A blue moon, defined as either the second full moon in a calendar month or the third full moon in a season with four full moons, occurs roughly every two to three years on average
. This means that about seven blue moons happen every 19 years, or about 41 times per century
. Occasionally, a rare event called a "double blue moon" happens, where two blue moons occur in the same calendar year; this happens about three to five times per century, with the last occurrence in 2018 and the next expected in 2037
. The phrase "once in a blue moon" reflects the relative rarity of this event, although blue moons are more common than the phrase suggests. The moon itself does not actually turn blue during these events unless unusual atmospheric conditions occur, such as volcanic ash or smoke in the air
. In summary:
- Blue moons occur about every 2 to 3 years.
- There are about 7 blue moons every 19 years.
- Two blue moons in one year (double blue moon) are rare and happen a few times per century.
- The next monthly blue moon is on May 31, 2026.
- The next seasonal blue moon is on August 19-20, 2024
This cycle is tied to the lunar phases, which average 29.5 days, causing occasional extra full moons in a calendar year.
