Blue Moon: what kind of phenomenon is this?

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According to NASA, “most Blue Moons look pale gray and white, indistinguishable from any other moon you’ve ever seen”. In 1883, after the volcano Krakatoa erupted, the moon turned green and blue all over the world due to the dust and NASA thinks that this is probably the origin of the term Blue Moon.

In fact, other events can make the moon turn blue, like dust storms and forest fires. With the exception of this kind of events, Blue Moons remain the same color as any other Full Moon.

The seasonal and the monthly Blue Moon

The Blue Moon is a phenomenon that can be defined in two different ways. The seasonal Blue Moon is the Full Moon in an astronomical season with four Full Moons instead of the the usual three. And the monthly Blue Moon, which is the second Full Moon in a month with two Full Moons.

Well, the seasonal Blue Moon is the original astronomical definition of a Blue Moon. The reason why that is a second definition of Blue Moon is due to an error on the calculation of the seasonal Blue Moon made by the amateur astronomer James Hugh Pruett.

Pruett wrote in an article published in the “Sky & Telescope” magazine, in 1946, that a Blue Moon was the second Full Moon in a month. After at least two decades of using the term Blue Moon to define the moon based on this calculation, it didn’t make sense to change it, and today this is considered a second definition rather than a mistake.

The monthly Blue Moon is more frequent than the seasonal one, that just occurs every two or three years. Usually, there are three Full Moons between each astronomical season, which is the time between each solstice and equinox. However, in some years there are four Full Moons in a season and, when this happens, the third Full Moon is called a Blue Moon.

blue moon

The next monthly Blue Moon: when?

According to NASA, the last monthly Blue Moon was in 2018, on March 31st and the next one happens in 2020, on October 31st. The last seasonal Blue Moon was in 2019, on May 18th, and the next one is in 2021, on August 22nd.

In 2018, there were two full moons in January and in March, which is an event that occurs just about three to five times in a century. The next time two full moons will happen in the same month will be in in 2037.

It is not possible to happen a double seasonal blue moon because this would require 14 full moons in the same year, which is not possible because the time between two full moons is approximately 29,5 days.

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