An international research team has revisited a precisely dated total solar eclipse observed in ancient China on July 17, 709 B.C.E., using historical geography and modern astronomical calculations to refine estimates of Earth’s rotation. The study also examines a later account that may describe the solar corona, suggesting that the Sun was emerging from a prolonged period of unusually low activity, although that interpretation remains uncertain.
Published in Astrophysics Journal Letter, the research provides more precise evidence about Earth’s rotation during the eighth to sixth centuries B.C. It may also offer an independent check on reconstructions of ancient solar activity based on radiocarbon measurements.
Ancient Chinese records preserve important eclipse clues
On July 17, 709 B.C.E., observers at the court of the Duchy of Lu witnessed a total solar eclipse. The event was recorded in the Spring and Autumn Annals, a historical chronicle compiled approximately 200 to 300 years later. Its original description stated that “the sun was completely eclipsed.”
Additional information appears in the Book of Han, which preserves passages written approximately seven centuries after the eclipse. One unusual description may refer to the solar corona, the Sun’s faint outer atmosphere that becomes visible to the naked eye during totality.
“What makes this record special is not only its age, but also a later addendum to the Book of Han, based on a quotation written seven centuries after the eclipse, which describes the eclipsed Sun as ‘completely yellow from top to bottom,’” said lead author Takashi Hayakawa, an assistant professor at Nagoya University’s Space-Earth Environmental Research Institute and Institute for Advanced Research.
“If this is true, this represents one of the oldest extant documents on the solar corona,” Hayakawa said.
At first, however, the historical account appeared to conflict with modern astronomical calculations. When the researchers used existing estimates of Earth’s ancient rotation to calculate the eclipse’s path, their results indicated that totality could not have been seen from the previously assumed location of Lu Gong in Qufu, the ancient capital of Lu.
The team traced the discrepancy to an error in the city’s geographic coordinates. By reviewing archaeological excavation reports and historical geographical evidence, the researchers determined that earlier calculations had placed the ancient town approximately eight kilometers from its actual location.
Correcting that relatively small error allowed the team to reconstruct the eclipse more accurately and extract information about both Earth’s rotation and the possible appearance of the corona.
“With this correction, we were able to accurately measure the Earth’s rotation during a total solar eclipse, calculate the orientation of the Sun’s axis of rotation, and simulate the appearance of the corona,” Hayakawa said. He holds a doctorate in solar physics and Eastern history.
Why ancient China recorded unusual celestial events
The survival of the observation reflects the importance of astronomy in ancient Chinese society. Unusual phenomena in the sky were often interpreted as omens, and successive dynasties employed experts to monitor celestial events.
This practice, known as omenology, was closely linked to political beliefs. Solar eclipses, aurora borealis and other unusual astronomical phenomena could be viewed as warnings that the emperor had behaved improperly.
Court astronomers therefore recorded changes in the sky carefully. Records preserved across successive dynasties have become some of the world’s most valuable historical sources for studying ancient eclipses and other astronomical events.
The researchers emphasize, however, that there is an important distinction between the original record of the 709 B.C.E. eclipse and the later descriptions of the corona.
The eclipse is considered the earliest clear written record of a total solar eclipse that can be reliably dated. It may also be associated with one of the earliest surviving descriptions of the solar corona, but that interpretation remains uncertain because the relevant passage appears in sources written centuries after the event.
Even without the corona interpretation, the reliably established timing of the eclipse provides a strong basis for investigating Earth’s historical rotation. The possible corona description may also offer a useful comparison with other studies of ancient solar activity.
How the eclipse revealed changes in Earth’s rotation
Earth does not rotate at a perfectly constant speed. Over long periods, its rotation has gradually slowed because of friction associated with ocean tides raised by the Moon’s gravitational pull.
As a result, Earth rotated slightly faster 2,700 years ago than it does today.
Using the revised position of the ancient town, the researchers obtained improved constraints on Earth’s rotation from the eighth through the sixth centuries B.C. Their calculations placed delta T, or ΔT, between 20,264 seconds and 21,204 seconds for the solar eclipse in 709 B.C.
Delta T is a measurement used to describe variations in Earth’s rotation over a given time scale.
“This new dataset corrects coordinate errors in previous studies of Earth’s rotation. It also improves the accuracy of dating and reconstruction of historical astronomical phenomena,” said co-author Mitsuru Soma of the National Astronomical Observatory of Japan.
Such measurements help scientists understand how the length of Earth’s day has changed over thousands of years. They can also improve calculations of when and where other ancient astronomical phenomena were visible.
Evidence that the Sun was emerging from a quiet period
Solar eclipses can reveal more than Earth’s rotational history. They can also preserve evidence of changes in the Sun’s activity.
Scientists have previously reconstructed ancient solar cycles by studying radiocarbon stored in tree rings. If the account from 709 B.C. does describe the corona, it could provide another way to test those reconstructions.
“This unique historical addendum on the possible structure of the Sun’s corona is important not only for providing a spot reference for reconstructions of solar activity from tree rings and ice cores, but also for providing independent validation of models of solar activity,” said co-author Matthew Owens, professor of astrophysics at the University of Reading.
Trees absorb carbon from the atmosphere during photosynthesis, including small amounts of radioactive carbon known as radiocarbon. Some of that material becomes trapped in tree rings, creating a natural record that researchers can study thousands of years later.
Radiocarbon levels are influenced by cosmic rays reaching Earth’s atmosphere. When the Sun becomes more active, its magnetic effects generally reduce the number of cosmic rays that reach Earth, changing the production of radiocarbon.
By measuring radiocarbon concentrations in successive tree rings, scientists can reconstruct changes in solar activity from periods long before modern instruments existed.
The Sun usually 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, however, the Sun enters a much longer period of reduced activity called a “grand minimum.” During such a period, sunspots become rare and the usual cycles become less pronounced.
One such interval, known as the “Neo-Assyrian Grand Minimum” or the “Homeric Grand Minimum,” lasted from about 808 to 717 B.C.
The 709 B.C.E. eclipse occurred shortly after this extended period of low activity ended. According to the researchers, the possible appearance of the corona is consistent with the Sun resuming its normal activity cycle. Its apparent structure suggests substantial magnetic activity and may indicate that the Sun was near the peak of its approximately 11-year cycle at the time.
This interpretation is consistent with reconstructions of early solar activity based on radiocarbon measurements from tree rings. However, it cannot be considered conclusive because of the uncertainties surrounding historical descriptions of the corona.
Ancient observations continue to inform modern science
The findings show how observations preserved for thousands of years can contribute to modern research. By combining historical documents, archaeological evidence, geographic studies and astronomical models, scientists can recover information that cannot be measured directly today.
In this case, correcting a location by only eight kilometers made the ancient eclipse record a more accurate source of information about Earth’s rotation and a possible window into the Sun’s distant past.
“Some of our ancestors were very skilled observers,” said co-author Dr. Meng Jing of the Lockheed Martin Solar and Astrophysical Institute. “Combining their careful records with modern computational techniques and historical evidence could uncover new information about the Earth and stars thousands of years ago.”
Source: www.sciencedaily.com