A woman holds a filter in front of the partially covered sun during a solar eclipse visible from Siberia in 2008.

Babak Tafreshi/National Geographic Creative

Three times scientists learned something from solar eclipses—and three times they were tricked

Next Monday’s solar eclipse, which should be visible across a wide swath of the continental United States, has inspired millions to travel to the path of totality, where the alignment of moon and sun will cast the land into darkness for up to 2 minutes and 40 seconds. For most scientists, though, the celestial phenomenon won’t be such a big deal. That’s because total solar eclipses happen pretty routinely, about once every 18 months. And even when they aren’t taking place, astronomers can still study the sun’s wispy atmosphere using coronagraphs, telescope attachments that obscure the surface glare of the sun.

But long before such high-tech instruments were available, eclipses helped lead to some key scientific discoveries. Here are three—plus one wild goose chase and two “findings” that turned out to be false alarms: 

A drawing depicting Hipparchus in his observatory in Alexandria. At the center is an armillary sphere, a model of the cosmos as ancient scholars perceived it.

Science History Images/Alamy Stock Photo

Estimating the distance from Earth to the moon

Just how far is our planet from its moon? Astronomers have tried answering that question since 4 centuries before the common era, starting with Aristarchus of Samos. Around 150 B.C.E., another Greek astronomer, Hipparchus of Nicaea, came up with his own calculation—using a solar eclipse. He learned that in northwestern Turkey one could see the moon perfectly aligning with the sun. But in Alexandria, Egypt, about 1000 kilometers away, only about 80% of the sun was blocked. Using this information and some simple trigonometry, he calculated the distance between Earth and the moon. He was a little off—about 20%. We now know that the moon is about 385,000 kilometers away from Earth—equivalent to walking around our planet 10 times.

A partial solar eclipse as seen from NASA’s Solar Dynamics Observatory, a spacecraft that has been observing the sun since 2010.

NASA’s Goddard Space Flight Center/SDO/Joy Ng, producer

“Discovering” the moon’s atmosphere

German mathematician and astronomer Johannes Kepler in 1605 suggested that the bright aura surrounding the sun, visible during a solar eclipse, was sunlight reflecting off the moon’s atmosphere. The only problem? The moon has virtually no atmosphere compared to Earth or the sun. In 1724, French-Italian astronomer Giacomo Filippo Maraldi figured out that the aura surrounded the sun, not the moon. And it wasn’t until 1806 that Spanish astronomer José Joaquín de Ferrer gave the aura—the sun’s atmosphere—the name corona ("crown" in Latin).

French astronomer Pierre Jules César Janssen  at the eyepiece of his telescope at Meudon Observatory in France in 1893.

Oxford Science Archive Heritage Images/Newscom

Discovering the new element helium

Up until the 1930s, when scientists developed telescope attachments that block out sunlight, it was possible to observe the sun’s outer atmosphere only during a solar eclipse, when the moon passing in front of the sun blocked its glaring light out. By looking at the sun’s atmosphere with a spectroscope, an instrument that separates white light into a wide spectrum of colors, French astronomer Pierre Jules César Janssen in 1868 saw an unknown line in the yellow part of the sun spectrum, which was later found to be produced by a new element, now known as helium.

A spectroscope used at Columbia University in the 1860s. 

National Museum of American History Smithsonian Institution/Flickr

Detecting coronium, the fake element

In another dalliance with spectroscopy, U.S. astronomers Charles Augustus Young and William Harkness in 1869 separately observed a line in the green portion of the solar spectrum that didn’t correspond to that of any known element. They dubbed the new substance “coronium.” But Young and Harkness’s discovery was taken down in the 1930s, when scientists found out that the coronium line was actually produced by iron at extremely high temperatures.

In 2016, Mercury passing in front of the sun appeared as a black dot to Earth-based sky watchers. 

NASA/SDO

Chasing a nonexistent planet

U.S. astronomers Maria Mitchell and James Craig Watson, together with inventor and Science founder Thomas Edison, set out to take advantage of the 1879 total solar eclipse to catch a glimpse of Vulcan, a never-before observed planet that scientists thought was causing Mercury’s irregular orbit around the sun. The trio didn’t succeed at spotting Vulcan—because the mysterious planet didn’t exist. It took Albert Einstein’s theory of general relativity, published in 1915, to show that warped space-time would be enough to explain Mercury’s unusual path.

One of Eddington’s photographs of the total solar eclipse of 1919.

F. W. Dyson, A. S. Eddington, and C. Davidson/Wikimedia Commons

Confirming Einstein’s theory of general relativity

Before 1919, Einstein was not yet a worldwide celebrity. But his theory of relativity predicted that an object’s gravitational field would warp the path of oncoming light by twice the amount predicted by Newtonian gravity. The effect is tiny, but a ray of starlight passing near the sun would bend by a slightly larger angle than previously thought. To test Einstein’s prediction, U.K. astrophysicist Arthur Eddington took pictures of a cluster of stars in the region around the sun, which were visible thanks to the darkness created by the eclipse. Eddington’s observations confirmed Einstein’s theory. The discovery instantly made headlines, turning the German physicist into an international figure.

A composite of photos of a solar eclipse visible from Germany in 2006.

blickwinkel/Alamy Stock Photo

Historical eclipses are still useful today, as they help scientists study the shape and motion of Earth. And even during the upcoming solar eclipse, researchers hope to collect valuable data. Atmospheric scientists may be able to learn more about gravity waves, ripples in Earth’s atmosphere caused by the moon’s shadow cooling the air in its path (not to be confused with gravitational waves, which are caused by two massive objects such as black holes spinning past each other). And to understand how the corona gets so hot—hundreds of times hotter than the sun’s surface—NASA astronomers will fly two jets over Missouri, Illinois, and Tennessee to capture the movement of the bright wisps shooting out from the corona.

Other scientists, like Anthony Aveni, an expert in historical astronomy at Colgate University in Hamilton, New York, will just gaze in awe at the sky. “A solar eclipse is an extraordinary phenomenon,” he says. “You feel like you’re witnessing the sublime.”

*Update, 17 August, 12:30 p.m.: An earlier version of this story stated that Einstein’s theory of general relativity predicted that an object’s gravitational field would slightly warp the path of oncoming light. In fact, it predicted that it would warp the path of oncoming light by twice the amount predicted by Newtonian gravity.