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What is meant by Blue Moon?
"Blue moon" as in the phrase "once in a blue moon" means a rare event.
This phrase most likely originated from the actual astronomical event known as a "blue moon".
Typically, we get one full moon a month -- the words month and moon are derivatives of each other. Hence, we normally get 12 full moons in a year. But once in a rare occasion, we get two full moons within a month. This second "extra" full moon is known as a blue moon.
So instead of the year having 12 full moons (which it typically does), we have 13 full moons. This happened in the year 2012. The 13th full moon appeared on December 28, 2012.
If a year has 13 full moons, that 13th full moon must occur on or between Dec. 21 and the 31st. However, that does not necessarily mean that December would have two full moons.
The extra full moon in the year 2012 sneaked in on the month of August that year, where we saw a full moon on August 2 and the second full moon on August 31. The moon on August 31, 2012 is a "blue moon" because it is the second full moon the month. [reference]
To be more precise, when I said full moon occurred in August 2nd, I meant the full moon occured on "2nd of August 2012 at 05:27:30 am Central European Time (CET)". But if you were in say in another part of the world, it could have easily occurred on August 1st your time.
To get a better understanding of the phases of the moon, watch this video below...
How Rare are Blue Moons?
Blue moons occurs about once every three years.
We saw a blue moon in the year 2012. The blue moon before that occurred in the year 2009. In 2009, that blue moon happened at the very last day of the year -- December 31st. This is known as a New Year's Eve full moon.
If a full moon occurs on December 31, there must be a full moon on December 1st or December 2nd. (In year 2009, it happened on December 2nd Central European Time).
So a New year's Eve full moon is always a blue moon. New Year's Eve full moon happens once every 19 years. The math just works out that way.
Blue moons (extra full moons) occur because the moon month is usually slightly shorter than calendar months.
You can see a list of full moons on fullmoon.info.
A Blue Moon Does not have to be blue
The term "blue moon" does not have anything to do with the color of the month. Although there are atmospheric conditions that would make a moon appear bluish, this has no bearing on whether the moon is a "blue moon" or not in the astronomic sense.
Can a Year Have Two Blue Moon?
Yes, it is possible.[reference] But that is rarer than a blue moon -- literally and figuratively.
The year 1999 had two blue moons. Full moons had appeared on ...
Saturday, 2 January 1999, 03:49:30 am
Sunday, 31 January 1999, 05:06:30 pm (blue moon)
Thursday, 1 April 1999, 12:48:54 am
Friday, 30 April 1999, 04:54:36 pm (blue moon)
The next time a year will have two blue moons will be in 2018. The full moon will happen on ...
Tuesday, 2 January 2018, 03:24:06 am
Wednesday, 31 January 2018, 02:26:48 pm (blue moon and an lunar eclipse too)
Friday, 2 March 2018, 01:51:24 am
Saturday, 31 March 2018, 02:36:54 pm (blue moon)
These are all Central European Time. Coincidentally, there will be a full lunar eclipse during the blue moon on January 31, 2018.[reference]
No, that does not mean that year 1999 and year 2018 will have 14 full moons. They still have 13 full moons. It would not be possible to have 14 full moons in a year. It just means that one of the months got short-changed and did not get a full moon at all. Usually that month is February with it being so short on number of days and all. And in fact, in both the years 1999 and 2018, February did not have a full moon at all.