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Physics Photo of the Week

March 24, 2006
Partial Lunar Eclipse

On March 14, 2006, the moon partially entered earth's shadow and experienced a partial lunar eclipse.  The right hand portion of the moon is noticeably darker than the left side of the moon. 

A partial lunar eclipse is not nearly as spectacular as a full eclipse, so I hadn't paid it much thought.  However, the weather was very nice on March 14, 2006 - clear and balmy - and I happened to see the moon soon after rising and noticed the darkened side.

The red color of the moon is due to absorption of light by the earth's atmosphere.  The moon had just risen above the horizon.  The optical path through the atmosphere is much longer than if the moon were high overhead.  As a result absorption and scattering of the blue colors causes the moon to appear red - similar to the red sun setting or rising.

A full eclipse is much more spectacular than the partial eclipse.  The last successful total lunar eclipse was recorded at Warren Wilson almost three years ago in May, 2003.

The picture at right is taken about 45 minutes later than the photo above.  Notice the overall color of the moon is not nearly as red as in the first photo.  This is due to the higher elevation of the moon in the sky - not nearly as much air to redden the apparent moon color.  The partial shadow is barely visible in the upper right quadrant.  The moon had moved relative to the earth's shadow in its continuing orbit around the earth.

A lunar eclipse is the only time when the moon enters the earth's shadow.  The lunar eclipse therefore can occur only when the moon is on the opposite side of the earth from the sun - i. e. when the moon is "full".  Usually the full moon misses the earth's shadow. 

Every month we get to view a crescent moon as in Molly sternberg's photo at left.  The crescent moon at left is featured in Physics Photo of the Week for Sept. 9, 2005.  The crescent moon was observed in the west not far from the sunset.  The angle between the sun, moon, and earth creates the crescent shape.  It is not the earth's shadow.  The moon can enter the earth's shadow only when it is in the full moon phase, and then only when the moon's orbital plane and the earth's orbital plane line up with the sun-earth direction.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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

Observers are invited to submit digital photos to: