Being able to witness a solar eclipse is a once-in-a-lifetime type of experience. While a total eclipse takes place every 18 months or so, it only occurs in one place on Earth, and only the people in that spot can see it. But every 360 to 410 years, a total eclipse occurs that can be observed from anywhere on Earth. Over time, these phenomena have turned from curiosities into well-understood astronomical events. Not only are scientists able to predict when they will occur, but the total eclipses provide vital information regarding changes in Earth's rotation.
Japanese researchers conducted a study, which is published in Publications of the Astronomical Society of the Pacific, with the hopes of better understanding the variability of Earth’s rotation throughout history. They searched for records of total solar eclipses during the Byzantine Empire, a time when total solar eclipse records are few.
“These records are an essential reference for constraining and reconstructing Earth's variable rotation (ΔT) prior to the 17th century,” writes lead study author Hisashi Hayakawa.
“However, ΔT reconstructions for the 4th–7th centuries have significant uncertainties, mainly owing to a data scarcity. Here, we analyze Byzantine historical sources with reports of total solar eclipses along the Eastern Mediterranean coasts and add probable ΔT constraints on their basis.”
"Here, we analyze Byzantine historical sources with reports of total solar eclipses along the Eastern Mediterranean coasts and add probable ΔT constraints on their basis."
Hisashi Hayakawa
Due to the lack of key information in the records, modern astronomers and scientists have struggled to pinpoint the correct times and locations of these eclipses.
"Although original eyewitness accounts from this period have mostly been lost, quotations, translations, etc., recorded by later generations provide valuable information," assistant professor of the University of Tsukuba and study co-author Koji Murata explains. "In addition to reliable location and timing information, we needed confirmation of eclipse totality: daytime darkness to the extent that stars appeared in the sky. We were able to identify the probable times and locations of five total solar eclipses from the 4th to 7th centuries in the Eastern Mediterranean region, in 346, 418, 484, 601, and 693 CE."
What is ΔT?
ΔT is the difference between the time measured according to Earth’s rotations and the time independent of Earth's rotation. In other words, changes in ΔT represent changes in the length of a day on Earth.
"Our new ΔT data fill a considerable gap and indicate that the ΔT margin for the 5th century should be revised upward, whereas those for the 6th and 7th centuries should be revised downward," says Murata.
The information and data collected in this study pinpoint changes in Earth’s rotation.
“These new data improve our understanding of Earth's variable rotation on a centennial timescale and ultimately contribute to further geophysical discussions, such as the long-term variability of sea level, global ice volumes, and core-mantle coupling,” writes Hayakawa.