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Scientists get another chance to study a solar eclipse mystery

SCOTT DETROW, HOST:

As millions of Americans look to the sky on Monday to witness the total solar eclipse, a group of University of Pittsburgh students will be chasing shadows in the Texas Hill Country. Sarah Boden with WESA in Pittsburgh reports that the young astronomers are on the verge of cracking the 200-year-old mystery of shadow bands.

SARAH BODEN, BYLINE: On a chilly afternoon in March, the Pitt students use duct tape and zip ties to close the opening of a weather balloon. The helium orb smells strongly of latex and is so large my arms would need to be more than twice as long in order to wrap around its circumference.

UNIDENTIFIED PERSON #1: Oh no, that is right. That was backwards.

BODEN: This is a practice launch for the actual experiment the students will be conducting to understand the phenomenon of shadow bands.

UNIDENTIFIED PERSON #2: There's only one tape, but go the other side...

BODEN: Shadow bands are thin, wavy lines of alternating light and dark that seem to race across the ground right before and right after the moon completely blocks out the sun. No one knows why this happens.

MATHILDA NILSSON: One concept is that the shadow bands are caused by turbulence, like gravity waves in the atmosphere.

BODEN: Mathilda Nilsson is a junior studying astronomy and physics. Since German astronomer Hermann Goldschmidt first wrote about the phenomenon in 1820, two leading theories have emerged. One, as Nilsson explains - just before totality, only a sliver of sunlight is visible. As that sliver travels through the Earth's atmosphere, it hits air pockets of different densities, and that causes refraction patterns to create the shadow bands. To test this, the students will send up their balloons equipped with weather instruments.

NILSSON: So there's that wire bit on one side. And then on the other side is the antenna, so that's transmitting the data.

BODEN: Data like humidity, temperature and barometric pressure. The second theory is shadow bands have nothing to do with the atmosphere, but rather it's sunlight bending around the moon.

NILSSON: It's also possible that when the moon's covering the sun, that, like, slight slit - and so it casts like this interference pattern on the ground.

BODEN: To test the slit theory, the students will also be launching separate high altitude balloons from Concan, Texas, a small community in the path of totality. These balloons float up to 90,000 feet above the Earth's surface. They'll have either a camera or light sensors that will detect if the shadow bands are visible before entering the atmosphere.

HOWARD MALC: It feels sometimes like we're going on a fool's errand here.

BODEN: Howard Malc is a senior studying physics. He says solving the mystery of shadow bands doesn't have a lot of practical applications.

MALC: There's not, like, a way to profit off of this. We're not doing research for R&D. We're doing research to figure out just the nature of the universe. But I guess that's, like, the truest type of science.

BODEN: Hopefully, everything goes well because North America won't see another total solar eclipse until 2044.

UNIDENTIFIED PERSON #3: Three, two, one. Launch.

BODEN: For NPR News, I'm Sarah Boden in Pittsburgh.

DETROW: All Eclipses Considered continues tomorrow. We'll look at how the ancient Mesopotamians observed and documented eclipse cycles and how much of a shadow, as it were, solar eclipses cast over Mesopotamian society. And since the moon is doing all of the work but not getting enough credit, we will talk moon as well.

UNIDENTIFIED PERSON #4: We're just seeing a snowball effect of activities on the moon, and they just keep coming. All of them taken together, that's the most exciting thing about lunar exploration right now.

DETROW: In fact, there is so much activity coming to the moon that the U.S. government has taken a step toward establishing a new lunar time zone. It's needed because - fun fact - time moves slightly differently on the moon than it does on Earth. Find out more tomorrow. Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Sarah Boden covers health, science and technology for 90.5 WESA. Before coming to Pittsburgh in November 2017, she was a reporter for Iowa Public Radio where she covered a range of issues, including the 2016 Iowa Caucuses.