What Causes a Blood Moon?


Quick Answer

A blood moon is a deep reddening of the moon caused by the Earth passing between the moon and the sun. During the period of the eclipse, light from the sun passes around the Earth and through its atmosphere. This scatters the short-wavelength blue light from the sun but allows the longer-wavelength red and orange light to reach the moon.

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Full Answer

Sunlight is scattered by the Earth's atmosphere. The scattering is not random, however, and it affects short wavelengths near the blue end of the spectrum more profoundly. This phenomenon, called Rayleigh scattering, is the reason the sky glows red and orange during sunrise and sunset. The longer the path the white light has to travel through air, the more blue light is scattered out of it and the redder it appears. During a blood moon, the only path sunlight has to the moon is around the curvature of the Earth and through much of the atmosphere. This effectively scrubs out blue light and permits the passage of only very deep reds. It is this red light that reaches the moon and is reflected back toward observers on Earth. The reflected light is further scattered as it passes through the air toward observers, leaving only a very deep red.

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