Page "Lunar phase" Paragraph 6

It might be expected that once every month when the Moon passes between Earth and the Sun during a new moon, its shadow would fall on Earth causing a solar eclipse.

Likewise, during every full moon one might expect the Earth's shadow to fall on the Moon, causing a lunar eclipse.

Solar and lunar eclipses are not observed every month because the plane of the Moon's orbit around the Earth is tilted by about five degrees with respect to the plane of Earth's orbit around the Sun ( the plane of the ecliptic ).

Thus, when new and full moons occur, the Moon usually lies to the north or south of a direct line through the Earth and Sun.

Although an eclipse can only occur when the Moon is either new or full, it must also be positioned very near the intersection of Earth's orbit plane about the Sun and the Moon's orbit plane about the Earth ( that is, at one of its nodes ).

This happens about twice per year, and so there are between four and seven eclipses in a calendar year.

Most of these are quite insignificant ; major eclipses of the Moon or Sun are rare.