Lunar eclipses occur when a full moon passes through Earth's shadow. Earth's shadow has two parts—the inner full shadow, or umbra, and the outer partial shadow, or penumbra. The moon darkens as it passes through the penumbra, then turns reddish-orange while within the umbra.

Though the moon is totally within Earth's umbra in the middle of this sequence, the moon is dimly lit by sunlight bending through Earth's atmosphere. The atmosphere filters out most of the blue light, making the moon appear a reddish color. The moon lightens again as it passes through the other side of the penumbra, then returns to normal brightness as it exits Earth's shadow.