Blue Moon A Blue Moon is the second full moon in a calendar month. Usually months have only one full moon, but occasionally a second one sneaks in. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month. A blue moon occurs about once in 2.5 years on average. A blue moon can occur in January and the following March if there is no full moon at all in February, as is the case in the years 1999, 2018, and 2037. Note that because the full moon occurs at different times (and therefore potentially on different calendar days and in different calendar months) in different time zones, the occurrence of blue moons is time zone (and daylight saving time) dependent Blue Moons 2001 to 2104

(Universal Time) 2001 , Nov 1, Nov 30 2004 , Jul 2, Jul 31 2007 , Jun 1, Jun 30 2009 , Dec 2, Dec 31 2012 , Aug 2, Aug 31 2015 , Jul 2, Jul 31 2018 , Jan 2, Jan 31 (!) 2018 , Mar 2, Mar 31 (!) 2020 , Oct 1, Oct 31 2023 , Aug 1, Aug 31 2026 , May 1, May 31 2028 , Dec 2, Dec 31 2031 , Sep 1, Sep 30 2034 , Jul 1, Jul 31 2037 , Jan 2, Jan 31 (!) 2037 , Mar 2, Mar 31 (!) 2039 , Oct 2, Oct 31 2042 , Aug 1, Aug 31 2045 , May 1, May 30 2048 , Jan 1, Jan 31 2050 , Sep 1, Sep 30 2053 , Jul 1, Jul 30 2056 , Mar 2, Mar 31 2058 , Oct 2, Oct 31 2061 , Aug 1, Aug 30 2064 , May 1, May 30 2066 , Dec 1, Dec 31 2069 , Sep 1, Sep 30 2072 , May 2, May 31 2075 , Apr 1, Apr 30 2077 , Oct 2, Oct 31 2080 , Jul 2, Jul 31 2083 , May 2, May 31 2085 , Dec 1, Dec 31 2088 , Sep 1, Sep 30 2091 , Jul 1, Jul 30 2094 , Jan 1, Jan 31 2096 , Oct 2, Oct 31 2099 , Aug 1, Aug 30 2102 , Jun 1, Jun 30 2104 , Dec 2, Dec 31 The mean time intervall is 2.577 years (2 years 7 months). Gregorian year = 365.2425 days lunation: 29.53058867 days 12,368277 lunations per year 0.368277 extra lunations per year 1 / 0.36827 years = 2.7153 years per extra lunations (2 years 8.6 months).