Eclipses explained

How ancient solar eclipse records shed light on Earth’s rotation

Five eyewitness records of ancient solar eclipses in the fourth, fifth, and seventh centuries A.D. are helping scientists refine their knowledge of the changing speed of Earth’s rotation.

It’s a story about the value of Delta-T, the observed rotational speed of Earth measured against what an atomic clock says is the length of one day.

Delta-T is a critical figure in calculating where the shadow of the Moon will fall on Earth during an eclipse of the Sun. If scientists don’t precisely know the rotational speed of Earth, then the path of totality will fall slightly to the west or east of where it’s predicted to.

Conversely, a historical observation of a total solar eclipse—specifically of totality itself—is useful for scientists to check their mathematics for Earth’s changing rotational speed.

A new paper published in the Publications of the Astronomical Society of the Pacific studied documents from the Byzantine Empire. They discovered official records of solar eclipses around the eastern Mediterranean that were not previously known about. The value of Delta-T is therefore not known precisely for that period.

Why are these ancient solar eclipse records important?

Because the Earth is slowing down over a very long period, but also various factors can cause its rate of change to vary over shorter lengths of time.

Earth’s rotation is being slowed by tides, volcanic activity, the melting of ice caps, and everything else that causes friction on its surface.

“Although original eyewitness accounts from this period have mostly been lost, quotations, translations, etc. that were recorded by later generations provide valuable information,” said Koji Murata, co-author and Assistant Professor of the University of Tsukuba in Japan. “In addition to reliable location and timing information, we needed confirmation of eclipse totality.”

Details of the times and locations of five total solar eclipses in the years 346, 418, 484, 601, and 693 were uncovered. On July 19, 418 stars were seen in the sky during the day as seen from what is now Istanbul in Turkey. That total solar eclipse was already known about, but the value for Delta-T presumed for that period of Earth’s history has the path of totality slightly to the southwest, missing Istanbul.

“Our new Delta-T data fill a considerable gap and indicate that the Delta-T margin for the 5th century should be revised upward, whereas those for the 6th and 7th centuries should be revised downward,” said Murata.

Source: The Variable Earth’s Rotation in the 4th–7th Centuries: New ΔT Constraints from Byzantine Eclipse Records