It’s a dance that’s been playing itself out for millennia. On average, once every year and a half, the moon slips directly between the Earth and the sun, punching a hole of darkness into the daytime sky. And whenever possible, there have been people below, looking up.

Experiencing a total solar eclipse is revelatory, especially for people who study them.

"Every eclipse gives you new information,” says Shadia Habbal, an astronomer at the University of Hawaii, originally from Syria.

For instance, 2,500 years ago, she says, the Babylonians used careful records of eclipses to learn how to predict them. Essentially, they used eclipses to infer basic laws of motion. 

The Babylonians "didn’t know anything about Newton,” Habbal says, “but you can say they preceded him by thousands of years.”

Newton, of course, was the guy who later derived those basic laws in the 17th century, building on the work of JohannesKepler, who had figured out a set of basic laws of planetary motion a few decades earlier.

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Habbal says neither Newton nor Kepler got help from solar eclipses, but that eclipses soon allowed other scientists to prove them both right.

So, roughly 400 years ago, eclipses confirmed fundamental new theories of how objects move — on Earth, in our solar system, and beyond.

And those breakthroughs, in turn, helped form the foundation of modern science and technology.

Fast-forward to 1868 and an eclipse in India. A French astronomer named Pierre Janssen trained an instrument on the corona — the sun’s atmosphere that’s still visible during an eclipse as a bright crown encircling the moon — and analyzed its light to figure out what the sun was made of.

What he found was super hot hydrogen, something already well known here on Earth for about a hundred years. It’s the H in H2O.

But there was something else, something that the English astronomer Norman Lockyer later determined was a new element, which he called helium, after Helios, the Greek god of the sun. It was unknown to science at the time.

“It took about 25 years before it was found on Earth,” Habbal says. Today, helium is used for everything from high-tech magnets to birthday party balloons. And we know about it thanks to a solar eclipse.

Related: A total solar eclipse was once all about fear, but it’s still an awe-inspiring event

A year later, during the eclipse of 1869 in Iowa, an American and a Scotsman separately detected what they thought was another new element glimmering in the corona. They called it coronium. But it later turned out there was no such thing — coronium was actually the element iron, stripped of 13 of its electrons.

But that discovery was a big deal, too.

“To lose 13 electrons meant the temperature [of the sun’s atmosphere] had to be at several million degrees,” Habbal says, hundreds of times hotter than its surface. 

"Personally, I think that was a landmark discovery,” Habbal says. It was totally unexpected, and it helped us learn more about how stars work.

Then there’s the eclipse that helped prove how the whole universe works.

It was in 1919, and it was observed by a team headed by British astronomer Arthur Eddington, who was seeking to prove a radical new idea — Albert Einstein’s theory of general relativity.

Central to the theory was the notion that space and time form a kind of fabric, and that massive objects like stars bend that fabric, affecting how other things move through space — things like other stars, planets, and even light.

Eddington believed that a solar eclipse could help prove both that phenomenon and Einstein's overall theory.

The sun, of course, is the most massive object close to earth, so in theory it presents the best opportunity to test Einstein's hypothesis. But under normal conditions, it's way too bright to measure any nearby starlight. 

Related: Helping the blind 'see' the solar eclipse

But by blocking out the sun's own light, Eddington believed a total eclipse would allow him to measure whether the light of stars very close in the sky was being bent ever so slightly, as Einstein had predicted.

Eddington sailed to the island of Príncipe in the Atlantic to observe the 1919 eclipse, and sent other members of his team to Brazil. When the moon blocked the sun, they took photographs, which they then used to try to determine whether starlight passing near the sun had in fact been shifted.

The result, Eddington claimed, was clear. The starlight had been shifted off its usual path. Einstein was right. 

Others weren't convinced, at least at first.

"It’s rather dubious, because the equipment they had wasn’t sensitive enough,” Habbal says. “I think many people agree that [Eddington's certainty] was more hype than reality.”

But before long, technology caught up to the challenge, and Eddington’s conclusion from the eclipse of 1919 was confirmed. Einstein’s theory of general relativity was correct. Some fundamental secrets of the universe were revealed.

And that’s the way it is with solar eclipses. The darkness illuminates our understanding — of our sun, our solar system and our universe. Of the power of science itself.

So, the question for this year’s eclipse is, what will we learn this time?

Related: We want your eclipse plans, stories and photos

From PRI's The World ©2017 PRI