Records of solar eclipses dating back a millennium and a half have allowed scientists to refine measurements of the Earth’s changing rotation.
A thorough review of historical documents from the Byzantine Empire has given scientists the times and locations for five solar eclipses. The results, while consistent with previous findings, put new and tighter constraints on Earth’s varying rotation rate, giving us a better understanding of how our planet is changing over time.
The length of a day seems to be a fairly reliable and invariant metric. Twenty-four hours 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 wishful thinking.
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.
Consider the long-term trend in which our days are getting longer and longer. From the fossil record, scientists have deduced that days lasted only 18 hours 1.4 billion years ago, and half an hour shorter than today 70 million years ago. It looks like we’re gaining 1.8 milliseconds per century.
Then there are the strange six-year oscillations: Scientists have discovered that Earth’s days undergo temporal variations of plus or minus 0.2 seconds every six years or so.
A wobble in the Earth’s rotation axis seems capable of producing anomalies, such as a peculiarly short day recorded last year. Just for something different.
From central activity, to atmospheric drag, to the Moon’s expanding orbit, several 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 clocks (universal time or UT) and a standardized metric accurately counted by atomic clocks (terrestrial time or TT), the most accurate timing devices we have, is a measure known as ΔT (delta-T).
ΔT becomes really important when dealing with 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 under UT. Therefore, you need to know the difference between the two times to be able to predict from where on Earth the eclipse will be visible.
But, it also works the other way around! If you have the exact time and location of a solar eclipse, you can calculate ΔT. Scientists have been able to determine ΔT from historical records from China, Europe and the Middle East.
Three scientists, Hisashi Hayakawa of the University of Nagoya, Koji Murata of the University of Tsukuba and Mitsuru Sôma of the National Astronomical Observatory of Japan, have now thoroughly examined historical documents of and from the Byzantine Empire to do the same.
This is to fill an important gap: from the 4th to the 7th centuries AD, there is a paucity of solar eclipse records. It’s a complicated job. Often, for example, details that are relevant to modern studies have not been included in the records. But the researchers were able to identify five solar eclipses from records that had not been previously analyzed.
“Although the original testimonies from this period have been mostly lost, quotations, 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 were able to identify the likely times and locations of five total solar eclipses from the 4th to the 7th centuries in the eastern Mediterranean region, AD 346, 418, 484, 601 and 693”.
To a large extent, the ΔT values the team was able to derive from these results were consistent with previous estimates.
However, there were some surprises. Based on the account of the eclipse that took place on July 19, 418 AD, researchers identified the observation site of the total eclipse as Constantinople.
The author, the historian Philostorgius, describes the eclipse: “When Theodosius [Emperor Theodosius II] had reached adolescence, on the nineteenth of July at about eight o’clock, the Sun was so completely eclipsed that the stars appeared.”
Philostorgius lived in Constantinople from 394 until his death in about 439 AD. Therefore, it is very likely that he saw the solar eclipse from there. The previous model for ΔT for this time would have placed Constantinople outside the eclipse’s path of totality, so the record has allowed the team to adjust ΔT for this time.
The other records also show slight adjustments.
“Our new ΔT data fill a considerable gap and indicate that the fifth-century ΔTmargin should be revised upward, while those of the sixth and seventh centuries should be revised downward,” says Murata.
While the tweaks may seem minor, they have considerable implications. They place tighter constraints on the variability of Earth’s rotation on century time scales and can inform future studies of other geophysical phenomena, such as modeling the planetary interior and long-term sea-level changes.
The research has been published in Publications of the Astronomical Society of the Pacific.