What is Earthshine?

August 17, 2019
Earthshine
NASA

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Ever noticed that you can sometimes see the part of the Moon that isn’t lit up?  That’s earthshine!  Historically, this is called “the new Moon in the old Moon’s arms.” Learn more about earthshine.

What is Earthshine?

When the crescent Moon appears in evening twilight, a strange but famous feature becomes visible: The dark portion of the Moon (the area unlit by the Sun) seems to glow.

This phenomenon is aptly called earthshine. 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.

What causes Earthshine?

This occurs because 38 percent of the sunlight that strikes Earth bounces back into space; some of this earthlight bathes the lunar surface. About 10 percent of that light bounces off the lunar surface (which is not very reflective) to create the visible glow (earthshine) on the Moon’s dark side.

The thinnest Moon crescents (both waxing and waning) display the brightest earthshine. This is due to the phase reciprocity of the Earth and the Moon: When the Moon appears thinnest from Earth, Earth would look full from the Moon. Conversely, Earth would appear to be unlit, or in its “new” phase, if viewed from the Moon during what we would see as the Moon’s full phase.

Don’t be fooled: The portion of the crescent Moon that is illuminated by earthshine appears to be part of a smaller orb than the sunlit crescent. This mirage is caused by our eyes’ response to the different light levels. It vanishes when you view the crescent through binoculars.

earthshine-poem.jpg

During what phases does Earthshine occur?

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!)

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.

The best time of the day to see earthshine is in the evening at dusk.

And the best time of the year to experience this phenomenon is in late winter and early spring. 

Cool Earthshine Facts

  • 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.
  • As a result of all of the sun- and earthshine bouncing, the crescent Moon’s earthlight is older than its sunlight.
  • If the Sun were to go dark suddenly, the crescent Moon would disappear at the same time, but earthshine would continue to illuminate the Moon faintly for three seconds.

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.

That’s why, 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