Byzantine recordings of solar eclipses made it possible to refine measurements of the Earth’s rotation

(ORDO NEWS) — Recordings of solar eclipses from 1,500 years ago allowed scientists to refine measurements of changes in the Earth’s rotation.

Painstaking analysis of historical documents from the Byzantine Empire allowed scientists to determine the time and location of five solar eclipses.

The results, while consistent with previous findings, place new, tighter limits on the Earth’s variable rate of rotation, giving us a better understanding of how our planet changes over time.

The length of the day seems to be a fairly reliable, unchanging metric. Twenty-four hours in a day: 86,400 seconds. This is how many of our clocks count day after day. This is the rhythm in which we live. But this is a small illusion.

The speed of rotation of our planet slows down and accelerates under the influence of many factors, both underfoot and overhead.

Consider a long-term trend in which our days are slowly getting longer. Based on fossil remains, scientists have concluded that 1.4 billion years ago, the length of the day was only 18 hours, and 70 million years ago it was half an hour shorter than today. It looks like we’re getting 1.8 milliseconds per century.

And then there are the strange six-year fluctuations: scientists have found that the Earth’s day undergoes time fluctuations of plus or minus 0.2 seconds every six years or so.

It appears that the Earth’s wobble is capable of generating anomalies, such as the unusually short day seen last year. Just for something else.

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

The discrepancy between the generally accepted length of day that we all set clocks to (Universal Time, or UT) and the standardized metric accurately measured by atomic clocks (Terrestrial Time, or TT), the most accurate timekeeping instruments we have, is 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 will fall on a planet operating in UT.

Therefore, you need to know the difference between these two times in order to predict where the eclipse will be visible on Earth.

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

Now, three scientists, Hisashi Hayakawa of Nagoya University, Koji Murata of the University of Tsukuba, and Mitsuru Soma of Japan’s National Astronomical Observatory, have studied historical documents from the Byzantine Empire to do the same.

This is necessary in order to fill a significant gap: from the fourth to the seventh century AD, there is a meager list of records of solar eclipses.

This is hard work. Often the records do not include details that are important to current research. But the researchers were able to identify five solar eclipses from records that had not previously been analyzed.

“Although the original eyewitness accounts of this period are mostly lost, quotations, translations, etc. recorded by later generations provide valuable information,” says Murata.

“In addition to reliable position and time information, we needed confirmation of the eclipse’s totality: daytime darkness to the point where stars appeared in the sky.

We were able to determine the probable time and location of five total solar eclipses from the 4th to 7th centuries in the Eastern Mediterranean region – in 346, 418, 484, 601 and 693 AD”.

The ΔT values ​​that the team was able to deduce from these results were largely in line with previous estimates.

However, there were also some surprises. Based on the report of the eclipse that occurred on July 19, 418 AD, the researchers determined the place of observation of the totality of the eclipse as Constantinople.

The author, the historian Philostorgius, describes the eclipse in this way: “When Theodosius [Emperor Theodosius II] reached adolescence, on the nineteenth of July, at about the eighth hour, the Sun was so completely eclipsed that the stars appeared.”

Philostorgius lived in Constantinople from about 394 until his death in 439. Therefore, it is most likely that he observed the solar eclipse from there. The previous ΔT model for this time would have placed Constantinople outside the eclipse totality path – so the recording allowed the team to adjust the ΔT for this time.

Other entries also show minor adjustments.

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

While these changes may seem minor, they have significant implications. They impose tighter limits on the variability of the Earth’s rotation on centenary time scales and can serve as a basis for future studies of other geophysical phenomena, such as modeling of the planet’s interior and long-term sea level changes.


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