The phrase “ Once in a Blue Moon ” fits perfectly with October of this year, which will witness the rare occurrence of a second full moon, known as a “ Blue Moon, ” on Saturday.
The usual monthly lunar phenomenon is that each month has a 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 math involved in all of this. “The length 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 be on the 1st or 2nd of the month, ”Paranjpaye explained. This extra time accumulates over the months. After about 30 months, there will be an additional full moon in the year, Paranjpaye said.
He added that it is also not possible to have a full moon in February because it has 28 days and 29 days in a leap year. N Rathnashree, director of the Nehru Planetarium, Delhi said the term Blue Moon is a calendar term. “It’s not too common to have a blue moon in a 30-day month,” she says.
The last blue moon in a month with 30 days was on June 30, 2007 and the next will be on September 30, 2050, Paranjpaye said.
There were two Blue Moons in 2018. The first took place on January 31st and the second Blue Moon followed on March 31st. “It’s because the previous February was only 28 days old,” Paranjpaye said.
The next Blue Moon will take place on August 31, 2023, he added.
TV Venkateswaran, a scientist at Vigyan Prasar, the communications wing of the Department of Science and Technology, and an astronomer enthusiast, said this phenomenon is largely limited to the Gregorian calendar which is used around the world.
“You will not see the phenomenon of two full moons in a month in Islamic or Tibetan calendars or those that are followed in large parts of India because they are based on lunar movements,” Venkateswaran said. The phrase “once in a blue moon,” denoting something that doesn’t happen very often, came from this phenomenon, Venkateswaran added.