Strange phenomenon of shadow bands captivates scientists as a total solar eclipse nears - CNN

Racing water. Phantom serpents. Rippling patterns of light and dark. Solar eclipse watchers have for centuries vividly described an eerie phenomenon known as shadow bands that appears just before and after totality — the period when the moon completely blocks the sun’s light and the sky goes dark. Long dark lines, separated by white spaces, race across the ground or sides of buildings. David Turnshek, a professor of physics and astronomy at the University of Pittsburgh, first witnessed shadow bands as a teenager. “I guess I would have been 14 years old. I went to see a total eclipse near Virginia Beach. It was 1970, and I had made my own telescope, and I went down there to try to photograph the eclipse,” said Turnshek, who is also the director of the Allegheny Observatory in Pittsburgh. “And while I was down there, I noticed right before the total eclipse, these bands of light were sweeping across the ground.” The human eye can easily detect shadow bands, but they are difficult to photograph and capture on video because of the subtle contrast. Turnshek’s research as an astronomer has primarily focused on galaxy formation and quasars, the luminous cores of distant, ancient galaxies. However, he has retained his youthful curiosity about shadow bands — a phenomenon with two competing theories but no definitive explanation. An upcoming total solar eclipse on Wednesday will provide scientists with another opportunity to study this mystery. Sky-gazers can watch for the spectacle when the path of totality sweeps through the northern tip of Russia, eastern Greenland, western Iceland, the northern half of Spain and northeast corner of Portugal. Surprising results In 2017, a total solar eclipse crossed the United States from coast to coast for the first time in 99 years. After securing NASA funding to study the occurrence, Turnshek traveled to Tennessee with a group of University of Pittsburgh undergraduates who called themselves the Pitt shadow bandits. Their goal was to test the leading theory, first put forward in the 1980s, that turbulence — the irregular motion of air currents in the atmosphere — caused shadow bands. When the moon obscures most of the sun just before totality, leaving only a thin sliver of sunlight, the turbulence becomes visible. It’s the same reason why stars, when viewed from Earth, appear to twinkle. However, in the case of the sun, the effect isn’t usually visible because of its massive size. The group placed light detectors called photodiodes on the ground and on board a high-altitude balloon, which collected data on the light patterns before and after totality. This data was then analyzed using spectrograms, a way of visually representing the strength of light waves and how they change over time. If the prevailing theory was correct, the team would observe shadow bands on the ground, but not on the balloon, which was 25 kilometers (15.5 miles) high — above most of Earth’s atmosphere. The results were surprising, Turnshek recalled. The researchers found a sustained signal of 4.5 hertz — a measure of how often a light wave passes a fixed point per second, at both high altitude and on the ground. “We saw this effect above the atmosphere and on the ground, which meant that the leading theory for shadow bands could be wrong in the sense that that wasn’t the only explanation for shadow bands,” he said. The findings from the 2017 eclipse suggested another explanation, known as the diffraction-interference, might be responsible for shadow bands. Diffraction is what happens to light waves when they encounter an obstacle. For example, when light bends around a sharp obstacle or boundary, such as a knife edge, the waves bend and interfere with one another to create bands of dark and bright light. “At one point in time it was the most popular theory, that it was some kind of diffraction-interference effect,” he said. “The thought is, ‘Well, we have an eclipse, and the moon, even though it has a curved edge, acts as some kind of a knife edge.” The team checked to make sure that the signal was not the result of faulty electronics that produced false readings. Turnshek and his student researchers looked for an opportunity to try and replicate the results, noting it didn’t necessarily mean that the prevailing theory was incorrect. “When you have a complicated situation in science, it’s often the case that two things are true at the same time, contributing to the phenomena,” Turnshek said. Nearly a decade later, during the April 8, 2024, total solar eclipse, Turnshek took another student research team to Concan, Texas, within the path of totality — and where the weather was expected to be clearest. A total solar eclipse passed over North America on Monday, putting on a dramatic show that was visible to millions of people.A total solar eclipse occurs when the moon passes between Earth and the sun, completely blocking the sun's face. Those within the path of totality, including 32 million people in the United States, saw the eclipse in its full splendor. People outside the path were still able to see a partial solar eclipse, where the moon blocks only part of the sun's face.Many people traveled to the path of totality to attend special viewing parties. Those along the very center line of the path saw an eclipse that lasted between 3½ and 4 minutes, according to NASA.For many Americans, this was the last chance to see a total solar eclipse for 20 years. The next one won't be visible across the contiguous United States until August 2044. Despite the researchers deploying more sensitive sensors and extending the scope of measurements, cloud cover meant that they did not detect shadow bands. The team deployed two high-altitude balloons with light sensors and 31 weather balloons to measure temperature, pressure, wind speed and humidity. “For the most part, it was cloudy on the ground, but we launched the two high-altitude balloons, and we didn’t see any signal whatsoever due to shadow bands despite the fact that our detector was much more sensitive,” Turnshek said. “Of course, I was incredibly disappointed.” The team was also able to deploy an additional set of light sensors on an aircraft that flew in Vermont, where the skies were clearer. The spectrogram from the aircraft, however, did not show a 4.5 hertz signal — the pattern they found on the high-altitude balloon during the 2017 eclipse. The researchers also weren’t able to make any corresponding ground-based shadow band observations in Vermont, making the data inconclusive. An open book Turnshek will travel to León, Spain, with some colleagues to see the upcoming solar eclipse, but he said he will be unable to replicate the same experiment because of the logistical complexity involved. This time, he’s primarily there as a tourist. “I decided initially I would just go to Spain and enjoy looking at the eclipse because it’s an incredible thing, but of course, I couldn’t help myself, and I’m taking one of our electronic detectors to Spain.” At best, Turnshek said he hopes to detect and capture video of the shadow bands, using the same ground-based equipment he deployed in Texas in 2024. But without measurements from above, using a plane or balloon, it won’t be possible to resolve the ambiguous results. “Maybe somebody will have read our 2017 and 2024 results, and will say, ‘I’m in Spain. I’ve got a plane.’” How to see the shadow bands Turnshek is not the only researcher looking to record shadow bands on Wednesday. Gordon Telepun, an eclipse chaser who worked as a NASA ambassador to share knowledge about eclipses in 2017 and 2024, will be on the Spanish island of Mallorca for the eclipse. He has seen shadow bands at five of the seven eclipses he has witnessed. “You’re not going to see them on grass or asphalt or something like that. You need a smooth, light-colored background,” said Telepun, who has a created an eclipse app that includes a reminder of when to look for the bands. “If you’re going to see them, you have to know to look for rows. Know in your mind what you’re looking for. They’re going to be very faint.” Shadow bands are easy to miss, he noted, because they happen just before totality — the most exciting time when viewers are wearing solar glasses and looking up at the final, thin crescent of the sun. Joe Conti, an independent researcher and physics enthusiast based in Massachusetts, is inviting eclipse observers across Iceland and Spain to capture the optical mystery on their smartphones as part of a citizen science project. Five groups have expressed interest in participating so far, he said. Conti recommends setting up a simple observation board made from cardboard and covering it with a white sheet. He said he hoped the data collected can perhaps help the broader scientific community close the book on what causes shadow bands. For those lucky enough to be in the path of totality on Wednesday and wanting to catch a glimpse of shadow bands, Turnshek’s advice is to be south of mountains and coast, where cloud cover is more likely to gather. He also recommends heading west to make sure the sun is as high as possible in the sky. The upcoming eclipse takes place in the evening local time when the sun will be setting. “Even in clear weather,” he said, “it’s cloudier near the horizon.”