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[ Music ]

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Like clockwork, the full Moon appears every month in our sky, a sight so familiar that we often take it for granted,

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but about twice a year, over the course of a few hours, the full Moon sports a decidedly different look.

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What causes this sudden change? A lunar eclipse occurs when the Moon passes through the Earth's shadow,

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just as a solar eclipse occurs when part of the Earth passes through the Moon's shadow.

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But the Moon circles the Earth every month as it cycles through its phases, lining up at both full Moon and new Moon.

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So why don't eclipses happen twice a month? The reason is that the Moon's orbit around the Earth is tilted

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relative to the Earth's orbit around the Sun. Although the Earth and the Moon always cast long shadows,

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they rarely shade each other thanks to the Moon's orbital tilt.

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But if that's the case, why do eclipses happen at all?

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Throughout the year, the Moon's orbital tilt remains fixed with respect to the stars, meaning that it changes with respect to the Sun.

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About twice a year, this puts the Moon in just the right position to pass through the Earth's shadow, causing a lunar eclipse.

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As the Moon passes into the central part of the Earth's shadow, called the umbra, it darkens dramatically.

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Once it's entirely within the umbra, the Moon appears a dim red due to sunlight scattered through the Earth's atmosphere.

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In fact, if you watched the eclipse from the surface of the Moon,

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you'd see the Sun set behind the entire Earth, bathing you in a warm red glow.

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Back home, you'll have to stay up late to watch a lunar eclipse, but if you do you'll see the Moon in rare form,

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and you'll catch a brief glimpse of our own planet's long shadow.

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[ satellite beeping ]