Be afraid mortals, for today the heavens align and the moon turns red as blood!

No, really. Tonight is not only the northern winter solstice, when the northern nights are longest since we are tilted away from the sun, but there is also a lunar eclipse tonight! I like lunar eclipses a lot, first of all, because they are much more common and long-lived than solar eclipses, but more importantly, the moon turns red.

Why does the moon turn red, you ask? Oh, nothing much, it’s just that every sunrise and sunset on earth is simultaneously being projected onto the moon’s surface. The earth is larger than the sun in the sky as seen from the moon, so it blocks all direct sunlight from hitting the moon. But, since we have an atmosphere, some sunlight just grazes the earth, is bent through our atmosphere, and ends up hitting the moon. As you know, our atmosphere scatters blue light more effectively than red light, so some of the same red light that you see as sunrise or sunset continues on into space. When there’s a lunar eclipse, the moon acts as a projector screen and is illuminated a deep red. It varies in brightness from eclipse to eclipse, but it’s always a cool sight.

So, if you can, step outside and take a look at the moon tonight. Even if the earth’s shadow hasn’t quite hit the moon yet, the moon will look super bright since the light is bouncing almost directly back toward the sun. Granular material like the moon’s surface is especially good at reflecting light back at its source, and effect which is called “opposition surge”.

A nice illustration from Wikipedia showing the optics of a lunar eclipse, with light passing through the earth's atmosphere and turning the moon red. (Note: I'm not sure why this graphic depicts an increase in blue light before the transition to red. See the discussion in the comments)

Here's a more accurate (but not as cool-looking) graphic of the reddening effect during a lunar eclipse.