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Want to see how a solar eclipse alters colors? Wear red and green on Monday

Pinhole shadows show crescent shapes in 2019 as the moon moves in front of the sun — one of several unique phenomena we can see during a solar eclipse.
Louis Kwok
AFP via Getty Images
Pinhole shadows show crescent shapes in 2019 as the moon moves in front of the sun — one of several unique phenomena we can see during a solar eclipse.

If you live along the path of totality for Monday's solar eclipse and you have any Christmas or Hanukkah gear, you might want to break it out. The celestial event will bring odd phenomena to our planet — including changes in how people see colors such as red, green and blue.

Here's a look at some of the unusual visual effects a solar eclipse brings to humans on Earth:

Red and green colors will look strange

That's partly due to the change in light when the moon blocks the sun, but also the way our eyes and brain adjust to and interpret that change.

As light dims, our eyes transition from photopic vision, associated with the retina's cone cells that deliver full colors and fine detail, toward scotopic night vision that relies on rod cells to detect objects in low light. In the middle is mesopic vision, the transitional phase where both rods and cones are active.

When the light's intensity dims in the eclipse, colors with longer wavelengths, like red, will look darker as cones become less active. But because rods are sensitive to the shorter blue-green wavelengths, those colors will have a chance to shine.

"This is pretty much a totality thing," Erika Grundstrom, director of astronomy labs at Vanderbilt University, told NPR, with only people in the eclipse's central path guaranteed to witness the phenomenon.

Also, she said, you shouldn't rely on just one red or green T-shirt to trigger the effect.

"You have to have lots of people (or colorful things) around to see it," Grundstrom said via email, adding, "the effect is the result of sudden dimness and your rods and cones trying to make sense of that dimness."

It's called the Purkinje effect.

The what?

The Purkinje effect, aka the Purkinje phenomenon or shift, was documented some 200 years ago by Johannes Evangelista Purkinje, a Bohemian scientist who noticed that when light passed through a prism in dimming conditions, the brightest spot moved — shifting away from red and toward blue, on the shorter end of the wavelength spectrum.

The effect has been studied in the years since — including during the famous 1919 total solar eclipse that gave scientists important observations affirming Albert Einstein's theory of general relativity.

The dimness should become more noticeable about 15 minutes before the eclipse reaches totality. To many people, the light takes on a metallic or silvery quality.

Some shadows will sharpen; others will alter

For one thing, your own shadow will be different.

"The change in lighting makes shadows look sharper on the ground, so it's possible to see individual hairs on your head in your shadow," according to the European Space Agency.

And if you see the sun's light coming through tight gaps in trees, you might notice lots of little crescents. As the ESA says, "The tiny gaps in the leaves will act like multiple pinhole cameras, projecting the Sun's image to the ground."

People in the eclipse's path can also see an odd shimmering called shadow bands.

"A minute or two before totality, ripples of light may flow across the ground and walls as Earth's turbulent atmosphere refracts the last rays of sunlight," as the EarthSky website said in 2017.

Other changes to expect include a drop in temperature — and the appearance of colors in the sky that will make it seem as if you're seeing a sunset (or sunrise) across the full 360 degrees of the horizon.

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Bill Chappell is a writer and editor on the News Desk in the heart of NPR's newsroom in Washington, D.C.