Why the Blood Moon Eclipse Turns Red

On Sunday (Sept. 27), skywatchers across the United States will be in for a special treat: The full "supermoon" (a full moon when the moon is closest to Earth in its orbit) will go into eclipse. A lunar eclipse occurs when the moon passes into the shadow of the Earth.

There are several "flavors" of eclipse. A penumbral eclipse happens when the moon just grazes the thinnest part of Earth's shadow. If the moon partly goes into the shadow, observers see a partial eclipse. A total eclipse is the most spectacular of the three: The moon goes fully into the shadow and appears either red or brown.

Sometimes an eclipse moon is called a "blood moon" because of this rusty color. But why does the moon turn red, and not simply darken to black when it goes into the shadow? As NASA explains, it's because the Earth's atmosphere extends beyond the planet, and sunlight passes through it, still reaching the moon.

"During a total lunar eclipse, white sunlight hitting the atmosphere on the sides of the Earth gets absorbed and then radiated out (scattered). Blue-colored light is most affected," NASA officials wrote online. "That is, the atmosphere filters out (scatters away) most of the blue-colored light. What's left over is the orange- and red-colored light."

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