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[music]

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notice that it looks slightly different every day. The change in its shadow is based on

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where the moon is in its orbit. We call this cycle the phases of the moon, and it occurs

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roughly once a month. At least twice a year, however, something quite different

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happens. The moon passes through the shadow cast by the Earth, causing it to look extremely

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unusual for a short period of time. From the Earth, the moon will appear to

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darken and turn a deep red before eventually returning to normal. This is called a lunar

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eclipse. If we were to look at what happens from space during an eclipse, it would go

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something like this. First, the moon passes through what's called the penumbra,

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where the Sun's light is only partially obscured. This results in only a slight darkening

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of the moon. As the moon continues along its path, however, it enters what's

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called the umbra, where all direct light from the Sun is blocked. But if the Sun is blocked,

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why does the moon turn red? When light from the Sun goes by the side of the Earth,

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it passes through a long and thick layer of Earth's atmosphere. Shorter wavelengths

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of sunlight, like blue, are scattered by the atmosphere, so by the time the light has

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finished its trip to the moon, more of the longer wavelengths, like red, are left over.

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On the Earth, the same thing happens at sunset as the ground you stand on gradually

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passes into night. As the eclipse ends, the moon leaves the umbra, returns

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to its normal color, and then leaves the penumbra, brightening and resuming its original

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cycle. Overall, the whole process lasts only from a few minutes to a few hours,

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so you'll have to be quick if you want to see it. But--as long as you're willing to stay

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awake--you'll catch the moon as you won't see it too often.

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[music, beeping]