Astronomy Essentials

Earthshine – what is it?

Earthshine: Bright narrow crescent moon with rest of moon showing pale gray.
The pale glow on the dark part of the moon – earthshine – 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.

See the young moon the next few days

In 2022, the March new moon fell on March 2 at 17:35 UTC. So you’ll want to start looking for a very slim crescent moon – low in the sky after sunset – on March 3. 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. If you’re in the Northern Hemisphere, the month of March, around the March equinox, is an excellent time to look for very young moons, that is, slim crescent moons in the west after sunset.

Southern Hemisphere? It’s autumn now in your part of the world. And the autumn angle of the ecliptic is always narrow with respect to the evening horizon. You’ll have to wait a day or two after March 3 to catch the slim crescent and earthshine.

Love watching the moon? Keep track of its phases for every day in 2022 with an EarthSky lunar calendar. Still available! Get yours now.

What is earthshine?

To understand earthshine, remember that the moon is a globe, just as Earth is, and moreover that the globe of the moon is always half-illuminated by sunlight just like Earth. 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.

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, 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!
Horizontal stripes of clouds with crescent moon and earthshine in unlighted part of moon.
Susan Gies Jensen caught this view of earthshine from Odessa, Washington.

Earthshine in an old ballad

Sometimes people refer to the very young moon as the new moon, as in this verse from the ballad Sir Patrick Spens, but in fact the new moon is not visible to the eye, and what the superstitious old mariner described was the young moon with earthshine “in her arms.”

I saw the new moon, late yestreen,
Wi’ the auld moon in her arm;
And if we gang to sea, master,
I fear we’ll come to harm.

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Bottom line: Earthshine, or the dim glow on the darkened portion of a crescent moon, is light from Earth shining on the night side of the moon. In 2022, from the Northern Hemisphere, start watching for it around the evening of March 3.

Read more: What’s the youngest moon you can see with the eye alone?

Posted 
March 2, 2022
 in 
Astronomy Essentials

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