Records of solar eclipses from a thousand and a half years ago have allowed scientists to refine measurements of the Earth’s changing rotation.
A painstaking review of historical documents from the Byzantine Empire has given scholars the timings and locations of five solar eclipses. The results, while consistent with previous findings, place new and more stringent constraints on Earth’s changing rate of rotation, giving us a better understanding of how our planet has changed over time.
The length of the day seems to be a reliable metric that does not change. Twenty-four hours a day: 86,400 seconds. This is what all of our hours are counting day in and day out. This is the pulse in which we live our lives. But this is a kind of illusion.
Our planet’s rotation rate is slowing and accelerating in patterns influenced by a variety of factors, both underfoot and above.
Consider the long-term trend in which our days are gradually getting longer. Based on the fossil record, scientists have concluded that the days were fair 18 hours 1.4 billion years ago, and Half an hour shorter than it was today 70 million years ago. Looks like we win 1.8 milliseconds of a century.
Then there is the stranger Six-year fluctuations: Scientists have discovered that Earth days undergo time changes of more or less 0.2 seconds every six years or so.
The oscillation in the Earth’s rotation axis appears to be capable of causing anomalies, like a strangely short day registered last year. Just for something different.
From basic activity, to atmospheric drag, to the expansion orbit the moonThere are a number of factors that can affect the actual length of Earth days.
The discrepancy between the accepted length of the day that we all set our clocks to (Coordinated Universal Time, or UT) and the standard scale precisely calculated by atomic clocks (Terrestrial Time, or TT)—our most accurate timekeeping device—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 below UT. So you need to know the difference between the two times in order to predict where the eclipse will be visible on Earth.
But it also works in reverse! If you have the exact time and location of the solar eclipse, you can work out for ΔT. Scientists were able to discover ΔT from historical records from China, Europe, and the Middle East.
Three scientists, Hisashi Hayakawa of Nagoya University, Koji Murata of the University of Tsukuba, and Mitsuru Soma of the National Astronomical Observatory of Japan, have now researched historical documents from the Byzantine Empire and done the same.
This is to fill in a large gap: from the fourth to the seventh centuries CE, records of solar eclipses are scarce. It’s a silly job. Often details relevant to recent studies were not included in the records, eg. But researchers were able to identify five solar eclipses from previously unanalyzed records.
“Although the original eyewitness accounts from this period have been lost, the quotations, translations, etc., recorded by later generations provide valuable information,” Morata says.
“In addition to reliable location and timing information, we needed to confirm total eclipses: daytime darkness to the point where stars appear in the sky. We were able to determine the likely times and locations of five total solar eclipses from the fourth to seventh centuries in the eastern Mediterranean region, at 346 and 418, 484, 601 and 693 AD.”
To a large extent, the ΔT values that the team was able to derive from these results were in line with previous estimates.
However, there were some surprises. From the account of the eclipse that occurred on July 19, 418 CE, researchers locate the observation site of the total eclipse as Constantinople.
The author, historian Philostorgius, describes the eclipse: “When Theodosius [Emperor Theodosius II] I came into my teens, on the nineteenth of July at about eight, the sun was so smashed that the stars came out.”
Philostorgius lived in Constantinople from about 394 AD until his death, around 439 AD. So it is likely that he saw the solar eclipse from there. The previous model of ΔT for this time had put Constantinople out of the path of the totality of the eclipse – so the record allowed the team to adjust ΔT for this time.
Other records show minor modifications as well.
Our ‘new data’ fills in a significant gap and suggests that ‘the margin of the fifth century should be revised up, while those of the sixth and seventh centuries should be revised down’, Morata says.
Although the tweaks may seem minor, they have big implications. They place stricter constraints on the variability of Earth’s rotation on century time scales, and may inform future studies of other geophysical phenomena, such as modeling of the inner planets, and long-term sea-level changes.
The search was published in Publications of the Astronomical Society of the Pacific.