Why lunar eclipses don't occur every month

Since lunar eclipses occur always at Full Moon, it makes sense to ask why each Full Moon does not generate one. Eclipses are relatively rare because the plane in which the Moon orbits around Earth is tilted 5 degrees compared to the plane of Earth's travels around the Sun, a plane that astronomers call the ecliptic. To visualize, think of commingling Hula Hoops—one big and one small—floating on the surface of a pool, and push the inner one down so that half of it is below the surface and half above. When the Moon gets into the ecliptic --right at the surface of the pool—during its full phase, then a lunar eclipse occurs. (The word "ecliptic" stems from the word "eclipse.") [Graphic] The geometry of any eclipse—the relative positions of the Sun, Earth and Moon—is eventually repeated during a set of complex cycles that each last just more than 18 years. This Saros cycle, as the whole thing is called, is behind the bunching of eclipses, too. Astronomers have figured it out and can predict eclipse timing and circumstances far into future.