The Synodical Month is 29.53 days long between any two Full Moons. There are 365.24 days in the year which equals 12.37 Lunar Months. The little bit that is left over means that there can be 13 Full Moons during years that are about 1/.37 = 2.7 years apart or so. The following list is the list of Blue Moons for the next 54 years, and a double Blue Moon will happen in 1999 and 2018!:
Date UT January 31 1999 16:07 March 31 1999 22:49 November 30 2001 20:50 July 31 2004 18:06 June 30 2007 13:49 December 31 2009 19:13 August 31 2012 13:59 July 31 2015 10:44 January 31 2018 13:27 March 31 2018 12:37 October 31 2020 14:50 August 31 2022 1:36 May 31 2026 8:46 December 31 2028 16:49 September 30 2031 18:59 July 31 2034 5:55 January 31 2037 14:05 October 31 2039 22:37 August 31 2042 2:03 May 30 2045 17:53 January 31 2048 0:15 September 30 2050 17:33We see that the only months with 30 or 31 days can have a second Full Moon ( A Blue Moon in April will not occur during this particular series). In this series, there also happen to be more Blue Moons in January, July and August than in the other months, but this is due to the statistics of sampling over only a limited time span.
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