The Saros Cycle

Solar eclipses happen when the Moon is directly between the Sun and the Earth. This allows the Moon to cast its shadow on the Earth. A solar eclipse can only occur during a New Moon. Likewise, Lunar eclipses happen when the Earth is directly between the Sun and the Moon. This allows the Earth to cast its shadow on the Moon. A Lunar eclipse can only happen during a Full moon.

But not every New Moon produces a Solar eclipse. And not every Full Moon produces a Lunar eclipse. The plane of the Moon's orbit is inclined about 5 degrees to the ecliptic, or the plane of the Earth's orbit around the Sun. Due to this inclination, the Moon is usually too high or too low for eclipses to happen. Although the Moon will cross the ecliptic plane twice per orbit, about every two weeks, it usually happens when the Moon is not full or new.

But about twice a year, the Moon crosses the ecliptic plane while it is full or new, creating the conditions necessary for an eclipse. The period of a couple weeks surrounding the Moon's ecliptic crossing is known as eclipse season. Eclipse seasons recur every 173 days. This is slightly less than half a year.

The orientation of the Moon's orbit is not fixed. It slowly rotates, changing the position where it intersects the ecliptic. This is why eclipse seasons are not exactly one half year apart. After 18 years, the orientation of the Moon's orbit makes one complete rotation, or one Saros cycle. Eclipse season will be centered around the same date as it was 18 years ago. This tends to create eclipses that are very similar as the ones 18 years prior. These eclipses are said to be part of the same Saros series. An entire Saros series lasts over a thousand years.

The simulation Saros.gsim shows the Moon completing a Saros cycle every 18 years. Watch as the orientation of the Moon changes direction and ultimately makes a complete rotation.

What causes the orientation of the Moon's orbit to precess, or change? Could it be the influence from the other planets, perturbing the Moon's orbit? Or could it be causes by the Solar tide through the Earth / Moon system? Maybe it's a combination of both these reasons. Or maybe it precesses on its own, without the need for an outside influence.

This is easy to check using Gravity Simulator. Open the simulation Saros.gsim and use the Edit menu to delete the planets. See what effect this has on the period of a Saros. Now try deleting the Sun. What is the effect now?