How often do solar eclipses occur?

Total solar eclipses occur roughly once every 18 months.

The Moon’s orbit is at a 5.15° angle of the plane of the Earth’s orbit around the Sun. The Moon crosses the ecliptic twice in it’s journey around the Earth, once on it’s way up (ascending node) and once on it’s way down (descending node). The Moon crosses the same node once every 27.21 days, an interval called draconic month.

When the New Moon is close to a node, which occurs twice a year the Moon’s shadow will hit the Earth. If the umbral shadow hits the Earth then a total eclipse will occur, otherwise it will be a partial or annular eclipse.

Soras series

Every 18 years, 11 days and 8 hours the Sun, Moon and Earth return to approximately the same geometry. This period is referred to as the saros.

Each time there is an eclipse (solar or lunar) you can expect that the same arrangement that produced that eclipse will reoccur 6585.3211 days later.

A saros series is the sequence of eclipses that result from approximately the same configuration every saros period.

The eclipse of August 21 2017 is the 22nd in a sequence of 77 solar eclipse in saros series 145

Exeligmos

A saros period is 6585.3211 days. There will be some adjustment in the eclipses in the saros series sequence, one notable difference is in the Earth’s rotation. Each subsequent eclipses commencing at roughly 120° advanced Earth rotation.

This means that after 3 eclipses in a saros series commence at close to the same longitude.

This period of 54 years and 33 days is known as an exeligmos.

An animation of an exeligmos series

If you were lucky enough to witness the eclipses of July 31st 1981, August 11 1999 and you are planning to observe the eclipse of August 21 2017 then you will be part of a rare group of people to complete an exeligmos set or a total exeligmos!