Four ways total solar eclipses helped us learn more about our universe

Eclipses force us to forgot our material obsessions and allow our minds to venture to the galactic abyss above our heads. One brought peace to a five-year ancient war. Another was used to unite Indigenous Native American tribes. It’s also been the source for scientific discoveries about Earth’s evolution, the workings of our host star and even how our universe operates. Many of these mysteries would have remained secret for much longer without observations from eclipses.

Here are a few of those big and small discoveries:

The biggest and most famous eclipse-related advancement is the experiment that proved Albert Einstein’s theory of general relativity, which revised how we think about gravity and catapulted the physicist into mainstream celebrity status.

Einstein’s theory explained not only how our solar system is bound together, but also predicted exotic phenomena like black holes. He proposed that gravity is not a force but a curving of space and time. Matter will distort time and space, where more massive objects will bend space around it more than a smaller objectwould.

Take a massive object like the sun, which should notably bend the space around it. To prove it, Einstein said to chart the position of the stars close to the sun’s edge when the sun is out and compare it to when the sun is not out (e.g. nighttime). General relativity predicted that the sun should deflect the starlight by a small, but observable, amount.

A total solar eclipse would be the perfect natural experiment to prove his idea. During the May 1919 eclipse, British astronomers Frank Dyson and Arthur Eddington gathered data on stars near the sun’s edge since the eclipse dimmed the sun’s bright, overwhelming surface. They successfully measured these changes — known as gravitational deflection — in starlight passing near the sun, proving Einstein correct.

“It’s a common opinion that nobody would have figured out general relativity for a long time if Einstein hadn’t gotten there first,” said John Thorstensen, an astronomer at Dartmouth College.

Before Einstein, there was Isaac Newton. As the story goes, Newton came up with the gravitational law after watching an apple fall from a tree. Why does the apple fall downward instead of sideways or even upward? He described a force called gravity, the pull between two objects. It took many eclipses to prove Newton’s idea wasn’t complete, opening the door for Einstein.

Newton’s law seemed to explain why we are tied to Earth, the impact the moon has on our tides and why planets rotate around the sun. He proposed that gravitational force is affected by the mass of the objects and how close they are, a formula we still use today.

But there was a nagging discrepancy that the law failed to explain. Mercury, Thorstensen said, had a little bit of an extra wobbly drift in its orbit compared to what was expected from gravitational forces of the sun and other nearby planets.

Scientists proposed that there must be another force pulling on Mercury, perhaps the pull of another planet. Calculations showed another planet inside the orbit of Mercury would explain its peculiar orbit, so researchers began searching for a planet called Vulcan.

Eclipses are the perfect opportunity to search for planets near the sun, as scientists can see objects around the sun’s outer edge in greater detail. For decades, astronomers searched for a hypothetical planet between the sun and Mercury during eclipses, but they never found one.

The oddity in Mercury’s orbit remained a mystery for centuries, until the 1919 eclipse experiment proved Einstein’s theory of general relativity. Einstein’s theory explained how the sun influenced Mercury’s wobbly orbit and debunked the existence of the planet Vulcan.

Centuries ago, scientists noticed that a glowing ring appeared around the sun during the moments the moon completely eclipsed the star. Some astronomers thought it could be part of the sun, or sunlight penetrating the Earth’s atmosphere or a lunar atmosphere. But the mystery began to unravel during the 1869 eclipse across the northeastern United States.

Astronomers from Dartmouth College and University of Rochester grabbed prime spots under the path of totality to study the sun. They used a new instrument called a spectroscope, which splits sunlight into a rainbow of colors that correspond to different elements. The scientists discovered continuous green emissions from the sun, concluding it was indeed part of the sun’s atmosphere. Today, we call that outermost layer the corona.

But what was the green emission? At the time, the green lines didn’t correspond to any known element. The astronomers thought they discovered a new element and called it “coronium.”

It took around seven decades before other scientists realized this was not a new element but iron stripped of half of its electrons. Such highly ionized iron would require an immense amount of energy to pull those electrons off, suggesting that the sun’s outer layer must be really, really hot. (While they didn’t find a new element, another eclipse did help with the discovery of the element helium.)

“It’s not ordinary iron, and that’s why they never saw it in the laboratory. It’s iron that’s at millions of degrees, much hotter than the surface of the sun,” said Thorstensen. The sun’s surface is about 9,940 degrees Fahrenheit. The corona is, literally, around 2 million degrees.

Researchers today still don’t understand why the corona is so much hotter than its surface. Scientists will continue to study the sun during the April 8 eclipse to understand this ongoing mystery, using telescope data collected from eclipse watchers along the path of totality. NASA’s Parker Solar Probe spacecraft is also currently collecting data about the corona, planning to make it closest approach to the sun this December.

Earth’s rotation is slowing down, causing the day to get longer by 1.8 milliseconds each century, according to an analysis of historic eclipse records. The difference is tiny on a short time scale, but it builds up to hours over millennia in a clock regulated by Earth’s rotation.

Throughout history, civilizations have documented eclipses because of the unexpected and otherworldly experience. Using observations as far back as 720 B.C., researchers created a catalogue of when and where eclipses occurred. The team used computer simulations to create past eclipses and assumed Earth’s current rotation, but the models did not match up with the observations. For instance, an eclipse on April 15, 136 B.C. was documented in ancient Babylon, but models showed the eclipse should have occurred 40 degrees west at Earth’s current rotation speed.

“It’s a long, ongoing story in the scientific world” to determine Earth’s rotational speed, said Leslie Morrison, a retired astronomer at the Royal Greenwich Observatory and an author of early research. “But it’s only in the past 50 years or so that it’s really firm down [and] got quite accurate.”

The slowdown is caused by a tumultuous tug between Earth and the moon. The moon pulls on Earth’s oceans and produces tides, which creates drag that slows down Earth’s rotation. There are also long-term transfers of momentum between different parts of Earth’s layers, which affects its rotational speed. As the Earth rotates, it also transfers its angular momentum to the moon and also pushes our lunar neighbor away.

“As you go forward, the [slowdown] is actually getting slightly less because the moon’s moving away and the drag on the Earth is slightly less,” said Morrison. Even so, “the human race will have disappeared long before” we notice more hours added to our day.