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The Physics of a "Blood Moon"

Once in a rare while, the moon turns red—because the sky is blue. That might sound like nonsense, but it's the simplest accurate way to explain what happened early this morning, when the moon disappeared from view before returning with an eerie, rusty cast to it.
An image of the "Blue Blood Supermoon" taken over Kobe, Japan last night—the phenomenon attracted lots of attention, for being a coincidence of three lunar phenomena: a supermoon (where the moon is full at perigee), a lunar eclipse, and a blue moon (the second full moon in a month).
Image Credit: Dennis Doucet, via Twitter
Earth and its moon both lie in the same orbital plane around the sun, which is why we get to see breathtaking astronomical events when the three bodies align; the shadow of the moon falling upon the earth creates a solar eclipse, and a lunar eclipse—like this morning's—occurs when Earth's shadow plunges the moon's surface into darkness. The most entrancing part of the whole spectacle, though, is the reddish tint that the moon takes on. So what puts the "blood" in a "blood moon"?

It might be weird to think of a gas casting a shadow, but that's precisely what you're seeing in the image above: the shadow of Earth's atmosphere falling on the moon! When light enters our atmosphere, it bounces around and scatters in all directions—but shorter, higher-energy wavelengths generally scatter more readily than long ones. This is why the sky is blue, but it's also—as we discussed in a previous post—why sunsets and lunar eclipses are red: you're looking at white light, minus the blue of the sky.

The eclipse happened early this morning, so most of us here in the states missed it. Fortunately, these things come around like clockwork; you'll have another chance to catch it—same time next year!

Stephen Skolnick


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