The eclipse of the moon occurs when earth gets in the way of the sun and moon. As you probably know, the moon does not emit any light - it simply reflects the light from the sun. Hence, when our planet gets in between the two, its shadow falls on the moon, resulting in what we call, lunar eclipse. The length of the eclipse depends on what part of earth the moon is passing through. Saturday's eclipse was short because it happened to be passing through the upper part of earth's shadow.