Lunar eclipses can only happen when the moon is directly opposite to the sun in our Earthly sky at the time of the full moon.  But we don't get a lunar eclipse every month — that is, at every full moon.  The reason for this is simple: the moon's orbit is inclined to the Earth's orbit by just over 5 degrees.  So more often than not, the full moon will pass either above or below the Earth's shadow. However, there are times when the sun, Earth and moon are aligned in such a way that the moon will pass either partially or totally into Earth's shadow, producing a lunar eclipse.