What is earthshine?

Sometimes, you can glimpse a pale glow on the unlit part of the crescent moon in the west after sunset or in the east before sunrise. It’s called earthshine.

Palm tree, and slim crescent moon next to it. The darkened part of the moon is glowing dimly with Earthshine.

View at EarthSky Community Photos. | Young moon with earthshine on March 8, 2018, from Dr Ski in Valencia, Philippines. If you were on the moon now, you’d see a nearly full Earth in your sky. Earth’s light is illuminating the lunar landscape, just as the light of a full moon illuminates our earthly landscape. Thanks, Dr. Ski!

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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.

Bright narrow crescent moon with rest of moon showing pale gray.

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. Photo by Aqilla Othman at Port Dickson, Negeri Sembilan, Malaysia. Thank you, Aqilla.

Horizontal stripes of clouds with crescent moon and earthshine in unlighted part of it.

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

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

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