As millions of Americans look to the sky on Monday to witness the total solar eclipse, a group of young astronomers from the University of Pittsburgh will be in a sparse pocket of the Texas Hill Country trying to crack a 200-year-old mystery.
Shadow bands are thin, wavy lines of alternating light and dark that seem to race across the ground in the minute or so right before, and right after, the moon completely blocks out the sun.
No one knows why this phenomenon occurs. But since German astronomer Hermann Goldschmidt wrote about it in 1820, a couple of theories have emerged.
The leading hypothesis is that shadow bands are caused by atmospheric turbulence. In the brief moments before and after the moon completely obscures the sun, just a sliver of light is visible. As that sliver travels down through the Earth's atmosphere, it hits air pockets of different densities. That causes refraction patterns to create the undulating shadows.
To test this theory, a team of Pitt undergraduates are launching weather balloons on Monday from Concan, Texas. The tiny community, roughly 80 miles west of San Antonio, sits in the path of totality.
Monday's launch is a return trip for the group. Concan was also in the path of last October's annular eclipse — it's a roughly 50-square-mile sweet spot that will witness both astronomical events.
The fall trip was mainly for practice. (During an annular eclipse, shadow bands are unlikely to appear because the moon is too far from the Earth to completely block out the sun.)
"We wanted to have kind of a controlled environment. So, nothing changed besides the time of year," said Peri Schindelheim, a senior studying physics and astronomy, who is the project's student.
On a chilly afternoon in March, Schindelheim and about a dozen others gathered on the lawn in front of the Allegheny Observatory for one final practice run before heading south. The equipment isn't working perfectly, but the students don't anticipate these problems once they reach rural Texas, where there will be less electrical interference.
The weather balloons, which the students inflate with helium, carry instruments that collect data on humidity, temperature, wind speeds and barometric pressure. The white orbs smell strongly of latex and are so large that Schindelheim's arms would need to be more than twice as long to wrap around its circumference.
There is a second theory that shadow bands have nothing to do with the atmosphere but rather are caused by sunlight bending around the moon.
"When the moon's covering the sun, that slight slit … casts this interference pattern on the ground," said junior Mathilda Nillson, who is majoring in astronomy and physics.
To test the slit theory, the students are also launching three high-altitude balloons, which are larger than the weather balloons and will float up to 90,000 feet above the Earth's surface. These balloons are equipped with either a camera or light sensors that will detect if the shadow bands occur before entering the atmosphere.
After the practice launch, research professor Sandhya Rao and the students debrief in the observatory's cramped, three-story library — portraits of Copernicus and other visionaries seem to peer down at the 20-somethings who munch on Sun Chips and peel clementines.
Rao's best guess on the provenance of shadow bands is that it's a combination of both theories — and she said she thinks the students are on the verge of finding out for sure.
"That's why you can hear the excitement," she said. "They know they're involved with something that's really quite extraordinary."
Still, there aren't practical applications for this knowledge or a way to profit off the work. Howard Malc, a senior studying computer engineering and physics, said that’s why the project sometimes feels like a fool's errand.
"We're doing research to figure out just the nature of the universe,” said Malc. “But, you know, I guess that's like the truest type of science."
Hopefully, everything goes well for the young astronomers, because the contiguous United States, Mexico and Canada won't see another total solar eclipse until 2044.
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