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Before the eclipse this past Monday, Marjorie Taylor Green post the following on X (formerly Twitter):
The replies were pretty brutal, in particular pointing out that we’ve known for decades when and where the eclipse would occur. How effective is a “sign from God” if you know when it’s going to happen? Seems to me that the element of surprise one of the more powerful aspects of an omen. The eclipse was a dead giveaway.
I took this as a good excuse for a quick once-over of the history of predicting eclipses. Spoiler alert: Babylonians had figured out a rough gauge for the repetition of eclipses sometime in the 6th Century BCE. While I’m sure this is not common knowledge, and nowhere near MTG’s forte (which seems to be screaming like a crazy person about fake issues), if the ancient Babylonians had the ability to roughly predict the repeat cycle for solar eclipses, that information is available to all modern people.
Nonetheless, solving this problem in ancient times was an exceptional feat. It required careful record keeping, and fairly precise measurement of the relative locations of the Sun and the Moon in the sky over long periods of time. In modern terms, the question comes down to three different periods of revolution regarding the Moon’s orbit. The first one is the most familiar, that is the period from one new Moon to the next. (After all, it’s the new Moon that eclipses the Sun.) This is just the period of the lunar cycle, which is about 29.53 days. (Babylonians probably didn’t know this period to this level of precision.) To describe the second cycle, it’s necessary to recognize that the orbit of the Moon around the Earth is not in the same plane as the orbit of the Earth around the Sun. If they were in the same plane, we’d have an eclipse every month. Instead, the Moon’s orbit is canted by about 5 degrees from the plane of Earth’s orbit around the Sun (called the ecliptic). The Moon can only block the Sun in an eclipse if its orbit sends it through the ecliptic at the moment that it becomes a new Moon. The period for when the Moon passes through the ecliptic at the same point is 27.21 days. Finally, there is the period that it takes for the Moon to traverse its elliptical orbit, to return to its original position: 27.55 days.
[You might wonder how it’s possible for, say, the first period to be different from the third period. Isn’t the Moon in the same place in it’s orbit from one new Moon to the next? The answer is no because in that period, the Earth has moved relative to the Sun, so the Moon has to travel further in its orbit to coincide with the Sun’s location in the sky from month to month. A similar argument applies to differences with the second cycle.]
The repeat period for a solar eclipse, then, is just when a whole number of each of these cycles (each of them different) occurs such that the relative positions of the Sun and Moon are the same (i. e. in the same spot in the sky as seen from the Earth). This corresponds to 6,585 days, which is a little more than 18 years. This period was dubbed by astronomer Edmund Halley (of Halley’s Comment fame) a “Saros,” which is Sumerian for “big number.” The Antikythera mechanism, an analog computer that predicted the locations of the Sun, Moon and known planets created by the ancient Greeks, incorporated the cycle for solar eclipses discovered by the Babylonians.
While the Babylonians were able to expect eclipses every 18 years (plus a few days), they couldn’t determine where the eclipse would occur. That calculation only occurred after Isaac Newton created classical physics and his universal theory of gravitation. Using these, in 1715, Edmund Halley was able to show the portions of Britain (including London) to be affected when an eclipse occurred on April 22 of that year. At that point, eclipses would have been seen as just a consequence of the celestial clockwork. While the experience of an eclipse can be thrilling or terrifying, the mechanics behind it are entirely ordinary and predictable. At that point, eclipses ought to lose their power as an omen or a “sign from God.” They’re just another aspect of the natural world.
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