A sliver of the moon and Venus on the side. Photo by Eric Fitzgerald

July 15 was a conjunction of the crescent moon and Venus. Luckily, our peripatetic photographer, Eric Fitzgerald, was on the job even on vacation in North Central New Mexico.

His photo shows Earthshine on the moon (Earth’s light is on the night side), with Venus to the left and a juniper tree in the foreground.

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 and it’s called earthshine.

To understand earthshine, remember that the moon is globe, just as Earth is, and that the globe of the moon is always half-illuminated by sunlight. 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 when 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. And that is earthshine. It’s light from the nearly full Earth shining on the moon.

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. Learn more at


No Comments Yet

Leave a Reply

Your email address will not be published.