This October will witness the rare occurrence of a second full moon called the 'Blue Moon' which abruptly suits the phrase 'Once in a Blue Moon'. The Blue Moon will occur on Saturday (October 31).

The usual monthly lunar phenomenon is that every month has one full moon and a new moon. However, there are unusual occasions when the same month gets two full moons.

October 1 saw a full moon and the next full moon falls on October 31, said Arvind Paranjpaye, director of Nehru Planetarium, Mumbai while quoted by News18.

When there are two full moons in a calendar month, the second full moon is called the Blue Moon, he said.

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. Therefore, to have two full moons in a month, the first full moon should take place on 1st or 2nd of the month, Paranjpaye explained.

This extra time accumulates over the months. After about 30 months there will be one extra full moon in the year, Paranjpaye said.

He added that it is also not possible to have a full moon in February as it has 28 days, and 29 days during a leap year.

The next Blue Moon will be on August 31, 2023.