Byzantine records of solar eclipses have refined measurements of Earth's rotation

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

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

A careful review of historical records from the Byzantine Empire has given scientists the times and locations of five solar eclipses. The results, while consistent with previous findings, place new, tighter constraints on Earth’s varying rotational speed, allowing us to better understand how our planet evolves over time.

The length of a day seems to be a fairly reliable and immutable measure. Twenty-four hours in a day: 86,400 seconds. That’s what all our clocks count, day after day after day. This is the pace at 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 variety of factors both below our feet and above our heads.

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

Then there are the weird six-year wobbles: Scientists have found that Earth’s days go through time swings of plus or minus 0.2 seconds every six years or so.

A wobble in the Earth’s axis of rotation appears to be able to produce anomalies, such as a particularly short day recorded last year. Just for something different.

From central activity to atmospheric drag to the expansion of the Moon’s orbit, a number of factors can influence the actual length of Earth’s days.

The discrepancy between the accepted length of a day to which we all set our watches (Universal Time or UT) and a standardized metric accurately counted by atomic clocks (Earth Time or TT) – the most accurate timekeeping devices that we have – is a measurement called ΔT (delta-T).

ΔT becomes very important when dealing with solar eclipses. This is because the Sun and Moon positions are calculated and predicted using TT, but the Moon’s shadow will fall on a planet operating under UT. So you need to know the difference between the two times in order to predict where on Earth the eclipse will be visible from.

But it also works in reverse! If you have the precise 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 scientists, Hisashi Hayakawa from the University of Nagoya, Koji Murata from the University of Tsukuba and Mitsuru Sôma from the National Astronomical Observatory of Japan, have now looked into historical documents from and the Byzantine Empire to make the same thing.

This is to fill an important gap: from the 4th to the 7th century AD, records of solar eclipses are rare. It is meticulous work. Often details relevant to modern studies have not been included in the records, for example. But the researchers were able to identify five solar eclipses from recordings that hadn’t been analyzed before.

“Although the original testimonies from this period have mostly been lost, the quotes, translations, etc., recorded by later generations provide valuable information,” Murata said.

“In addition to reliable location and timing information, we needed confirmation of the totality of the eclipse: daytime darkness as 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.”

For the most part, the ΔT values ​​the team was able to derive from these results were consistent with previous estimates.

However, there were a few surprises. From the account of the eclipse that took place on July 19, 418 CE, scholars have identified the site of observation of the totality of the eclipse as Constantinople.

The author, the historian Philostorgius, describes the eclipse: “When Theodosius [Emperor Theodosius II] had reached adolescence, on July 19 around the eighth hour, the Sun was so completely eclipsed that stars appeared.”

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

The other recordings also show slight adjustments.

“Our new ΔT data fills a considerable 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.

Although the adjustments may seem small, they have far-reaching implications. They impose tighter constraints on the variability of Earth’s rotation on centuries-scales and may inform future studies of other geophysical phenomena, such as modeling the planetary interior and long-term level changes. of the sea.

The research has been published in Publications of the Astronomical Society of the Pacific.

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