How We Learned to Predict Solar Eclipses

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Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and Space Radio, and author of “Your Place in the Universe.” Sutter contributed this article to Space.com’s Expert Voices: Op-Ed & Insights

Ancient astronomers had a really tough time predicting solar eclipses, sometimes with dire results. For a particularly gruesome example, we have a story passed down to us of Hi and Ho, the court astrologers (which for a long time, were indistinguishable from astronomers) of the emperor K’ang of ancient China. Apparently they were out drinking one night and somehow neglected to predict the eclipse of 2159 B.C. The emperor was not pleased. They were beheaded. End of story. 

Hi and Ho were not exactly fully to blame. Solar eclipses were notoriously hard to predict. While they occur roughly every couple of years somewhere on the Earth, for any region the size of a country you had to rely on complex and intricate cycles within cycles within cycles to even come close to finding a rhythm. That is, until the ultimate Power Couple of gravity came onto the scene.

Related: Best Photos of the 2019 Total Solar Eclipse

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What a downer

Everyone who was anyone had known about gravity for a long time. It’s the thing that makes objects fall to the Earth. But Isaac Newton figured out something absolutely remarkable about gravity that no one else had ever realized: that it’s universal.

According to Isaac himself, the thought struck him when he watched an apple fall from a tree as he was chilling at his mom’s house one lazy afternoon. He saw the apple fall in a straight line toward the Earth. We’ve all seen this exact same thing, but in that moment Newton saw something stunning. He saw the apple accelerate from being still to moving. And all accelerations require a force. So, the Earth was applying a force to the apple even though it wasn’t touching it.

So far, so good, and so far, partially mundane. But old Isaac also knew that every action has an equal and opposite reaction. If the Earth is applying a force to accelerate the apple, then the apple must be applying a force to accelerate the Earth. Whatever this force of gravity is, it must be mutual and in both directions. The reason the apple moves more than the Earth is because the mass of the Earth is so much greater — it just appears like that apple is doing all the moving and the Earth is doing all the work, when in Newton’s suddenly much clearer vision, all things were equal when it comes to forces.

So, if the Earth is applying gravity to the apple and the apple is applying gravity to the Earth, then this force of gravity must be operating with all pairs of objects simultaneously all across the universe. In other words, gravity must be universal.

Best gravitational buds

Newton worked out the implications of this newfound universal force of gravity, and instantly many things clicked into place. He was able to predict the speed of the moon in its orbit. He was able to rederive Kepler’s laws from much simpler principles. And he was able to explain the motions of all the planets and all the moons around those planets. And then he took all this fantastic work, wrote it in a book, and put the book on a shelf and forgot about it

It sat there for years until one of his friends, Edmond Halley, started agitating for him to publish it. Apparently, for Newton, unlocking the secrets of universal gravity was just an idle afternoon pastime and not something worthy of serious academic interest. But Halley knew better. He constantly pressed (and probably annoyed) Newton until he finally published his work.

Edmond Halley then took this newfangled theory of universal gravitation and went crazy with it, solving almost every single problem known to plague astronomers. Most notably, he figured out the regular pattern of a particular comet that now bears his name by digging into the historical records and using that data to feed into calculations of a prediction of its reappearance.

But perhaps even more importantly, by digging into the ancient records, Halley was able to predict an upcoming total solar eclipse over his home city of London. Using Newton’s theory of universal gravitation, Halley predicted the eclipse of May 3, 1715, to an accuracy of a scant four minutes.

That’s right. Four minutes. Without any calculator or computer. Just using historical records and Newton’s laws, Halley nailed the first-ever accurate prediction of a solar eclipse, something that had bedeviled astronomers ever since the unfortunate days of Hi and Ho thousands of years ago. It’s a shame those guys didn’t have a buddy like Newton.

Learn more by listening to the episode “What can we learn from solar eclipses?” on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Michael M., Crai W., and Robert M. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter. 

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