Since lunar eclipses occur always at Full Moon, it makes sense to ask why each Full Moon does not generate an eclipse.

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 floating on the surface of a pool, and push 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.")

Next: So just how often do they occur?