Millions of people equipped with cameras, eclipse glasses, welder’s helmets and homemade projectors (but hopefully not their own unprotected eyes) took in the total eclipse on Monday. At its peak along the path of totality, the moon became a dark disc, encircled all the way around by a bright, white light. The sun’s corona seemed to stretch out, as if trying to grasp the stars and planets that were suddenly visible.
“I had expected it was going to look like nighttime,” said Susana Martinez-Conde, a neuroscientist who studies visual illusions at SUNY Downstate Medical Center, and who watched the eclipse at the Greenville Zoo in South Carolina. “The sky remained dark blue. It wasn’t day, but it wasn’t night either.”
For centuries, people have tried to portray total solar eclipses like this one in illustrations, photos — you might have taken some snapshots of your own — and scientific imagery. Why don’t those come close to the real thing?
The answer lies, in part, in how confusing it is for human and man-made machines to make sense of the extreme light and dark of a total solar eclipse. Depending on what’s doing the processing, different perspectives emerge.
Take these three images, for instance, Richard Woo, a scientist who studies the corona at NASA’s jet propulsion lab at Caltech, suggested in a 2015 paper. In the first, a painting praised for its authentic rendering of what humans saw during an eclipse in 1932, a corona engulfs the whole moon. Long streamers extend out from it in a slate blue sky. In the next, a photograph of the same eclipse as seen in Canada (this one taken by an astronomer named G. Harper Hall), a globular, ghostly corona emerges in a sea of black. And in the final image, of a 1994 eclipse in Chile, processed by the High Altitude Observatory at the University Corporation for Atmospheric Research with a special camera-telescope to capture the corona’s reach, streamers are present, but light disappears in holes at the sun’s poles.
The variation can be explained by a mismatch between the range of brightness that humans and cameras can detect and the far-greater range of brightness that exists during a total solar eclipse.
In a dark room, humans can detect the dim light of a single photon, and the bright light of up to one million photons at the same time. This dynamic range, as it’s called, explains why we can see in a moonlit forest and on a sun-soaked beach — and distinguish the difference between the light in both. The thing is, we don’t have to look at them at the same time, which is kind of what we have to do during a total solar eclipse. It makes our eyes go a bit nuts.
To manage this, our eyes shift their ranges to accommodate the brightest light, sacrificing our ability to see the dimmest. That’s why we see the streamers that aren’t visible in a globular photo made with regular camera.
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