Early Wednesday morning, just before the sun rises, the full moon will turn red in a total lunar eclipse. But why red?

Every month the moon circles Earth in uneventful fashion. However, about twice a year, it passes through the shadow of the Earth — the umbra — which prevents direct sunlight from reaching the surface of the moon. This is the total lunar eclipse.

Related: Details on Wednesday morning’s lunar eclipse



The lunar eclipse over the Washington Monument on February 20, 2008. (Kevin Ambrose)

Even though the moon is in the Earth’s shadow, some light does manage to hit the lunar surface during the eclipse, and this is what gives the moon its crimson glow, for the same reason that the sky is blue, and sunsets on Earth appear orange, pink, and red.

As light passes through the atmosphere of the Earth, it is scattered in all directions. The molecules in the atmosphere are an excellent size for scattering short wavelength blue light, which makes our sky appear blue when the sun is overhead.

But as sunlight passes through long, dense portions of the atmosphere, like it does at sunrise and sunset, longer wavelengths of light, which we see as red, are filtered out the least.

Just like at sunset, during a lunar eclipse the sun’s rays have a lot of atmosphere to pass through around the edges of the Earth. And in addition to the red-colored scattering, the light is also refracted, or bent, sending the red light straight toward the moon.

In fact, if you were standing on the surface of the moon during the eclipse, you would see a halo of red around the Earth as it passed in front of the sun, like a 360-degree sunset.

While all lunar eclipses end up being some shade of red, the exact color can vary based on the nature of the particles in our atmosphere. Temperature, humidity and dust can all play a role in the intensity of the moon’s redness, which can vary from orange to deep crimson.