Enjoying EarthSky? Subscribe.

102,260 subscribers and counting ...

By in
| Space on Jan 15, 2010

When can you see earthshine on a crescent moon?

That pale glow on the unlit part of a crescent moon is light reflected from Earth.

Sometimes when there is a crescent moon shortly after sunset or before sunrise, you can see not only the bright crescent of the moon, but also the rest of the moon as a dark disc. Why can you see the dark disc of the moon? Shouldn’t it be pitch dark like space?

That pale glow on the unlit part of a crescent moon is light reflected from Earth. It’s called “earthshine.” 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 – and illuminates the lunar landscape. Next time you see a waxing crescent moon after sunset or before sunrise, think about the fact that the sun is just below the horizon.

And when you’ve got that sense of the moon and sun in space, expand your thinking – to include the Earth under your feet. You might get a sense of the sun, shining on Earth’s day side – and see the glow on the unlit portion of the moon for what it really is – sunlight reflected from Earth.