Lunar Eclipse Frequencies
Why don't we have eclipses every month?
Eclipses occur about every 5½ months as the nodes (the intersections) of the
Earth's and the Moon's orbits align with the 'line of sight' between the
Earth and the Sun, and as long as the Moon arrives at or near that spot at
the same time.
The reason for this length of time, rather than 6 months, is because the
Moon's orbit around the Earth twists and makes a complete 'backward twist'
once every 18 years, 10 or 11 days (depending on how many leap years there
are in the intervening time).
Most months, the combining effect of the Earth's tilt (23½ degrees) plus the
Moon's orbital tilt compared with the Earth's equator (5 degrees) means that
both the Earth's and Moon's shadows miss the other body, hence no eclipse
(solar or lunar), but when the nodes and the Moon are in our line of sight
of the Sun then as many as 3 eclipses can occur (spaced two weeks apart, a
span of four weeks). The first and the third would be partial and both would
be solar or lunar (but not one of each). Between these two, there would be a
total eclipse of the other type.
There is a maximum of 7 eclipses per year and a minimum of 2.
Even though the Moon revolves around the Earth every month, in order for an
eclipse to occur, the Earth, Moon and Sun all have to line up. A paritial
eclipse will occur every 18 months or so, but a total eclipse will only
occur every 100 years and only on a small area of the Earth.
Click here to return to the Astronomy Archives
Update: June 2012