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[music] Narrator: If you looked at the moon over the course of a few weeks, you'd probably
notice that it looks slightly different every day. The change in its shadow is based on
where the moon is in its orbit. We call this cycle the phases of the moon, and it occurs
roughly once a month. At least twice a year, however, something quite different
happens. The moon passes through the shadow cast by the Earth, causing it to look extremely
unusual for a short period of time. From the earth, the moon will appear to
darken and turn a deep red before eventually returning to normal. This is called a lunar
eclipse. If we were to look at what happens from space during an eclipse, it would go
something like this. First, the moon passes through what's called the penumbra,
where the sun's light is only partially obscured. This results in only a slight darkening
of the moon. As the moon continues along its path, however, it enters what's
called the umbra, where all direct light from the sun is blocked. But if the sun is blocked,
why does the moon turn red? When light from the sun goes by the side of the Earth,
it passes through a long and thick layer of Earth's atmosphere. Shorter wavelengths
of sunlight, like blue, are scattered by the atmosphere, so by the time the light has finished
its trip to the moon, more of the longer wavelengths, like red, are left over.
On the Earth, the same thing happens at sunset as the ground you stand on gradually passes
into night. As the eclipse ends, the moon leaves the umbra, returns
to its normal color, and then leaves then penumbra, brightening and resuming its original
cycle. Overall, the whole process lasts only from a few minutes to a few hours,
so you'll have to be quick if you want to see it. But, as long as you're willing to stay awake,
you'll catch the moon as you won't see it too often.
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