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| Space on May 26, 2014

What is earthshine?

That glow over the unlit part of a crescent moon – called earthshine – is light reflected from Earth.

The moon and Venus over  North Carolina by Ken Christison.

The moon and Venus over North Carolina by Ken Christison.

When you look at a crescent moon shortly after sunset or before sunrise, you can sometimes see not only the bright crescent of the moon, but also the rest of the moon as a dark disc. That pale glow on the unlit part of a crescent moon is light reflected from Earth. It’s called “earthshine.”

The moon and Venus over  North Carolina by Ken Christison.

The moon and Venus over North Carolina by Ken Christison.

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Photo credit: carolune

To understand earthshine, remember that the moon is globe, just as Earth is, and that the globe of the moon is always half-illuminated by sunlight. When we see a crescent moon in the west after sunset, or in the east before dawn, we’re seeing just a sliver of the moon’s lighted half.

Now think about seeing a full moon from Earth’s surface. Bright moonlight can illuminate an earthly landscape on nights when the moon is full.

Likewise, whenever we see a crescent moon, a nearly full Earth appears in the moon’s night sky. The full Earth illuminates the lunar landscape. And that is earthshine. It’s light from the nearly full Earth shining on the moon.

So next time you see a crescent moon, expand your thinking – to include the Earth under your feet. See the glow on the unlit portion of the moon for what it really is – sunlight reflected from the nearly full Earth shining in the moon’s sky.

View larger. | Matt Burt wrote:

View larger. | Matt Burt wrote: “My son and I took a walk after dinner to a nearby hilltop to get this image of the waning crescent moon and the comet Pan-STARRS.” Comet and moon with earthshine seen in spring 2013. Thank you, Matt!

Moon with earthshine and planet Venus. Photo credit: fdecomite