Total eclipses offer scientists a way to see all the way down to the sun’s surface
S. Habbal et al.
The sky will go dark. The temperature will drop. Stars will shine in the middle of the day. For the first time in nearly a century, millions of Americans from coast-to-coast will witness a total solar eclipse. Those who have watched the sun suddenly snuff out say it’s an otherworldly feeling. It can be humbling. It can be spiritual. It can change the course of history (SN: 5/13/17, p. 29).
But as the moon passes in front of the sun during the Aug. 21 Great American Eclipse, scientists will be doing some serious work.
“Everybody’s taken in by the beautiful dark sky,” says Padma Yanamandra-Fisher of the Space Science Institute’s branch in Rancho Cucamonga, Calif., who will observe the eclipse from Carbondale, Ill. “But that part is so dominant that people don’t always appreciate the nuances of the science you can actually do.”
Many of the scientific questions researchers are after have to do with a big doughnut of space around the sun observable only during a total eclipse. This doughnut, the inner corona, is the part of the sun’s tenuous atmosphere that starts right at the sun’s surface and extends out to about 2½ solar radii. It happens to be where a lot of the most interesting solar physics happens. It’s where the solar wind originates (SN: 12/16/08), where loopy magnetic structures are anchored (SN: 2/12/00, p. 101) and where coronal mass ejections and space weather (SN: 7/31/04, p. 74) that can knock out power grids on Earth get their start.
Using a disk called an occulter to block out the sun’s brightness, space telescopes like NASA’s Solar and Heliospheric Observatory watch the outer corona all the time. But it’s hard to hold a telescope steady. If the occulter were exactly the size of the sun in the telescope’s view, any drift or jitter would let bright solar photons in. These particles of light can ruin the data and possibly damage the telescope.
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Coast to coast
The August 21 eclipse will cross the United States in about an hour and a half. People in the region of totality (shaded in yellow/orange) will see the sun disappear completely for about two minutes, and will see a partial eclipse for about 90 minutes before and after totality. The rest of the continent will also see a partial eclipse.
There’s also the issue of diffraction, or the way light bends and spreads when it passes across an edge. A too-small occulter could make sunlight bend right into the camera. And you can’t just hold up your thumb on any bright day and block the sun’s disk yourself. Sunlight scattering in the atmosphere — the same effect that makes the sky blue — ensures that the corona is washed out by the sun’s brilliance, almost no matter what.
So telescopes have oversized occulters, blocking a disk with a radius about twice that of the sun. That keeps data and telescopes safe, but also blocks most of the inner corona.
The moon dodges these problems, because it’s so far away and outside the atmosphere. And by lucky coincidence, it can appear exactly the right size in the sky to block out the solar disk, and no more. “The moon makes a better eclipse than humans have been able to do,” says Jay Pasachoff of Williams College in Williamstown, Mass., who will observe this eclipse — his 34th total one — from Salem, Ore.
That’s why eclipses are so scientifically special in general. What’s so great about this particular eclipse?
A total solar eclipse is visible from somewhere on Earth every 18 months or so — so these events are not really that rare. But most of those eclipses go almost entirely unseen. Often the narrow zone of totality — where the sun is completely blocked out — falls over the oceans, and cloud cover blocks those precious minutes of totality over land.
This eclipse, on the other hand, will offer 93 minutes of totality on solid ground (although each location only gets about two minutes). That means more chances for land-based observers to have clear skies. It also gives a rare opportunity to watch the corona dance and sway over the full hour and a half it takes the eclipse to traverse from Oregon to South Carolina (SN: 8/20/16, p. 14).
The Citizen Continental-America Telescopic Eclipse (Citizen CATE) project, led by Matt Penn at the National Solar Observatory in Tucson, Ariz., will take full advantage of that time. Volunteer observers will deploy 68 identical telescopes and digital camera systems along the path of totality in an eclipse relay — as soon as one site’s two minutes of totality are up, the next site’s will begin.
“You can take an event that’s only two minutes long and make it a 90-minute event,” says Yanamandra-Fisher, who is adding a polarization filter to two of the CATE telescopes.
If you want to find out more about what her team — and many others — hope to learn, check back in the coming days. And join us as we count down to the Great American Eclipse.