Every so often, the silvery orb of the full moon undergoes a dramatic transformation. Darkness slips across the lunar face, and for a while, the entire moon may be colored a deep blood red.

This is what's known as a total lunar eclipse, when the moon, the sun, and Earth line up in just the right way for the moon to be engulfed in Earth's shadow. This cosmic phenomenon has delighted or terrified humanity throughout recorded history, and astronomers have learned how to track celestial motions to predict when a lunar eclipse will occur years to centuries in advance.

Lunar eclipses happen when the moon moves behind Earth, from the sun's perspective. When the moon is completely swallowed