The total lunar eclipse on 31 January, 2018 is considered quite rare. Why is this you may ask? Total lunar eclipses alone are not all that rare. A total lunar eclipse occurs on the Earth roughly once or twice a year on average. When they do occur, half of the globe can see it.
The eclipse in January however coincides with a supermoon when the Moon is at perigee (closest point to the Earth along its orbit around our planet). Even this is not that rare. Supermoon eclipses occur every few years or so.
The main element that makes this lunar event quite rare is the fact that the full Moon will be a ‘blue Moon’. The term blue Moon can be misleading as it has nothing to do with the Moon’s colour. A blue moon is when a full Moon occurs twice in the same calendar month. This is possible because the Moon orbits the Earth in 29 days on average. Blue Moons are ‘squeezed’ in to one calendar month once every 2.7 years.
The term blue Moon is often cited as being a piece of old folklore. Although it sounds as though it could be, in actual fact the term came from a mis-interpreted Almanac. Originally, in the early 1900s in places such as the Maine Farmer’s Almanac, the term “blue moon” was used to refer to a related phenomenon, when four full moons occurred within a given season, instead of the typical three. However, in 1946, amateur astronomer by the name of James High Pruett incorrectly interpreted the term in an article he wrote in Sky & Telescope using the meaning we know today.
So, a total lunar eclipse happening during a supermoon and a blue Moon is quite rare indeed. The last time we had a blue Moon total eclipse was March 31, 1866 and the next blue Moon total eclipse will be Dec. 31, 2028!