Solar and Astrophysics Laboratory.