What is earthshine?

When you see the crescent moon after sunset this week, notice the pale glow on its unlit portion. This glow is called earthshine.

Aqilla Othman at Port Dickson, Negeri Sembilan, Malaysia caught the March 29, 2017 young moon. The pale glow on the dark part of the moon is light from a nearly full Earth, seen from the Earth-facing side of the moon. Thank you, Aqilla.

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 disk. That pale glow on the unlit part of a crescent moon is light reflected from Earth. It’s called earthshine.

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.

Susan Gies Jensen caught this view of earthshine from Odessa, Washington on April 8, 2016.

Susan Gies Jensen caught this view of earthshine from Odessa, Washington.

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Bottom line: Earthshine – the dim glow on the darkened potion of a crescent moon – is light from Earth cast on the night side of the moon.