Every calendar month has a full moon. This July will see two. The first occurred on July 2, the second July 31. And this is called a blue moon. The first full moon has to appear at or near the beginning of the month so that the second will fall within the same month.
Why does this happen
The explanation is simple. It occurs simply because the lunar cycle and the calendar year are not perfectly synched. And the moon will not turn blue.
This rare phenomenon happens once every two or three years. This will be the first such occurrence since August 2012.
Where does the expression blue moon come from?
The expression has been around for more than 400 years, according to Dr Philip Hiscock, a specialist in languages, folklore and popular culture.
Hiscock says the earliest use of the term came with sayings such as the moon was made of green cheese. Both were odd. The saying “he would argue the moon was blue” was taken by people in the 16th century to mean that “he would argue that black is white”.
The idea that a blue moon was absurd gave rise to it meaning “never” by the 18 century, such as “I will pay you the money owed when the moon is blue.“
But the moon is blue!
There are examples in history of when the moon literally turned blue.
When Krakatoa, the Indonesian volcano erupted in 1883, it threw up dust in the atmosphere and “turned sunsets green and the moon blue around the world for the best part of two years”, says Hiscock.
The late arriving Indian monsoons in 1927 meant a long dry season which blew up enough dust to cause the moon to have a blue hue. The same thing happened when huge forest fires in western Canada threw up smoke particles into the sky.
So blue moons are quite rare but they do happen from time to time – a fairly infrequent event- and this gives rise to the saying “once in a blue moon”.
The next blue moon, or two full moons in one month, is predicted to take place in 2018. In that year there will be two such events – in January and March.