Earthshine Is Coming—How to See the Dark Part of the Crescent Moon in Spring 2021

Kazushi_Inagaki/iStock via Getty Images
Kazushi_Inagaki/iStock via Getty Images / Kazushi_Inagaki/iStock via Getty Images

Two weeks before April's full moon (or pink moon) on the 27th, sky-gazers will have another reason to looking up at our planet's satellite. The days leading up to and following the new moon on April 12, 2021, are great times to observe Earthshine, which happens when the unlit disk of the crescent moon becomes visible. Here's what you need to know about the phenomenon.

What Is Earthshine?

When only a small portion of the moon is in the right position to reflect the sun's light back to Earth, it resembles a crescent shape. The full disk of the moon is still there, but because most of it isn't illuminated, it's usually hard to see.

Earthshine is the best opportunity to look for this unlit part of the moon. When it occurs, the glow of the sun's light reflecting off Earth is strong enough to reach the moon. The Earth's glow doesn't fully illuminate the moon like direct sunlight would, but it does fill out the dim gray disk around the bright crescent.

When to See Earthshine

Earthshine occurs at varying degrees throughout the year, but the best time of year to see it is in April and May. In the days surrounding the new moons on April 12 and May 12, the unlit surface of the moon will look especially pronounced. To catch the event, look for the moon around twilight. Though a telescope or binoculars will show the dark surface of the celestial body in greater detail, you can also observe it with your naked eye.