What is Earthshine?

March 14, 2019

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When you see the thinnest of crescent Moons—just before and after the new Moon—you may see a glorious phenomenon called Earthshine!

What is Earthshine?

Ever noticed that you can sometimes see the part of the Moon that isn’t lit up?  That’s earthshine!  How does it happen?  The Sun’s light rays are reflecting off Earth’s surface and back onto the Moon! It gives the dark, unlit portion of the Moon an eerie radiance.

This is unique in all the universe: Only the Moon is near enough to reflect back our own light for our narcissistic enjoyment.


When to See Earthshine

Earthshine is most apparent one to five days before and after a New Moon when you’ll see a thin crescent moon in the night sky.

When is the New Moon this month? Look on your Moon Phase Calendar!

The best time of the year to experience this phenomenon is in late winter and early spring. And the best time of the day to see earthshine is in the evening at dusk.

Cool fact: The phases of Earth and Moon are complementary. When the Moon is a skinny crescent, lunar colonists would see Earth in their sky as nearly full.  Conversely, when the Moon is full, then our world is “new” in the Moon’s sky, and they only see our night hemisphere.

Why does Earthshine seem so bright? Only when the Moon is a thin crescent does its sunless portion receive the brilliance of a virtual full Earth. And we are simply dazzling. With four times the Moon’s diameter and 3 ½ times its shininess, our world is over a hundred times more brilliant than the Moon appears to us, even when it’s full.

Credit: NASA

The Old Moon in the New Moon’s Arms

In the olden days, earthshine was called “the old Moon in the new Moon’s arms.” You’ll also hear it as, “the new Moon in the old Moon’s arms.” Earthshine is sunlight that’s taken a detour. Sunshine hits us, bounces to the Moon, then enough bounces back to our eyes to enable us to see it. It’s a three way trip, making the glow on the Moon’s dark side “older” than the direct sunlight illuminating the brightly lit crescent.

Therefore, if the Sun were to blow up, the bright crescent would vanish first, but the dimmer earthshine would linger for an additional three seconds—light’s roundtrip time from Earth to the moon. If you see this happen, grab the phone and sell your stocks. This knowledge will give you a few seconds’ head start over everyone else.

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe