What Causes an Eclipse of the Moon?

Lunar Eclipse, 21 December 2010.

Image: Jiyang Chen/Creative Commons Attribution-Share Alike 3.0 Unported license.

Unless you’ve been living under a rock, you know there will be a lunar eclipse tonight. This eclipse will be completely visible over Africa and central Asia, visible whilst rising over South America, western Africa and Europe, and whilst setting over eastern Asia. In western Asia, Australia and the Philippines, the lunar eclipse will be visible just before sunrise. The North Americans will miss out.

This particular lunar eclipse will be special because unlike most lunar eclipses, the center point of Earth‘s shadow will fall on the moon. This is known as a central total lunar eclipse, and it will have an unusually long duration.

For those who wish to learn more about lunar eclipses, and especially about this lunar eclipse, I’ve found a series of videos about lunar eclipses that you might enjoy. These videos answer a few questions about what causes lunar eclipses, why the moon turns red during an eclipse and I’ve even found a very short video that describes an experiment that NASA will be conducting during this eclipse:

Visit SpitzerScienceCenter‘s YouTube channel.

I am surprised that the previous video didn’t explain why the moon (usually) turns red during a lunar eclipse. (Particularly since christians adopted the ancient myth that a blood-red moon is an indication of the second coming of Christ, thus leading to panic among those who don’t know the real, scientific explanation for this phenomenon.)

Basically, the moon‘s red colour is all about the Earth‘s atmosphere: if Earth had no atmosphere, the moon would be completely dark during an eclipse. The moon‘s red or copper hue is caused by scattering, or refraction, of sunlight passing through Earth‘s atmosphere into the shadow that it casts onto the moon. Shorter wavelengths of light are most scattered by air molecules, air pollution, dust and other small airborne particles. By the time the photons of light have passed through the atmosphere, only the longer wavelengths remain because they are least scattered. These longer wavelengths are red in colour.

This very short video clip gives you a visual idea of what’s happening (no sound):

Visit djxatlanta‘s YouTube channel (NASA/Goddard Space Flight Center Scientific Visualization Studio).

As I mentioned, tonight’s lunar eclipse will provide scientists a unique opportunity to examine the moon. While the sun is blocked by the Earth, NASA‘s Lunar Reconnaissance Orbiter (LRO)’s Diviner instrument will gather temperature readings from the lunar surface. Since different rock sizes cool at different rates, scientists will use these data to infer the size and density of rocks on the moon.

This animation shows the lunar eclipse viewed along the EarthMoon line (no sound):

Visit djxatlanta‘s YouTube channel (NASA/Goddard Space Flight Center Scientific Visualization Studio).

And, last but certainly not least, if you happened to be standing on the side of the moon facing the earth and sun when the lunar eclipse occurs, here’s what you might see (no sound):

Visit djxatlanta‘s YouTube channel (NASA/Goddard Space Flight Center Scientific Visualization Studio).


email: grrlscientist@gmail.com
twitter: @GrrlScientist

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