Strange phenomenon captivates scientists as total solar eclipse nears

PITTSBURGH - 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. 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.