Why does the moon look red during a lunar eclipse?

A total lunar eclipse could be seen in cloud-free regions across most of the United States and Canada on Saturday morning, Dec. 10. In a total lunar eclipse the sun, Earth and moon line up and the Earth casts its shadow on the moon. The moon is always a full moon and it never goes completely dark during a total lunar eclipse. It appears reddish for the same reason that sunsets and sunrises often have a red tint.

The Earth’s shadow has two parts: the umbra and the penumbra. A small amount of the sun’s energy shines directly within the penumbra. Within the umbra, none of the sun’s rays shine directly in this part of the shadow, so it is the darkest part of the shadow. A total lunar eclipse occurs when the moon travels through the Earth’s umbra.

The umbra is not pitch black because of the Earth’s atmosphere. Sunlight is scattered, or redirected, in all directions by the Earth’s atmosphere into the penumbra and umbra. As light passes through the atmosphere, the blue colors are scattered out of the path. Red and orange light pass through the atmosphere and are scattered into the shadow zone. If our planet had no atmosphere, then the moon would be completely dark during a total lunar eclipse.

If you missed seeing this recent total lunar eclipse, the next total lunar eclipse that is viewable from our area occurs in 2014.

Category: Phenomena
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