Total Solar Eclipses help us gain more knowledge about Ancient History: How?

Total solar eclipses, like the one that will occur on April 8th, occur at reliable times that we can calculate back to the past. This information helps researchers date events considered mysterious in ancient history.

eclipse solar total; IA
Archaeoastronomy uses the rare moments and locations of previous total solar eclipses to help us measure history.

In 648 BC, the Greek poet Archilochus wrote that "nothing can be surprising, impossible or miraculous anymore, now that Zeus, father of the Olympians, made it from noon to night, hiding the light from the shining sun".

Total solar eclipses have fascinated and terrified people for centuries. Currently, we know that total solar eclipses - such as the eclipse that will occur on April 8 - are caused by a cosmic coincidence, when the Moon interposes between the Earth and the Sun, momentarily blocking the view of the Sun. However, in the past the cause was unknown.

However, the peoples of those times left written reports. From all corners of the Earth, the stories of the day that turns into night or the sun that is consumed abound, and these records are opening a new branch of study.

Astroarchaeology - also called archaeoastronomy - uses astronomical records to help date important moments or events in history. Of all the astronomical phenomena, total solar eclipses are among the best measuring instruments, because they are only visible at a given time and place.

Total solar eclipses are rare enough that a certain point on Earth can only see one of 375 in 375 years (on average). And when an eclipse happens, it only appears as total for those who are along a narrow path on Earth.

Identification of dates of past eclipses

This combination of rare time and place helps researchers determine the exact date when ancient peoples saw an eclipse. Additional clues, such as the time of day when the eclipse occurred (morning, noon or night), the time of year (season) or the presence of bright planets can also help identify the exact eclipse.

For example, a record of a total solar eclipse that occurred near dawn in ancient Chinese texts relating to King Yi helped to identify the year in which his reign began.

One of the oldest eclipses recorded is on a clay board in the city of Ugarit, in present-day Syria. The city was knocked down shortly after the eclipse, which makes the board one of the last things written by someone in this city. The inscription on the board says: "... new moon day in ḫiyaru the Sun set, his guardian was [Rashap]."

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The word ḫiyaru refers to a time of year around February/March, and Rashap is probably a planet. With this information and with the knowledge that the city disappeared in the Bronze Age, the researchers dated the board and the eclipse of March 5, 1222 BC, more than 3000 years ago, with the planet Mars visible near the eclipsed sun. Thanks to this eclipse, we know that Ugarit fell shortly after March 5, 1222 BC.

Is it possible to accurately predict an eclipse?

To accurately predict future eclipses or trace the trajectories of historical eclipses, it is necessary to know the positions of the Sun, Moon and Earth. Computers can follow the movements of each one, but the challenge here is that these movements are not constant.

As the Moon influences the tides in the Earth's oceans, the process also causes the Moon to slowly move away from Earth and the length of the day on Earth slowly increase. Essentially, the length of a day on Earth is increasing by about 18 microseconds per year, or one second every 55,000 years. After hundreds or thousands of years, this fraction of a second per day corresponds to several hours.

The change in Earth day also affects the dating of historical eclipses - if the difference in the length of the day is not corrected, the calculations may be inaccurate by thousands of kilometers. As such, when eclipses are used to date historical events, a correction should be applied. Uncertainties in the correction can make the identifications of old eclipses more difficult to determine, in the absence of additional information that helps reduce the possibilities.

The change in the length of the day

By marking the eclipses of the last 2000 years, researchers mapped the length of the Earth's day over that same period. The value of 18 microseconds per year is an average, but sometimes the Earth slows down a little more and other times a little less.

Total solar eclipses allow us to observe not only our own history, but also the history of the Earth itself.

Currently, we can measure the change in the duration of a day on Earth with instruments, but we would not be able to capture that change hundreds or thousands of years ago in time without a precise measuring rod and records of eclipses over millennia and around the world. Total solar eclipses allow us to observe not only our own history, but also the history of the Earth itself.