Using ancient Chinese records, scientists have calculated Earth’s variable rotational speed back to 709 BCE, based on the earliest datable total solar eclipse.
The team’s work sought evidence to support the recently developed reconstruction of the solar cycle stretching back to the 8th century BCE. To do so, they analyzed descriptions of the solar corona produced in the ancient Chinese Lu Duchy’s capital, Qufu, which provided new information about the Earth’s rotational speed over time, as revealed in a new paper in the Astrophysical Journal Letters.
Eclipse Records of the Lu Duchy Court
Written in the Spring and Autumn Annals of the Lu Duchy court, it is reported that on July 17, 709 BCE, “the Sun was totally eclipsed.”
Ancient Chinese rulers filled their courts with ancient astronomers, tasked not with understanding the physical nature of the universe but with interpreting the signs and portents the night skies might hold. Notable astronomical events, such as auroras and eclipses, were believed to provide insight into whether cosmic forces approved of an emperor’s decision-making. The scrutiny placed on interpreting the meaning of these events led China to keep the ancient world’s best astronomical records.
A later addendum to the record in the first century CE states that the eclipse “penetrated the center of the Sun, and it was completely yellow above and below.” However, there is no documented provenance to support the claim that this quote originated with a witness to the event.
“What makes this record special isn’t just its age, but also a later addendum in the ‘Hanshu’ (Book of Han) based on a quote written seven centuries after the eclipse,” explained lead author Hisashi Hayakawa, Assistant Professor from the Institute for Space-Earth Environmental Research and Institute for Advanced Research at Nagoya University.
“It describes the eclipsed Sun as ‘completely yellow above and below.’ This addendum has been traditionally associated with a record of a solar corona,” Hayakawa continued. “If this is truly the case, it represents one of the earliest surviving written descriptions of the solar corona.”
Locating Qufu from an Eclipse
The work not only revealed information about the Earth and Sun, but also about the geography of ancient China. Where the Lu Court at Qufu was believed to reside at the time would not have had the view of the eclipse described in the annals. The implication, therefore, is that earlier historical research obviously must have had the city’s location wrong.
Matching historical sources and modern archaeology to discern the actual location was a task unto itself. Eventually, the researchers pinpointed a new site some 8 kilometers from the previously purported location of Qufu. With his PhDs in both solar physics and Asian history, Hayakawa was uniquely suited to the task.
“This correction allowed us to measure the Earth’s rotation during the total eclipse accurately, calculate the orientation of the Sun’s rotation axis, and simulate the corona’s appearance,” Hayakawa said.
Our planet has not always rotated at the same speed. At the time of the observation, 2,700 years ago, due to a variety of factors, including the Moon’s pull on the tides, it rotated faster than it does today. By correlating the revised location of Qufu with the date, the researchers were able to accurately measure the Earth’s rotational speed between the 8th and 6th centuries BCE.
“This new dataset fixes coordinate errors in previous Earth rotation studies. Additionally, it improves the accuracy of dating and reconstructing historical astronomical events,” Mitsuru Sôma, coauthor from the National Astronomical Observatory of Japan, said.
“This unique historical addendum for the possible solar coronal structure is critical for providing a spot reference on solar activity reconstructions from tree rings and ice cores, as well as providing independent validation of solar activity models,” explained Mathew Owens, coauthor and professor of Space Physics at the University of Reading.
Solar Minimum
Like the rotation of our planet, solar activity has also been unsteady over time. According to radiocarbon analysis, the 8th century BCE was a period in which the Sun became more active, following a lengthy quiet period.
Tree rings serve as a natural record of solar activity because they absorb cosmic rays. The effects of these cosmic rays on tree rings can be measured at much later dates and correlated with radiocarbon dating to reveal a year and the degree of solar activity. While typical solar cycles are roughly 11 years in duration, from 808 to 717 BCE, the Neo Assyrian Minimum, also known as the “Homer Grand Minimum,” saw an extended period of low activity.
The quoted observation of the eclipse, therefore, aligns with what radiocarbon dating reveals, adding to its potential veracity. With the Sun emerging from a period of low activity and then reaching the peak of an 11-year cycle, an observer in 709 BCE would likely have witnessed a significant amount of yellow corona surrounding an eclipse.
“Some of our ancestors were very skilled observers,” said co-author Dr. Meng Jin, of the Lockheed Martin Solar and Astrophysics Laboratory. “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.”
The paper, “Analyses of the Ancient Chinese Report on the Total Solar Eclipse in 709 BCE: Implications for the Contemporaneous Earth’s Rotation Speed and Solar Cycles,” appeared in The Astrophysical Journal Letters on December 2, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.