Solar eclipses in Byzantium helped to clarify the uneven rotation of the Earth
(ORDO NEWS) — Examining the Byzantine chronicles, scientists found references to total solar eclipses that occurred “not quite” in the calculated places. The analysis of deviations made it possible to clarify the non-uniformity of the Earth‘s rotation around its axis.
Solar eclipses are famous for their predictability. Thanks to astronomers, observers know exactly the location of the bands of total eclipses, which are usually only a couple of hundred kilometers wide.
Worth missing – and instead of the unforgettable spectacle of the night sky, which hangs a black disk framed by the solar corona, the observer will see a much less impressive partial solar eclipse.
The regularity of the movement of celestial bodies makes it possible to calculate solar eclipses for many hundreds of years forward and backward.
Astronomers know and calculate the positions of the planets, their satellites and spacecraft with fantastic accuracy – where does the uncertainty come from?
The point is the rotation of the Earth around its axis. We recently released detailed material about the various reasons for its unevenness, and here we will briefly talk about them.
There are no constants in nature, except for the fundamental physical constants and their derivatives – there is insufficient measurement accuracy.
With the advent of atomic clocks, the standard of time based on days had to be abandoned: their length is subject to both well-predictable regular shifts and chaotic fluctuations.
The first includes the influence of lunar tides , which increases the length of the day by 1.7 – 2.3 milliseconds per century (the value depends on the calculation method, some details can be found here ).
Chaotic fluctuations are caused by redistributions of mass relative to the axis of rotation of the Earth: air and ocean currents, seismic and many other phenomena.
If part of the mass moves away from the axis, for example, when the polar ice sheets melt, the Earth slows down its rotation, like a ballerina with outstretched arms, and vice versa.
The secular deceleration of the Earth’s rotation causes the Earth time to deviate from the universal time. The first is determined, in fact, by the earth’s rotation, and the second – by atomic clocks: there are always exactly 86400 reference seconds in a day.
This deviation grows in proportion to the square of the elapsed time, and in two thousand years it reaches about a couple of hours.
The total “incursion” from chaotic deviations also increases with increasing time interval – over long periods, the contribution of increasingly “slow” processes on the Earth’s surface and in its bowels becomes noticeable.
Historical evidence of total solar eclipses helps to reconstruct the history of the Earth’s daily rotation. The width of the lunar shadow is usually a couple of hundred kilometers, and the linear speed of rotation of the earth’s surface reaches almost half a kilometer per second.
If the Earth “does not turn back” by only ten minutes by the time of the eclipse, the entire band will shift away from the calculated position.
Exploring ancient and medieval evidence of solar eclipses, astronomers reconstructed the rotation of the Earth over the past three thousand years, but in this “chronicle” there was a gap of the early Middle Ages, from which too little chronicle evidence has been preserved.
In the year 1000, the total deviation was half an hour, at the turn of the new era – as much as three hours, and in the interval between these dates, thanks to the unknown contribution of random fluctuations, it could be an hour or even two.
Scientists from the University of Tsukuba, led by Hisashi Hayakawa, filled this gap by analyzing Byzantine chronicles from the fourth to the seventh century AD.
The full text of their work, with scans and transcripts of the original chronicles, is in the public domain. In the annals, it was possible to find evidence of total solar eclipses in 346, 418, 484, 601 and 693, and one of them became especially revealing.
Modern calculations predicted only a partial eclipse in the capital of Byzantium – Constantinople – in 418, but the chronicler Philostorgius claims that on that day it became dark as night, and stars were visible in the sky.
Other chronicles made it possible to recreate the full picture: in the fifth century, the deceleration of the Earth’s rotation slightly slowed down, and in the next two centuries, on the contrary, it accelerated. Thanks to this, the eclipse in Constantinople really turned out to be total.
Most likely, such long-term fluctuations in the rate of deceleration of the Earth’s rotation are associated with “slow” processes: the melting of glaciers or changes in the circulation regime of molten iron in the Earth’s core.
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