105 Years Ago, This Astrophysicist Used a Solar Eclipse to Prove Einstein’s Theory Of Relativity

And other ways a solar eclipse can shed light on how our planet, and the universe, work.

The sun's corona is visible as the moon obscures the sun during the Great American Solar Eclipse at ...
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As the Moon’s shadow creeps across the Sun on April 8, physicist Aroh Barjatya will launch a trio of 60-foot-tall rockets into the highest layer of Earth’s atmosphere.

The short, sudden darkness of a solar eclipse sends changes rippling through the electrically charged layer of Earth’s atmosphere, called the ionosphere, and Barjatya’s three sounding rockets will carry instruments to measure those changes before, during, and after the peak of the eclipse.

“We know when and where it is going to happen, thus creating a nice and easy experimental setup to study basic physics,” Barjatya, who directs the Space and Atmospheric Instrumentation Lab at Embry-Riddle Aeronautical University in Daytona Beach, Florida, tells Inverse.

Astronomers and physicists have used solar eclipses as a way to do experiments for centuries, and past solar eclipses have helped scientists discover new chemical elements, search for hidden planets, and field-test the rules that describe how everything in the universe works. This year, teams of scientists will use the darkness of the eclipse to shed light on the Sun’s ever-changing outer layer and how the Sun affects our planet’s atmosphere. Here’s a look back on how eclipses have helped advance science and how they are continuing to push innovation forward.

The sun's corona is visible as the moon obscures the sun during the Great American Solar Eclipse at Madras High School in Madras, Oregon, on Monday, August 21, 2017.

San Francisco Chronicle/Hearst Newspapers via Getty Images/Hearst Newspapers/Getty Images

1868: Discovering a New Element

It's hard to imagine not knowing that helium exists. It's the second-most abundant element in the universe, and here on Earth, we use it for everything from cleaning out rocket engines to cooling the superconducting magnets in magnetic resonance imaging (MRI) machines to filling party balloons. But the lighter-than-air element was unknown until 1868, when astronomers Norman Lockyer and Pierre Janssen pointed their telescopes at the Sun during a solar eclipse.

During an eclipse, the Sun's outermost layer is visible as a pale, wispy ring of light peeking around the edges of the Moon. The Moon blocks most of the Sun's light, giving astronomers a rare chance to focus on its outermost layer, the corona. On August 18, 1868, Janssen used an instrument called a spectroscope to split the faint light of the corona into the individual wavelengths that made it up.

Several wavelengths of light were clearly missing from the Sun’s spectrum. A few years earlier, chemist Josef von Fraunhofer suggested those wavelengths of light were missing because they were being absorbed by chemicals in the Sun’s outer layers. Each chemical element emits and absorbs its own specific colors of light, and by 1868 chemists had identified the elements absorbing most of the missing lines in the solar spectrum — except the one Janssen saw, with a wavelength of about 587.5 nanometers (a very tiny fraction of an inch).

It didn't match the light absorbed by any chemical element scientists knew about at the time. Meanwhile, observing the eclipse from a different vantage point, Lockyer noticed the same bright yellow line in the spectrum and came to the same conclusion. It was Lockyer who named the element, which seemed to make up a lot of the Sun's outermost layer, helium after the Greek sun god Helios.

In 1882, physicist Luigi Palmieri spotted the same bright yellow spectral line in lava from Mount Vesuvius, in Italy, and realized it was also helium (so the story of helium's discovery includes both a solar eclipse and killer volcano, which may make it the most awesome element on the periodic table).

The presence of dark lines in the spectrum of sunlight was first detected by a scientist named Fraunhofer.


1919: Proving Einstein Right

A few decades later, astrophysicist Arthur Eddington used a solar eclipse to prove that Einstein's theory of relativity was correct, especially about how gravity could warp spacetime. Einstein had predicted that an object's mass should warp spacetime around it. Gravity happens because spacetime curves toward massive objects so nearby objects moving through spacetime find themselves moving along those curves. Einstein's predictions meant that light also follows the curves and warps in spacetime.

