You might think that, after thousands of years of observing total solar eclipses, science-minded folks would have exhausted what can be learned from this awesome natural spectacle.
You would be wrong.
"I get asked all the time, why are we still doing eclipses for scientific purposes," says Jay Pasachoff, an astronomer at Williams College in Massachusetts who chairs the International Astronomical Union's working group on solar eclipses. "There [are] still whole parts of the sun that can't be seen from satellites and that we just see better at eclipses."
That's why, later this month, many astronomers and citizen-scientist volunteers will gaze up toward the sun and join a long line of eclipse scholars that includes Plutarch, the Babylonians, ancient imperial astronomers in China, and even Thomas Edison.
One focus of attention will be the sun's outer atmosphere, called the corona, which is revealed when the moon slips in front of the sun and blocks its blinding light. The corona gets its name from the fact that it looks like a glowing ring, or crown.
"The corona, I think, is the most beautiful thing you can see in the sky," says Matt Penn, an astronomer the National Solar Observatory in Arizona. "The corona is just fantastic and filamentary and delicate and awesome."
Now, scientists have ways to look at the corona any old time. They can put a disk in their telescopes to create an artificial eclipse. But the moon can do something those little disks can't: When it perfectly aligns to block the sun, it exposes the innermost part of corona.
The inner corona is particularly interesting to researchers because there's a lot of activity there related to so-called space weather, which can affect Earth's electric grids and communications systems.
But the moon moves more quickly than researchers would like. From any given spot on the ground, Penn says, the view of the inner corona lasts for just a couple of minutes while the moon is totally blocking the sun. "And the corona is big. It changes, but slowly, and in two minutes you can't really see changes that we want to study in the solar wind," he adds.
That's why Penn has been organizing Citizen CATE: the Continental-America Telescopic Eclipse experiment. Trained volunteers will be positioned at 68 sites across the entire 2,500-mile path of the total eclipse. As the moon's shadow crosses the country, the volunteers will use identical telescopes to take photos of the corona.
"With that many telescopes, you can get continuous coverage of the eclipse from coast-to-coast during totality," says Bob Baer of Southern Illinois University, another member of the CATE team.
When the images get stitched together into a continuous movie, Penn says, "we can observe the corona for 93 minutes and therefore see changes that we wouldn't normally otherwise detect."
In addition to ground-based telescopes, researchers working on other projects will watch the eclipse with the help of satellites, airplanes and high-altitude balloons.
More than 50 balloons will send back live video of the eclipse from the edge of space, and balloons will also collect information about the Earth's atmosphere, says Angela Des Jardins of Montana State University, who runs the Eclipse Ballooning Project.
"Normally the atmosphere experiences change with night and day," she says, "but the eclipse coming across the country, in a dark shadow at an average of about 1,500 miles per hour, is going to set up waves in the atmosphere." She says the balloons will monitor changes in temperature and other atmospheric features.
Scientists are also taking this opportunity to send up some harmless, but hardy, bacteria. "It turns out that high altitude, where the balloons are going to be, is an analogy in temperature and pressure to the surface of Mars," Des Jardins says. During an eclipse, NASA says it's even more Mars-like, so scientists want to test how life reacts to such an alien environment.
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