Once in a Blue Moon? You mean once every two and a half years. Blue moons are rare occurrences, but are not as rare as people think.
Tonight, the world will experience a “blue moon”, according to the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA).
But in this case, the moon doesn’t literally turn blue. The moon is called blue when it’s the second full moon within a month. Usually, there is only one blue moon in a month, with of course, the exception of blue moons.
The lunar cycle is 29 days long, which means that eventually, there will be an appearance of two moons in one month. This usually happens when a full moon appears at the very start of the month, either on the first or second day.
A blue moon happens roughly once every two and a half years on average, the last two happened in August 2012 and July 2015. In rare cases, there are two blue moons in one year. The “double blue moon” occurred last 1999, and will happen again this year – one tonight, and another in March. On the other hand, when double blue moons occur in January and March, February does not have a full moon, partially because it only has 28 days.
Bluer than Blue
There have been cases of an actual “blue moon,” which are rarer than its conventional meaning. The moon changes hue when there are volcanic eruptions or large fires that leave particles in the atmosphere.
One of the longest times a blue moon occurred was when the Indonesian volcano Krakatoa erupted in 1883, equal to the blast of a 100-megaton nuclear bomb. People reported to have heard a cannon-like noise up to 600 kilometers away. Ash and particles about 1 micrometer wide rose up to the Earth’s atmosphere, causing selective light to pass through and reach the surface. The moon “turned blue” for days in areas near Krakatoa.
Reported sightings of a “blue moon” also happened after Mount Pinatubo erupted in 1991. Forest fires are also a cause for blue moons because of the smoke and particles they create. In these occurrences, “lavender suns” are also reported to be seen, also caused by particles in the air.
Originally, the blue moon was considered the third out of four full moons in a season (winter, spring, summer, fall). Each season usually experiences 3 full moons, hence the appearance of a 4th moon, or the Blue Moon, which came rarely and is considered the 13th moon in a year. This was based on Maine Farmer’s Almanac from 1819, which farmers used as reference for agricultural purposes.
However, in 1946, an article on Sky & Telescope misinterpreted the blue moon as the 2nd moon in a month, inferring from the idea that the blue moon appeared as the 13th full moon in a year. The article was titled “Once in a Blue Moon”, a phrase which integrated itself into pop culture meaning something that happens very rarely.
From this misinterpretation, a blue moon can be considered either of the following:
1. It is the extra full moon within a season, which usually has three moons (Maine’s definition); or
2. It is the second full moon within a month (Sky & Telescope’s definition).
The latter is the more popular and commonly used definition for a blue moon nowadays, with the other definition practically defunct.
Catch the blue moon tonight, peaking at 8:51 PM (Philippine Standard Time).