Several time zones on earth are expected to witness the rare astronomical phenomenon called Blue Moon on October 31, the event that gave rise to the phrase ‘once in a blue moon’.
What you need to know upfront is that the moon will not really turn blue in colour – that’s a separate phenomenon about which we will come to later in this piece.
But a Blue Moon is referred to as such when there is a second full moon in a single calendar month.
Making the phenomenon rarer is the fact that October 31’s Blue Moon – which will start becoming visible at about 8.19 pm in several time zones -- coincides with 'Halloween'.
The first full moon of October took place around October 1 and October 2.
Besides being an astronomical event, the Blue Moon is also associated with various folklores and horror stories. Here’s how the phenomenon came to be called Blue Moon.
Origin of the term Blue Moon
The calendar year, based on the solar cycle, has months, which are similar to the lunar cycle but not the same. This means that there will be years when there are 13 full moons in a year, instead of 12, with one month have two full moons. This phenomenon was first described in a March 1946 issue of Sky and Telescope magazine in an article named ‘Once in a Blue Moon’.
The rarity of the phenomenon also manifests itself in seasons – sometimes a season comprising three months may have four full moons. Thus, you can sometimes have a monthly Blue Moon, or the seasonal Blue Moon.
Blue Moons take place once every few years. The last monthly Blue Moon was in March 2018 while the last seasonal Blue Moon took place in May 2019.
Does the moon appear blue in colour?
While the moon will not appear blue in colour on Blue Moon days, there are other times it may do so.
This happens when Earth’s atmosphere contains dust or smoke particles of a certain size, slightly wider than 900 nanometers. The particles of this size are responsible for scattering red light. When these particles are present in the air, the moon shines through them and may appear blue.
The particles are often present in the air after a large fires or volcanic eruptions. For instance, it is said that the moon appeared from several parts of the world for a full two years following the Krakatoa eruption in 1883, one of the deadliest volcanic explosions in recorded history.