Why the total eclipse has solar physicists very excited

A total solar eclipse is a rare occasion to marvel at nature, contemplate life, and think about the cosmos. But the total solar eclipse on August 21 will also be an important moment to gather scientific data about the sun as the moon covers it completely for an hour-and-a-half journey across the United States.

To this day, many aspects of the sun remain a mystery: What causes solar flares, when massive amounts of energy and plasma are ejected from the sun? Why is the corona, the solar atmosphere, actually hotter than the surface?

The eclipse provides a natural experiment to test some of these questions. The moon is perfectly sized to block out the entire surface of the sun, leaving the corona, which is some million times fainter in our view.

“Even with our best instruments today we cannot recreate those observations,” Ryan Milligan, a solar physicist who works with NASA, says. The space agency has two spacecrafts, called the Solar and Heliospheric Observatory and STEREO, that can — with a disc — mimic an eclipse to study the sun.

But the moon provides a deeper view. With the moon, “we can see the corona almost all the way down to the surface,” Milligan says.

It’s essential for us understand the sun. For one, knowing more about our own star helps us understand all the other stars in the universe just a bit better. But we also need to understand the sun because of the dangers it poses to our civilization. One solar storm pointed toward Earth could take out or disrupt much of our communication infrastructure.

During this coming solar eclipse, there will be several experiments to make the most of this rare moment. One involves NASA sending up two WB-57F jets to trace the path of totality and capture the corona with specially mounted telescopes. The hope is to create a time-lapse video of the activity in the corona.

“These could well turn out to be the best-ever observations of high frequency phenomena in the corona,” Dan Seaton, a University of Colorado solar physicist leading the project, said in a press statement. Specifically, the planes will be searching for “nano flares” — relatively small pulses of energy that are hard to spot but might explain the superheating of the corona.

Other efforts will be ground-based. One NASA-funded experiment, called Citizen Cate, will link 68 telescopes (manned by high schools and universities) across the path of totality in the hope of creating a 90-minute video of the corona.

“Some of these cameras [attached to the telescopes] can take 100 frames per second, and from that, you could actually see waves propagating from coronal structures,” Milligan says. And charting those waves, again, could give clues to how energy is transferred from the surface of the sun to the corona. Another, separate project will launch dozens of balloons equipped with cameras and sensors in the path of totality to capture the event above the threat of clouds.

If these efforts are successful, they’ll join the canon of scientific knowledge accumulated from eclipses past.

For instance, helium — the second most abundant element the entire universe — was discovered during a solar eclipse.

In 1868, a total eclipse was passing over southern India, and scientists had what was cutting-edge technology at the time: a spectroscope. The spectroscope is basically a prism — a device to separate light into its different wavelength components. When you point a spectroscope at burning gases, you can determine which element is burning in the flames by looking at the pattern of light that comes out of the prism.

The sun is, essentially, a ball of burning gases. In 1868, astronomer Pierre Janssen used a spectroscope to analyze the composition of the sun’s atmosphere during a total eclipse, and he found a strange spectral pattern. Further analysis revealed the element burning in the corona was like nothing else on record. The element was named “helium” (for helios, the Greek word for “sun.”) “Helium is the only element that was ever discovered somewhere else rather than on Earth first,” Steve Ruskin, an astronomer and science historian, says. And it’s all thanks to the solar eclipse.

One of the most famous scientific theories of all time was also proven during a solar eclipse: Einstein’s theory of general relativity. The theory, first published in 1915, said that gravity literally warps the space and time surrounding massive objects. The sun, the most massive object in the solar system, in this sense should act like a lens, bending light around it. In 1919, during an eclipse in South America, astronomers took photographs of the stars surrounding the sun during totality. And they found the evidence: Stars that should have appeared near the sun were shifted ever so slightly in the sky, proving spacetime warps around our star.

All this science is possible because of a cosmic coincidence: The apparent size of the moon is the same as the sun in our sky. There’s no scientific reason for this to be the case. We’re just lucky.

And we won’t be lucky forever. The moon is slowly getting farther and farther away from the Earth. It will take a billion years or more, Vox’s Joss Fong reports, but eventually there will be a final total solar eclipse.

By then, we hope, humanity — or whatever comes to replace it — will have finally figured out the secrets of the sun.