When such a rare phenomenon occurs, to say when there are two full moons in one calendar month, the second full moon is called the Blue Moon - but disappointingly this term does not describe the colour of the moon.
This phenomenon is so rare that it gave birth to the proverbial phrase “once in a blue moon”, which is used to denote something that does not occur frequently.
Generally every month has one full moon and one new moon, but sometimes a calendar month has two full moons. In 2018, there were two Blue Moons – first on January 31, second on March 31.
The last time there was a Blue Moon in a month with 30 days was on June 30, 2007, while the next will be seen on September 30, 2050. Meanwhile, the next Blue Moon will be seen on August 31, 2023.
Such an event happens about once every 2.7 years on average; however, a “true” Blue Moon is rare and happens when the cycle of lunar phases causes the full Moon to occur within a few days after an equinox or solstice, Sky and Telescope noted. The last such occurrence was in February 2019, and the next after this month will be in August 2024.
This year’s Blue moon is set to appear near Jupiter and Saturn and will be visible from sunset at 8.19pm, but for best visibility check back between 10pm and midnight.