Analysis of historical records of solar eclipses reveals that the Earth’s rotation speed is erratic and does not fully correspond to established models. On July 19, 418, Constantinople (present-day Istanbul) witnessed a complete solar eclipse that, theoretically, was not due.
Analyzing the historical records of solar eclipses, an investigation of the University of Nagoya (Japan), led by Hisashi Hayakawahas established that, 1,600 years ago, the Earth rotated faster than previously thought, and that 1,400 years ago its rotation was much slower.
He has come to this conclusion from the places and times when Byzantine witnesses reported observing a total solar eclipse. The results of this research are published in the journal Publications of the Astronomical Society of the Pacific.
For much of human history, time has been measured in terms of the 24-hour day-night cycle (86,400 seconds). This cycle is based on the speed that our planet reaches when it rotates on its axis, it is very regular and practically does not register perceptible variations.
Current methods for measuring the Earth’s rotation are based on telescopes, atomic clocks, and laser measuring devices, which have only existed since modern times.
looking to the past
But, if we want to know the oldest rotation speed of the Earth, we use historical records of where solar eclipses were observed over time.
Taking into account that the location from which the solar eclipse can be viewed changes according to the rotation speed of the Earth, if there is a record that transmits the exact location, time and appearance of the observation, it is possible to determine the speed rotation of the Earth at that time in the past.
However, not all eras have reliable records of this information, stand out the researchers. In particular, there are very few records of total solar eclipses that can be examined between the 4th and 7th centuries AD. C., at the same time that there are many unknowns about what the rotation speed of the Earth was at that time.
The new study examined the historical record of total solar eclipses that occurred during the Byzantine empire, which existed between 330 and 1453 on the eastern coast of the Mediterranean.
This analysis has allowed researchers to clarify past changes in the Earth’s rotation rate: it suggests that the slowdown in Earth’s rotation rate may have been more erratic than previously thought.
Specifically, they analyzed the historical records of five solar eclipses that occurred between the 4th and 7th centuries, a time when the Earth’s rotation could only be calculated by approximations.
The team analyzed texts from the Byzantine Empire, which at the time covered the entire eastern Mediterranean. Specifically, they looked for references to eclipses that must have been visible in this region in the years 346, 418, 484, 601 and 693.
And they found enough records of the five solar eclipses, which allowed them to deduce what until now had not been possible: the rotation period of the Earth at those times.
Those records suggest that estimates of Earth’s rotation from the 5th century should be revised upwards, while those from the 6th and 7th centuries should be adjusted downward, the researchers explain.
And they give an example: the records indicate that on July 19, 418, Constantinople (present-day Istanbul) saw stars in the sky in broad daylight. That means that their people were watching the solar eclipse that we positively know took place that day.
However, based on the available model for Earth’s rotation at the time, Constantinople would have had to be outside the path of the eclipse, so the model needs to be adjusted, the researchers say.
They add that if we were to investigate more historical records of total solar eclipses, we could more accurately reconstruct the Earth’s long-term rate of rotation.
This information can be used not only to assess the credibility of past historical astronomical records, but also to provide information on long-term changes in global sea level, as well as changes in the internal structure of the Earth, changes in the polar ice caps , etc.
What’s going on?
It may also be useful for understanding global environmental changes related to changes in the Earth’s rotation rate that are occurring today, according to the researchers.
While in recent decades the planet’s average rotation speed has steadily decreased, today the picture is completely different: in 2020, the rotation of the Earth on its axis started to speed up, causing the shortest days since records are had. Now, the trend has reversed againaccording to atomic clocks, and promises to intensify in the long term.
Among the factors that directly impact the changes in the speed of the planetary spin, the influence of the Moon or the levels of erosion can be indicated, as well as the snow on the most important mountains on Earth.
In that sense, a study published in 2015 linked the change in the rotation of the Earth with global warming. If this relationship is confirmed, what will happen when global warming further reduces snow cover at high altitudes? How far could Earth’s rotation go in that case, and how will it affect predicted estimates of the visibility of upcoming solar eclipses?
The Variable Earth’s Rotation in the 4th–7th Centuries: New ΔT Constraints from Byzantine Eclipse Records. Hisashi Hayakawa et al. Publications of the Astronomical Society of the Pacific, Volume 134, Number 1039. DOI:https://doi.org/10.1088/1538-3873/ac6b56