Why doesn't a lunar eclipse occur once a month?

Answer 1

The phase of the moon (new, half, full, etc) is a function of how much of its lit surface we can see from the earth. Half the moon is always sunlit, half is dark, but as the moon orbits the earth, more or less of the lit face is pointed towards us here on earth.

The moon's orbital period around the earth is 28 days, which more or less coincides with the length of a month. Once in each of its orbits, its lit face is directly oriented towards earth -- a full moon.

Because the 28-day orbital period is less than a full month, on relatively rare occasions we can have two full moons within one calendar month. The second of those full moons is referred to as a "blue moon" -- thus the phrase, once in a blue moon, which means "rarely."

Answer 2

It's because the Moon's orbit lies in a plane that is inclined to the plane of the Earth's orbit, otherwise called the ecliptic. Usually at Full Moon and New Moon the Moon passes above or below the Sun because of the inclined orbit.

Twice in each lunar month the Moon passes a node, which means it crosses the ecliptic, from south to north at the ascending node, or north to south at the descending node.

Eclipses only occur when New or Full Moon happens to occur at the same time as the Moon is at one of its nodes. Then all three bodies are lined up and there is an eclipse.

Answer 3

Because of the way the earth goes round and tilts on its axis.

Answer 4

Because the moon is off-center in its orbit most of the time. It either passes above or below the preumbra and the umbra (the Earth's shadow path of totality).