Solar eclipse archives show the Earth’s changing rotation

A solar eclipse is a rare occurrence that happens as the moon passes between the Earth and Sun, leaving many in awe. This phenomenon doesn’t happen quite often, especially a full solar eclipse.

Upcoming solar eclipse

We will actually be experiencing our next upcoming partial solar eclipse, a little less rare, on October 25 of this year. This eclipse will be visible in Europe, northern Africa and western Asia.

This leads to the question, what can we learn from ancient solar eclipse records?

Eclipses during the Byzantine Empire

Past records on solar eclipses reveal the change in rotation of the Earth over a short time frame. Researchers studied historical documents from around 1,500 years ago, and identified five total solar eclipses seen around the Eastern Mediterranean during that time, from the fourth to seventh centuries A.D. They pinpointed the probable times and locations of each eclipse. The five eclipses were in A.D. 346, 418, 484, 601 and 693.

Research and discoveries

The new study was published in The Publications of the Astronomical Society of the Pacific and it identified solar eclipses during a time period when it was previously difficult to pinpoint them. Researchers reviewed records during the Byzantine Empire so they could find total solar eclipse data around the Eastern Mediterranean in the fourth through seventh centuries. They used these documents to see how the Earth’s rotation has changed over thousands of years.

However, due to the historical nature of this study, some important details may have been left out by those who recorded the events many centuries ago, data that could be vital to present day astronomers, including accurate locations and times.

“For each case, we investigated the historical source texts to identify the most reliable reports and reproduce their original texts and provide their English translations,” the study mentioned. “From these records, we further identified the dates and sites of the reported solar eclipses, confirmed the eclipse totality with the appearance of stars, and computed their observational time following the descriptions and informants' locations.”

Accuracy within the study

There was a huge effort to make sure the study proved accurate, reliable and precise, despite the records being from many years ago.

“Although original eyewitness accounts from this period have mostly been lost, quotations, translations, etc., recorded by later generations provide valuable information,” said Koji Murata, assistant professor at the University of Tsukuba in Japan.

The key factor that this new study helps to explain is a formula for delta T, described as "the difference between time measured according to the Earth's rotation and time independent of the Earth's rotation. The variations in delta T represent the length of a day on Earth."

Such a detailed account of the ancient eclipses with this key variable allowed the researchers to give an example of an exact eclipse record from thousands of years ago. They identified one that took place on July 19, 418 in Constantinople, at that time the capital of the Roman Empire, and Istanbul now in modern-day Turkey.

Future global findings

The detailed information of the Earth’s rotation can help scientists learn more about other historical global occurrences, including changing sea levels and shifting volumes of glacial ice across the globe.

This study can also help researchers predict phenomena with exactness by using both the knowledge from the past and the learnt, revised information to foresee the future.