Watch the sky! October is a Blue Moon month!

The phrase 'Once in a Blue Moon' is aptly suited to the month of October this year, which will witness the rare occurrence of a second full moon, called the 'Blue Moon', on Saturday.

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.

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.

N Rathnashree, the director of Nehru Planetarium, Delhi said the term Blue Moon is a calendrical term.

It is not too common to have a Blue Moon in a month of 30 days, she said.

The last Blue Moon in a month with 30 days was on June 30, 2007 and the next one will be on September 30, 2050, Paranjpaye said.

There were two Blue Moons in 2018. The first one was on January 31 and the second Blue Moon followed on March 31. This was because the preceding month February had only 28 days, Paranjpaye said.

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

T V Venkateswaran, scientist with Vigyan Prasar, the communication wing of the Department of Science and Technology, and an astronomy enthusiast, said this phenomenon is largely restricted to the Gregorian calendar which is being used worldwide.

You will not see the phenomenon of two full moons in a month in Islamic or Tibetan calendars or the ones that are being followed in large parts of India as they are based on lunar movements, Venkateswaran said.

The phrase 'Once in a Blue Moon', denoting something which does not happen very often, came from this phenomenon, Venkateswaran added.