When do eclipses occur?

Apr 21, 2016

The Moon's orbit is inclined at ${5}^{\circ}$ to the ecliptic.

Explanation:

An eclipse can only occur when the Sun, Moon and Earth are lined up. A solar eclipse can only occur on a new moon and a lunar eclipse can only occur on a full moon.

As the Moon's orbit is inclined at ${5}^{\circ}$ to the ecliptic, at most full and new moons the Moon is too far above or below the plane of the ecliptic for an eclipse to occur.

The Moon is in the plane of the ecliptic on two occasions in each orbit. These are called the ascending node and the descending node.

An eclipse can only occur within an eclipse season which is when the line connecting the two nodes is pointing within a few degrees of the Sun. There are two eclipse seasons each year each lasting about 34 days.

This means that there are between 2 and 5 solar eclipses every year. Most of these are partial eclipses.