Physics Photo of the Week

April 15, 2011

Earthshine on the Crescent Moon

When the Moon is just past new or just before new we only see a thin crescent illuminated by the Sun.  This image was taken on April 5, 2011 soon after the Sun had set below the horizon. 

Of special interest is the visibility of the features on the dark side of the Moon.  This faint illumination comes from sunlight reflected from the Earth - Earthshine.  Earthshine on the Moon is exactly analogous to Moonshine on the Earth (not an illegal beverage...).  If we were standing on the Moon facing the Earth, we would see an almost full Earth, very bright, hence the surrounding moonscape would also by illuminated by the Earthlight.  Likewise when we are outdoors on Earth during the full Moon, the Moon is very bright and we can see our shadows in the Moonlight.

The timelapse photo of the moon's apparent motion in the sky is shown at right.  Notice that the Moon's setting path is at an angle to the horizontal.  This angle depends on our latitude on Earth.  Compare with the recent sunrise time (PPOW for March 25, 2011).  When observed from the northern hemisphere setting objects (Sun, Moon, and stars) move down and toward the right (north).  Rising objects move up and toward the left (south).

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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