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What is special about a crescent moon?

Although crescent shapes surround us if we think of bananas or croissants, the Moon is the only crescent in the sky—even if telescope users can sometimes see Venus and Mercury that way, too.

Crescents only appear within a few hours of sunrise or sunset. Cartoonists often depict a thin crescent Moon hovering above errant teenagers sneaking home at midnight, but this is impossible!

We all find beauty in a crescent Moon. Perhaps its dreaminess comes from its uniqueness. Interestingly, the crescent’s orientation changes with the seasons.

• The crescent Moon will appear as a smile at nightfall in the spring and an archer’s bow in the summer.
• At no time or location does the Moon appear to be frowning. An upside-down smile is simply not possible, except as an elusive blue-sky daytime challenge.

See your local Moon phase time.

The crescent Moon and Earthshine

Whenever a thin crescent Moon appears, a strange but famous feature materializes: The dark portion of the Moon (the area unlit by the Sun) prominently glows, a phenomenon called Earthshine.

It’s an accurate label. It happens because 38% of the sunlight that strikes Earth bounces back into space, and some of it hits the Moon. About 10% of that light reflects back to our eyes. So, Earthshine is sunlight that has made a series of bounces.

The thinnest crescent displays the brightest earthshine. This is due to phase reciprocity, one of those terms it’s oddly fun to use as much as possible, even when it’s not appropriate. 

It boils down to this: When the Moon appears thinnest from Earth, Earth then looks most full from the Moon.  So that’s when our own brilliance in the lunar sky maxes out to paint the lunar surface brightly. It happens the next few evenings! 

Learn more about Earthshine.