A lunar eclipse occurs when the sun, Earth, and moon align. During total lunar eclipses, the entire moon is engulfed in Earth's darkest shadow. But during partial eclipses, the moon never completely goes dark or turns red—only a portion of its disk appears to darken slightly. (Read about a total lunar eclipse in 2011.)

"This is an eclipse where the circumstances place the moon only inside of the very light outer shadow of the Earth called the penumbra, rather than the darker inner shadow known as the umbra," said Larry Ciupik, an astronomer at Adler Planetarium in Chicago, Illinois.

"That outer shadow cone is so light that one normally barely notices the darkening moon as the eclipse progresses."