Eddington realized that if Einstein was right, our Sun's mass should curve spacetime around it. Light from distant stars, hidden behind the Sun (from our point of view) should follow that curved path and arrive at a point in front of the Sun, where we can see it. On May 29, 1919, Eddington and a team of other astronomers watched the Sun during a total solar eclipse, and they saw the reflected image of several stars (whose light was normally drowned out by the brighter glow of the Sun).

Today, the same mechanism lets astronomers use whole galaxy clusters as giant cosmic magnifying lenses, revealing distant galaxies that our best telescopes otherwise couldn't see.

NASA Hubble Space Telescope image of the galaxy cluster CL1358+62 released 30 July has uncovered a gravitationally-lensed image of a more distant galaxy located far beyond the cluster. The gravitationally-lensed image appears as a red crescent to the lower right of center.

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2024: Making Waves in the Atmosphere

More than a century after a solar eclipse proved Einstein right about spacetime, Barjatya and his colleagues hope to learn more about how eclipses and other events affect the ionosphere, an electrically charged layer of the atmosphere 37 to 190 miles above Earth’s surface.

“This layer reflects and refracts radio signals and impacts satellite communications as the signals pass through,” says Barjatya. Understanding and modeling it will help enable it to run smoothly.

A solar eclipse is a perfect opportunity to study how the ionosphere reacts to disturbances, like the sudden disappearance of the Sun's constant bombardment of radiation. The Sun rises and sets every day, but those changes are much more gradual, and impact a much wider swath of Earth’s atmosphere at one time. But a solar eclipse is like sunset and sunrise happening in a matter of minutes, along a very narrow strip of the atmosphere, and that creates disturbances, sometimes over vast distances.

“The waves and dynamics created by the eclipse shadow racing across the ionosphere travel more than 1000 kilometers (620 miles),” says Barjatya.

Instruments aboard the rockets will measure electricity, magnetic fields, air density, and temperature in the ionosphere. During the annular eclipse in October 2022, the team observed some changes; as the eclipse shadow passed, the density of charged particles in the ionosphere decreased sharply. Since charged particles are the reason the ionosphere reflects radio waves, it's important to understand the mechanics behind those changes. Barjatya and his colleagues hope their April 8 can reveal whether the changes happen because the Moon is blocking high-energy ultraviolet light from reaching the ionosphere, or because the sudden darkness causes lower layers of the atmosphere to cool, creating waves in the air that disrupt the ionosphere.

More than 750 students will launch weather balloons from sites around the U.S. to study how the atomspheres reacts to the sudden vanishing of sunlight during an eclipse.


Studying the Eclipse from the Ground Up

Along the path of the total eclipse, other teams of scientists will be gathering their own data using weather balloons, high-altitude planes, and networks of radar dishes.

NASA’s sleek WB-57 high-altitude research planes will chase the shadow of the eclipse from 50,000 feet above the ground. The high-altitude chase will keep the planes — and the instruments they carry — in darkness for an extra two minutes. Amir Caspi, of the Southwest Research Institute, and his colleagues hope their visible and infrared images will reveal new details about the structure of the Sun’s corona, and maybe even spot previously undiscovered asteroids orbiting close to the Sun. Shadia Habbal, of the University of Hawai’i, and her colleagues will also use cameras and spectrometers to study the corona’s structure; they’re especially interested in how solar wind forms in the Sun’s outermost layers.

Teams of students across the U.S. will also launch hundreds of weather balloons to measure how the atmosphere responds to the sudden cold and darkness of an eclipse.

On the ground, radar dishes in the Super Dual Auroral Radar Network (SuperDARN) will measure radio waves bouncing back from the ionosphere during the eclipse. A NASA-funded project led by Virginia Tech physicist Bharat Kunduri will compare the SuperDARN measurements to computer simulations to learn more about how the ionosphere reacts to the rapid changes that happen during an eclipse.

And the upper atmosphere isn’t the only place that changes dramatically during an eclipse. If you close your eyes and listen, you may hear animals around you responding to the sudden darkness. Animal behavior during an eclipse — especially the sounds they make — will be the focus of another NASA-sponsored study on April 8.

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