A ‘Blue Moon’ is an infrequent phenomenon that involves the appearance of an extra full moon within a given period. Recently, the term Blue Moon has been applied to the second full moon in a single calendar month. There are roughly 29.5 days between full moons, making it unusual for two full moons to fit into a 30- or 31-day-long month.
October this year will witness the rare occurrence of a second full moon, called the ”Blue Moon”, about which we talked above, on Saturday. October already has seen a full moon and the next full moon falls on 31 October, said Arvind Paranjpaye, director of Nehru Planetarium, Mumbai.
“When there are two full moons in a calendar month, the second full moon is called the Blue Moon,” he said.
What’s The Maths Behind The Blue Moon?
There is a bit of mathematics involved in this. “Duration of a lunar month is 29.531 days or 29 days, 12 hours, 44 minutes, and 38 seconds. Hence, in order to have two full moons in a month, the first full moon should be on the 1st or 2nd day of the month,” Paranjpaye explains. This extra time accumulates over the months. After about 30 months there will be one extra full moon in the year, Paranjpaye said.
The director of Nehru Planetarium, Delhi, N Rathnashree said this term is a calendrical term. “It is too uncommon to have a Blue Moon in a month of 30 days,” she further said. The last Blue Moon in a month with 30 days was on June 30, 2007, and the next Blue Moon will be on September 30, 2050, Paranjpaye said.
There were two such occurrences in 2018. The first one was on January 31 with the second followed on March 31. “This was because the preceding month February had only 28 days,” Paranjpaye said. He further told that the next Blue Moon will appear on August 31, 2023.
Why Can’t India Predict It?
A scientist with Vigyan Prasar, T V Venkateswaran, an astronomy enthusiast, said this phenomenon is largely bounded to the Gregorian calendar which is used worldwide.
Venkateswaran said, “You will not see this appearance of two full moons in a month in Tibetian or Islamic calendars or the ones being followed in large parts of India because these are based on lunar movements”.
The phrase ”Once in a Blue Moon”, denotes some event that does not happen very frequently came from this phenomenon, Venkateswaran added.