A lunar eclipse occurs when the Moon passes behind the Earth such that the Earth blocks out the sunlight and casts it shadow over the Moon.
A lunar eclipse only occurs when there is a full Moon. This is because a full Moon only occurs when the moon is directly opposite the Sun. In this instance if the Moon passes through the Earth’s shadow a lunar eclipse takes place.
A total lunar eclipse will be observed when the entire Moon passes through the umbra, the inner region of the shadow where the Earth blocks out all the sunlight reaching the Moon. A partial lunar eclipse is observed when part of the moon passes the umbra shadow. As the Moon moves across the penumbra, the outer shadow where the Earth partially blocks the sunlight, a penumbral lunar eclipse is observed.
A lunar eclipse does not occur at every full Moon because the Moon’s orbit is tipped 5 degrees to the Earth’s orbit around the sun. This means the moon passes either above or below the Earth’s shadow most of the time.