If you're lucky enough to find yourself under a cloudless sky tonight, you'll be able to see one of our solar system's great wonders, a full lunar eclipse, also known as a 'blood moon.' The spooky nickname derives from the reddish hue the moon takes on when Earth casts its shadow upon it. Featured here is a blood moon over the Swiss Alps. A full lunar eclipse occurs when the Earth and moon align perfectly with the sun, and the moon falls directly behind Earth's shadow. When Earth falls behind the moon's shadow, a solar eclipse occurs.