When someone says “Once in a blue moon”, you know what they mean: an occurrence which is rare and seldom. They do not have the exactitude in mind that the term has come to acquire in astronomical circles: the second full moon in a calendar month. It has nothing to do with the colour of these "blue moons", which remains pearly grey.

Mostly, every Calendar month has one full moon. But because full moons on an average are separated by 29.53 days, while most of our calendar months (except February) are 30 or 31 days, these extra one-half or one-and-one-half days per month accumulate over the year, resulting in some years to have 13 full moons rather than 12. This happens every two and a half years, on average. Because of the way these add up, "once in a blue moon" one could end up with years such as 1999 when there were two full moons each in January (2nd and 31st) and March (2nd and 31st), with none in February.

The last "blue moon" was in 2012 August (2nd and 31st) 2012. There was a full moon on July 2nd. The next, on July 31st, is by definition therefore a Blue Moon. The next will be in January 2018 (2nd and 31st).

This widely accepted meaning has come into being from 1946 when the Sky & Telescope magazine in 1946 misinterpreted the Maine Farmer's Almanac and labelled a blue moon as the second full moon in a month. Most people when they say "once in a blue moon" even then did not even mean what the Maine Farmers' Almanac actually defined it as: the third full moon in a season with four full moons, not the usual three.

For most people, however, "once in a blue moon" still means something so rare you'd be lucky (or unlucky) to see in your lifetime. As NASA's National Space Science Data Center explains it, rare natural phenomenon could cause smoke or dust particles to rise to the very top of the atmosphere, such as during a cataclysmic volcanic eruption, as a result of which the colour of moon could take on a bluish hue.

For instance, in 1883, people saw blue moons almost every night. Full moons, half moons, crescent moons – they were all blue, except some nights when they were green. NASA explains it as a result of an explosion of Indonesian volcano named Krakatoa exploded. "Scientists liken the blast to a 100-megaton nuclear bomb. Fully 600 km away, people heard the noise as loud as a cannon shot. Plumes of ash rose to the very top of Earth's atmosphere. And the moon turned blue."

"Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide – the right size to strongly scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green."

Blue moons persisted for years after the eruption, according to NASA. "People also saw lavender suns and, for the first time, noctilucent clouds. The ash caused 'such vivid red sunsets that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration,' according to volcanologist Scott Rowland at the University of Hawaii."

In 1983, the eruption of the El Chichon volcano in Mexico, Mt St Helens in 1980 and Mount Pinatubo in 1991 also are said to have resulted in such blue moon sightings. The key to a blue moon is having in the air lots of particles slightly wider than the wavelength of red light (0.7 micron)— and no other sizes present. This is rare, but volcanoes sometimes spit out such clouds, as do forest fires, explains NASA.

Is that likely today? Likely not. But who knows – check out when the moon rises at 6.43 pm towards the east and reaches its fully illuminated total phase at 4.13 pm.