A blue moon is the appearance of the third full moon in a season that has four full moons, instead of the usual three. It is never visually blue. Because a blue moon occurs only every two or three years, the term blue moon is used colloquially to mean a rare event, as in the phrase “once in a blue moon”.
Why a Blue Moon ?
One lunation (an average lunar cycle) is 29.53 days. There are 365.26 days in a solar year. Therefore, about 12.37 lunations (365.26 days divided by 29.53 days) occur in a solar year. In the widely used Gregorian calender, there are 12 months (the word month is derived from moon) in a year. Each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. The extra moon necessarily falls in one of the four seasons, giving that season four full moons instead of the usual three, and, hence, a blue moon.
When to see a Blue Moon ?
- 2012: Aug 31
- 2015: Jul 31
- 2018: Jan 31
- 2018: Mar 31
- 2020: Oct 31
- 2023: Aug 31
- 2026: May 31
- 2028: Dec 31
- 2031: Sep 30
- 2034: Jul 31
- 2037: Jan 31
- 2037: Mar 31
- 2039: Oct 31
- 2042: Aug 31
- 2045: May 30
- 2048: Jan 31
- 2050: Sep 30
- 2053: Jul 30
- 2056: Mar 31
- 2058: Oct 31
- 2061: Aug 30
- 2064: May 30
- 2066: Dec 31
- 2069: Sep 30
- 2072: May 31
- 2075: Apr 30
- 2077: Oct 31
- 2080: Jul 31
- 2083: May 31
- 2085: Dec 31
- 2088: Sep 30
- 2091: Jul 30
- 2094: Jan 31
- 2096: Oct 31
- 2099: Aug 30
- 2102: Jun 30
- 2104: Dec 31