The Science behind Blue Moons
Blue Moon is a term used to describe the third Full Moon of a season that has four Full Moons. It is also commonly used to refer to the second Full Moon in a calendar month.
What Is a Blue Moon?
The phrase once in a Blue Moon is colloquially used to suggest that something is very rare. But just how rare depends on your definition.
Seasonal Blue Moons
- Spring: March Equinox to June Solstice
- Summer: June Solstice to September Equinox
- Fall/autumn: September Equinox to December Solstice
- Winter: December Solstice to March Equinox
When one of the seasons in a year has four Full Moons, instead of the usual three, the third Full Moon is called a Blue Moon, or seasonal Blue Moon.
Monthly Blue Moons
These days, the second Full Moon in a calendar month is also referred to as a Blue Moon, or a monthly Blue Moon. This particular use was popularized due to a miscalculation published in a 1946 article in Sky and Telescope magazine.
How Rare Are Blue Moons?
Blue Moons occur around once every two or three years. In the 1100 years between 1550 and 2650, there are 408 seasonal Blue Moons and 456 monthly Blue Moons.
Not Really Blue
A Blue Moon does not look blue. However, blue colored Moons do rarely occur when dust or smoke particles in the air are of a specific size. Such particles help create a blue colored Moon by scattering away red light.
Moons which appear red can be caused by other sizes of dust particles or by a total lunar eclipse. These are much more common than Blue Moons.
Why Is it Called Blue Moon?
There are different accounts of why the third Full Moon of a season of four Full Moons is called a Blue Moon.
The month of Lent contains the Lenten Moon. The first Full Moon of spring – also known as the Easter Moon or the Paschal Moon – falls a week before Easter. In order to ensure that Lent and Easter coincide with the phases of the Moon, the calendar has termed the third Moon of the season as the Blue Moon.
Another version of this is that since each Full Moon of a normal year already has a given name, for instance, Harvest Moon, the 13th nameless Full Moon in a year was named a Blue Moon. This way the lunations and calendars were aligned to make sure celebrations and customs would still fall during their "proper" times.