A blue moon, defined as the second full moon in a single calendar month, occurs about once every two to three years on average. More specifically, it happens roughly once every 33 months or about seven times every 19 years
. There is also a seasonal blue moon, which is the third full moon in a season that has four full moons instead of the usual three. These seasonal blue moons also occur about seven times in every 19 years
. Sometimes, two blue moons can occur in the same calendar year, a phenomenon known as a "double blue moon," which happens about three to five times per century
. Despite the name, the moon does not actually turn blue during these events; the term "blue moon" refers to the rarity of the event rather than the moon's color. Actual blue-colored moons are extremely rare and caused by specific atmospheric conditions like volcanic ash or smoke
. In summary:
- Blue moons happen about every 2 to 3 years (once every 33 months).
- About 7 blue moons occur every 19 years.
- Double blue moons in one year are rare, occurring a few times per century.
- The moon does not literally turn blue during a blue moon.
This explains the phrase "once in a blue moon" as meaning something that happens rarely but not impossibly
