NEW YORK – Next Saturday the 31st of March, people on Planet Earth will be able to observe the second and last Blue Moon of 2018. The next catch will only take place on October 31, 2020, Earth Sky.org reports.

There are two definitions for “Blue Moon”; there is a Seasonal Blue Moon which is the third Full Moon in an astronomical season with four Full Moons, instead of the usual three. While the second definition is a Monthly Blue Moon which refers to the second Full Moon in a month with two Full Moons.

The phrase has nothing to do with the actual color of the moon. Nevertheless, the moon can become blue during rare atmospheric conditions like volcanic eruptions and forest fires which release particles of slightly wider than 900 nm to preferentially scatter red light in the atmosphere, this phenomenon is called Mie scattering.

Blue Moons happen once every two or three years. In the 1,100 years between 1550 and 2650, there are 408 seasonal Blue Moons and 456 monthly Blue Moons.

There can never be a double seasonal Blue Moon, as that would require 14 Full Moons in the same year, which isn’t possible because the time between two Full Moons is approximately 29.5 days.

A solar calendar year only embraces two Blue Moons if there are 13 full moons in one calendar year – and, in addition, February has no full moon at all.

That’s exactly what happens now in 2018. Double Blue Moons happen only about three to five times in a century. February didn’t have a full moon, while January and March both have two full moons.

February is the only calendar month that can go without a full moon. Prior to 2018, we last had a February with no full moon 19 years ago, in 1999. And – as you might expect – we also had two Blue Moons in 1999, on January 31 and March 31.

Nineteen years from now, in the year 2037, February again will have no full moon, and the months of January and March will each feature two full moons.

Moreover, between 1550 and 2650 there are 20 years which have one seasonal and one Monthly Blue Moon. The next time is in 2048 while the previous time was in 1934.

Triple Blue Moons, a combination of one seasonal and two monthly Blue Moons in the same calendar year, happens 21 times in the same time span. The next is in 2143, while the last time was in 1961.