Physics Photo of the Week

February 22, 2008

Total Lunar Eclipse
.  Discussion by Forrest Brown.
Despite the call for clouds last Wednesday many people turned out to view the complete lunar eclipse, a phenomenon only possible during the full Moon, when the Earth comes between the Moon and the Sun.  This picture shows the completely eclipsed Moon.  The Moon entered the Earth's shadow, becoming a reddish hue. This coloring is caused by the absence of the shorter wavelengths of light, such as blue.  The blue light is scattered as it tries to penetrate the Earth’s atmosphere. Only the longer red light wavelengths are able to completely penetrate the Earth’s atmosphere and reach the Moon.  See diagram below.

The picture at left shows the Moon as it begins to enter the Earth's shadow.  The shadow of the Earth’s curve can be seen on the Moon as it obscures the Sun’s light. Here the moonlight appears normal because the full spectrum of sunlight is reaching it. The Earth has yet to fully block the Moon.

Finally the Moon begins to emerge from the Earth’s shadow. In the picture at right it can be seen that the bottom of the Moon is emitting the full spectrum of light, while the rest is still illuminated by only red light wavelengths. This picture was taken at a longer exposure so that the dark eclipsed portion of the Moon is visible, unlike the picture above.

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

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