MadSci Network: Astronomy

Re: Why does the Moon turn red during a Lunar Eclipse?

Area: Astronomy
Posted By: George Delange, Secondary School Teacher Science/Math, Phoenix ZUnion High School District
Date: Sun Feb 16 20:20:13 1997

Hi Nicole

You have asked a very good question. Yes, on occasions the moon does appear to turn slightly red during a lunar eclipse! On occasions it even is known to turn what is called a "blood red" color.

The reason for this color is exactly the same reason that we have red sun rises and sun sets on the earth. It is caused by the dust and other pollutions that are in the air of the earth.

As the sunlight travels through the earth's atmosphere, it picks up a red color from the pollutions and dirt and then the sunlight travels on through the earth's atmosphere and it stops on the surface of the moon during an eclipse! Thus, the red color we have in our sunrises and sunsets in now being projected upon the moons surface! Therefore, the moon will have a red to red brown color.

The reason it only happens during eclipses is that is the only time that the allignment of the sun, earth, and moon will allow the sunlight to go through the earths atmosphere and to be directly projected upon the moon.

On other occasions (ie. moon phases), the reflected light from the earth strikes the moon, but that does not give the moon a red color. It must be light that passes through the earths atmosphere and the light must then pass directly to the moon. It can not be reflected light.

I have observed this change in color on many occasions. It becomes more red in color after eruptions of very large volcanoes on the earth which throw large amounts of smoke and dust into the earths air. There are even some changes in color after large burnings of forests due to large amounts of smoke in the earths air. These events can also make it difficult to observe very faint objects in space such as stars and galaxies.

By the way, the upcoming partial lunar eclipse will present a rare opportunity to see comet Hale Bopp at the same time since the moon and the comet will appear to be near each other and the eclipse will darken the sky enough to see the comet!

Clear skies and happy observing.

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