Byzantine records of solar eclipses have refined measurements of the Earth’s rotation

Records of solar eclipses a millennium and a half ago have allowed scientists to refine measurements of Earth’s changing rotation.

A careful review of historical documents from the Byzantine Empire has provided scientists with dates and locations for five solar eclipses. Although the results are consistent with previous findings, they place new, tighter constraints on Earth’s variable rate of rotation and give us a better understanding of how our planet is changing over time.

The length of a day seems to be a fairly reliable, unchanging metric. 24 hours in a day: 86,400 seconds. That’s what all our watches count for, day after day after day. This is the beat by which we live our lives. But it’s a bit of an illusion.

The speed at which our planet spins slows and speeds up in patterns influenced by a multitude of factors both beneath our feet and above us.

Consider the long-term trend that our days are gradually getting longer. Based on the fossil record, scientists have concluded that days 1.4 billion years ago were only 18 hours long, and 70 million years ago half an hour shorter than today. We seem to be gaining 1.8 milliseconds per century.

Then there are the strange six-year oscillations: Scientists have found that Earth’s days undergo time fluctuations of plus or minus 0.2 seconds about every six years.

A wobble in Earth’s spin axis appears capable of producing anomalies, such as a particularly short day recorded last year. Just something different.

From core activity to atmospheric resistance to the Moon’s expanding orbit, a number of factors can affect the actual length of Earth’s days.

The discrepancy between the accepted length of a day to which we all set our clocks (Universal Time or UT) and a standardized metric accurately counted by atomic clocks (Terrestrial Time or TT) — the most accurate timekeeping devices we have a measurement known as ΔT (Delta-T).

ΔT becomes really important when it comes to solar eclipses. This is because the positions of the Sun and Moon are calculated and predicted using TT, but the Moon’s shadow falls on a planet operating below UT. So you need to know the difference between the two times to predict where on Earth the eclipse will be visible.

But it also works the other way around! If you know the exact time and location of a solar eclipse, you can calculate ΔT. Scientists were able to calculate ΔT from historical records from China, Europe and the Middle East.

Three scholars, Hisashi Hayakawa from Nagoya University, Koji Murata from Tsukuba University, and Mitsuru Sôma from the National Astronomical Observatory of Japan, have now trawled through historical documents from the Byzantine Empire to do the same.

This is intended to close a significant gap: From the fourth to the seventh century AD there is a lack of solar eclipse records. It’s fiddly work. Often, for example, details relevant to modern studies have not been included in the records. But the researchers were able to locate five solar eclipses from records that hadn’t been previously analyzed.

“Although original eyewitness accounts from this period have largely been lost, citations, translations, etc. recorded by later generations provide valuable information,” says Murata.

“In addition to reliable location and timing information, we needed confirmation of the totality of the eclipse: daytime darkness to the extent that stars appeared in the sky. We have been able to identify the probable times and locations of five 4th- to 7th-century total solar eclipses in the eastern Mediterranean in AD 346, 418, 484, 601 and 693.

The values ​​for ΔT that the team was able to derive from these results were broadly consistent with previous estimates.

However, there were some surprises. From the account of the solar eclipse that occurred on July 19, 418 AD, researchers identified the observing location for the entirety of the eclipse as Constantinople.

The author, the historian Philostorgius, describes the eclipse: “As Theodosius [Emperor Theodosius II] adult, at about eight o’clock on July 19, the sun was so completely eclipsed that stars appeared.

Philostorgius lived in Constantinople from around AD 394 until his death around AD 439. It is therefore very likely that he observed the eclipse from there. The previous model for ΔT for this time would have placed Constantinople outside the eclipse totality path – so the record allowed the team to adjust ΔT for this time.

The other recordings also show slight adjustments.

“Our new ΔT data fills a significant gap and indicates that the ΔT margin for the 5th century should be revised upwards, while those for the 6th and 7th centuries should be revised downwards,” says Murata .

While the tweaks may seem minor, they have a significant impact. They impose tighter constraints on the variability of Earth’s rotation on centennial timescales and may inform future studies of other geophysical phenomena, such as modeling the planet’s interior and long-term sea-level changes.

The research was published in Publications of the Astronomical Society of the Pacific.