Blue moons occur roughly every two to three years. This frequency applies to the common definition of a blue moon as the second full moon in a calendar month, which happens about seven times in every 19 years
. Another type of blue moon, called a seasonal blue moon (the third full moon in a season with four full moons), also occurs about seven times every 19 years, or roughly every two to three years
. In terms of months, a blue moon happens on average once every 33 months (about 2 years and 9 months)
. February can never have a blue moon because it is shorter than the lunar cycle, and sometimes February has no full moon at all, which can lead to two blue moons in adjacent months in the same year-a rare event occurring about 3 to 5 times per century
. Occasionally, two blue moons can occur in a single year, which is quite rare and happens about four times per century
. The next instance of two blue moons in one year will be in 2037
. In summary:
- Blue moons occur about once every 2 to 3 years.
- There are two main types: monthly blue moons (second full moon in a calendar month) and seasonal blue moons (third of four full moons in a season).
- Both types occur about seven times every 19 years.
- Two blue moons in one year are rare, happening roughly four times per century.
- February never has a blue moon due to its short length.
This frequency explains why the phrase "once in a blue moon" means something rare but not extremely uncommon in astronomical terms
