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The Moon

What causes a red moon?

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April 16, 2014 6:13PM

The orange and red tints that the Sun and Moon sometimes take on are caused by the particles in the Earth's atmosphere.

When light (or more specifically, packets of light called photons) from an astronomical object passes through the Earth's atmosphere, it scatters off of particles in the latter. It turns out that these particles like to scatter blue light more than they do red light; so "bluer" photons (those with shorter wavelengths) tend to get scattered, and "redder" photons (those with longer wavelengths) pass through. So, astronomical objects look redder from Earth than they would from space, because the redder wavelengths from the objects penetrate the atmosphere better than the bluer ones. Incidentally, this is why the sky is blue: blue light from the Sun is scattered in all directions on its way to the Earth.

But how does this explain the occasional redness of the Moon or the Sun? Your son may have noticed that they always occur when the Sun or Moon is close to the horizon. If you think about it, sunlight or moonlight must travel through the maximum amount of atmosphere to get to your eyes when the Sun or Moon is on the horizon (remember that that atmosphere is a sphere around the Earth). So, you expect *more* blue light to be scattered from Sunlight or Moonlight when the Sun or Moon is on the horizon than when it is, say, overhead; this makes the object look redder. In other words, the Sun or Moon tends to look orange or red when it is rising or setting because that's the time when the light has to travel through the most atmosphere to get to you. The effect is exacerbated when there are thin clouds in front or behind the Sun or Moon: the clouds themselves often glow bright pink as well, because they are so good at scattering blue light.