We recently received an e-mail from a customer who hoped her night sky print would have “a crescent moon with the dark part illuminated.” 

Have you ever seen this phenomenon?  It’s called “earthshine,” and tonight (Thursday, December 5) is a perfect time to observe it.

Earthshine is exactly what it sounds like.  Just as the moon reflects the sun’s light, the surface of the Earth reflects the sun’s light too.  Right after or just before the new moon, the moon appears in the Earth's sky as a slender crescent.  For a few hours, the part of the Earth that is still experiencing daylight reflects the sun's light out into space, and as it lights up the darkened orb of the moon, here’s what you see:

The intensity of earthshine depends on the reflective quality of the Earth’s surface.  Clouds, for instance, reflect more sunlight back into space than water.  So earthshine observed in California when there’s no cloud cover over the Pacific will be not as great as on those evenings when there are clouds far out to sea.

If there aren’t clouds obscuring your view tonight, take a moment to see the moon’s earthshine.  Poets have called this phenomenon the “old moon in the new moon's arms.”  Beyond the urge to wax lyrical, you’ll be rewarded with an extra visual treat as ‘Evening Star’ Venus – now at its brightest evening apparition – makes a lovely pairing with the young moon, illuminated about 12%, both of them shining amid the stars of Sagittarius.