Five things we learned from last year’s Great American Eclipse

It’s been a year since the total solar eclipse of August 21, 2017, captured millions of imaginations as the moon briefly blotted out the sun and cast a shadow that crisscrossed the United States from Oregon to South Carolina.

“It was an epic event by all measures,” NASA astrophysicist Madhulika Guhathakurta told a meeting of the American Geophysical Union in New Orleans in December. One survey reports that 88 percent of adults in the United States — some 216 million people — viewed the eclipse either directly or electronically.

Among those were scientists and citizen scientists who turned their telescopes skyward to tackle some big scientific mysteries, solar and otherwise. Last year, Science News dove deep into the questions scientists hoped to answer using the eclipse. One year out, what have we learned?

1. The eclipse sent ripples through Earth’s atmosphere.

Normally, the sun’s radiation splits electrons from atoms in the atmosphere, forming a charged layer called the ionosphere, which stretches from 75 to 1,000 kilometers  up. But when sunlight briefly disappears during an eclipse, the electrons rejoin their atoms, creating a disturbance in the ionosphere that is detectable with receivers on the ground (SN Online: 8/13/17).

The moon’s supersonic shadow produced a bow wave of atoms piling up in the ionosphere, similar to the wave at the prow of a boat, Shun-Rong Zhang of MIT’s Haystack Observatory in Westford, Mass., reported in December. Although such bow waves were predicted in the 1960s, this was the first time they were definitively observed.

The eclipse also sent a wave traveling through the thermosphere, an uncharged layer of the atmosphere about 250 kilometers high, that was observed from as far away as Brazil nearly an hour after the eclipse ended (SN: 5/26/18, p. 14). And measurements of temperature, wind speed and sunlight intensity showed that the eclipse briefly changed the weather along the path of darkness.

2. Showing Einstein was right is not so simple.

Physicists chased the moon’s shadow to redo the iconic experiment that showed Einstein’s theory of general relativity was correct (SN Online: 8/15/17). In Einstein’s view, the sun’s mass should warp spacetime enough that the positions of stars should appear to be slightly different during an eclipse. During the May 1919 solar eclipse, British astronomer Arthur Stanley Eddington took photographs that proved Einstein right.

During the 2017 eclipse, almost a century later, amateur astronomer Donald Bruns of San Diego made similar measurements with modern equipment and came to the same conclusion as Eddington: Stars visible during the eclipse were all askew. Bruns published his results in Classical and Quantum Gravity in March.

But astrophysicist Bradley Schaefer of Louisiana State University in Baton Rouge and others had far more difficulty reproducing the measurement with enough precision to show that Einstein was right. “‘Bummer’ is an understatement,” Schaefer says. “This all may have been for naught.”

Schaefer had enough trouble that he thinks it may have been impossible for Eddington to get the precision he claimed. The earlier astronomer may have hit upon the right answer by luck, not because he actually measured it.

3. Infrared light will help measure the corona’s magnetic field.

Some eclipse experiments didn’t revolutionize our understanding of the sun on their own, but will enable future ones to pull back the veil. One of these was the first infrared observations of the sun’s corona, the shimmering halo of hot, diffuse plasma that is only visible in its entirety during a total solar eclipse. The shape and motion of all that plasma are guided by magnetic fields, but the corona’s magnetic field is so weak that it has never been measured directly (SN Online: 8/16/17).

Previous studies suggested that infrared wavelengths of light might be particularly sensitive to the corona’s magnetic field. So two groups chased the August 2017 eclipse in airplanes to get some infrared observations. Amir Caspi of the Southwest Research Institute in Boulder, Colo., and his colleagues took the first infrared image of the entire corona.

Flying in another aircraft, Jenna Samra of Harvard University measured the corona in five specific wavelengths, one of which had never been seen before. Comparing those results with observations taken from the ground in Casper, Wyo., (where I watched the eclipse) showed that those wavelengths are bright enough that a telescope now under construction in Hawaii will be able to help map the corona’s magnetism (SN Online: 5/29/18).

4. Figuring out what heats the corona will take more work.

Almost every experiment aimed at the eclipsed sun last August had some bearing on the biggest solar mystery of all: Why is the corona so hot? The solar surface simmers at around 5500° Celsius, but the corona — despite being farther away from the solar furnace and made of much more diffuse material — rages at millions of degrees.

One year after the Great American Eclipse, scientists are still scratching their heads. Caspi’s team searched for waves rippling through the corona, which could distribute energy far from the solar surface. Those waves could also help comb out magnetic tangles in the corona and explain its well-ordered look (SN Online: 8/17/17).

In a complementary measurement, the group in Wyoming saw signs of neutral helium atoms in the corona, says solar physicist Philip Judge of the National Center for Atmospheric Research in Boulder. Those uncharged atoms probably represent cool material embedded in the corona (SN Online: 6/16/17).

Similar cool spots have been seen during earlier eclipses, although it’s hard to imagine how the cool atoms survive in the searing heat, like ice cubes remaining solid in hot soup. But collisions between charged ions and neutral atoms could help convert ordered motions, like Caspi’s waves, into coronal heat.

The results so far are interesting, but inconclusive, Caspi says. “It’s certainly possible we will get some very interesting results from this set of observations alone,” he says. But for such a big problem as coronal heating, eclipse observations may play a supporting role to more direct measurements, such as those that the recently launched Parker Solar Probe will make (SN Online: 8/12/18).

5. People are already looking to the next eclipse.

A survey done by researchers at the University of Michigan found that eclipse watchers sought more information about eclipses and the scientific questions involved an average of 16 times in the three months following the event.

Several research groups are planning observations for the next total eclipses, visible in South America in July 2019 and December 2020 (SN: 8/5/17, p. 32). Caspi and Samra’s teams both hope to fly through those eclipses in aircraft again.

And amateurs and pros alike are preparing for the Great American Eclipse version 2.0, which will cross from Texas to Maine in 2024.

“Everybody’s eyes are on 2024,” Caspi says.