Why doesn't a lunar eclipse happen every time there is a full moon?
Because the moon's orbit is slightly inclined from the plane of the ecliptic. The ecliptic is the path that the sun appears to follow in the sky, caused by the path of the earth's orbit around the sun. The moon's orbit is inclined about 5 degrees from the plane of the earth's orbit around the sun. The moon crosses the ecliptic about twice per month. If this happens during a new moon, a solar eclipse occurs, during a full moon, a lunar eclipse occurs. http://csep10.phys.utk.edu/astr161/lect/time/moonorbit.html http://en.wikipedia.org/wiki/Ecliptic http://en.wikipedia.org/wiki/Moon