Question

Why don't we get an eclipse every month?

Answer 1

We don't get an eclipse every month because the Moon's orbit is inclined at about `5^@` to the ecliptic.

Explanation:

If the Moon's orbit was in the same plane as the Earth's we would experience two eclipses every month. A solar eclipse would happen every new moon and a lunar eclipse would happen every full moon.

As the Moon's orbit is inclined at `5^@` to the ecliptic, an eclipse can only happen when the Moon is near once of its nodes which are the points where the Earth and Moon orbits intersect. The line joining the nodes also has to be pointing towards the Sun.

An eclipse can only occur during an eclipse season which is a 34 day interval during which the Moon's nodes are suitably aligned. There are two eclipse seasons every year. If a full moon or a new moon occur during an eclipse season there will be an eclipse. There are always 2 or 3 eclipses every eclipse season.

The are actually between 2 and 5 solar eclipses every year. Most are partial eclipses which don't attract much interest.

For a total solar eclipse to occur there must be a new moon around the middle of the eclipse season. The Moon also needs to be close enough to the Earth to completely cover the Sun. This occurs on average every 18 months.