Byzantine solar eclipse records shed light on mysterious history of Earth’s rotation

Newswise – Tsukuba, Japan – Watching a total solar eclipse is an unforgettable and perhaps the most impressive experience in history before we could accurately understand and predict its occurrence. But the historical records of these remarkable astronomical sightings are more than just a curiosity – they provide invaluable information about changes in Earth’s motion.

In a new study in Publications of the Astronomical Society of the PacificJapanese researchers combed records from the Byzantine Empire to identify and determine the total solar eclipse observed around the eastern Mediterranean in the fourth and seventh centuries CE, a period when records of previously identified solar eclipses were particularly rare.

These records are essential to understanding the diversity of Earth’s rotation throughout history. However, because the people who recorded these events in antiquity often left out key information of interest to modern astronomers, determining the correct times, locations, and ranges of historical eclipses is hard work.

“Although the original eyewitness accounts from this period have often been lost, the quotations, translations, etc., recorded by later generations provide valuable information,” explains Associate Professor Koji Murata, Associate Professor at the University of Tsukuba. “In addition to reliable location and timing information, we needed to confirm total eclipses: daytime darkness to the point where stars appear in the sky. We were able to determine the likely times and locations of five total solar eclipses from the fourth to seventh centuries in the eastern Mediterranean region, at 346 and 418, 484, 601 and 693 AD.”

The main variable that this new information highlights is ΔT, the difference between time is measured according to the Earth’s rotation and time is measured independently of the Earth’s rotation. Thus, the differences in ΔT The differences represent the actual length of a day on Earth.

If we take the eclipse of July 19, 418 AD as an example, an ancient text reported a solar eclipse so complete that stars appeared in the sky, and the observation site was identified as Constantinople. Previous ΔT The model for this time would have put Constantinople out of the college course for this eclipse. Therefore, ΔT The fifth century AD can be modified based on this new information.

Our new ΔT The data fill a large gap and indicate that ΔT The margin of the fifth century should be revised upwards, while the margin of the sixth and seventh centuries should be revised downward”, says Dr. Morata.

This new data sheds light on the variation of Earth’s rotation on a centigrade time scale, and thus helps to better study other global phenomena throughout history, such as sea level and ice volume variability.

This work was supported in part financially by JSPS Grant-inAids JP15K05038, JP19K13389, JP20K22367, JP20K20918, JP20H05643, and JP21K13957, JSPS Overseas Challenge Program for Young Researchers, ISEE Leadership Fund for Financial Year 2021, Agriculture (Young Leader Program), and The YLC collaboration project of Nagoya University, and Tokai Pathways to Global Excellence (Nagoya University) for the Strategic Professional Development Program for Young Researchers (MEXT). We thank the Bibliothèque Nationale de France and the British Library for allowing us to access and reproduce MS Coislin 249 and MS Or 818.