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Second Blue Moon on Saturday PDF Print E-mail
Written by Steve Glennie-Smith   
Monday, 26 March 2018 19:44

Easter Saturday - March 31st - presents the second and final Blue Moon of 2018.  By popular acclaim, a Blue Moon is defined as the second of two full moons to occur in one calendar month.  January had two full moons.  If this happens, March usually will as well.

Two Blue Moons in one year seem to belie the famous statement: Once in a Blue Moon - though Blue Moons are not that rare.  However, it is rare to have two Blue Moons in the same calendar year.  It last happened in 1999 and won't happen again until 2037.

February is the only month that can go without a full moon.  Prior to 2018, we last had a February with no full moon 19 years ago, in 1999.  This resulted in two Blue Moons that year; on January 31st and March 31st.  Nineteen years from now, in 2037, February again will have no full moon, so the months of January and March will each feature two full moons.

There was a total eclipse of this year's January 31st Blue Moon (though not visible in the UK).  Nineteen years later, in 2037, there will be another total eclipse of the Blue Moon on 31st January that year - this will be visible from the UK.

There is a reason for this.  Every 19 years, the phases of the moon fall on or near the same calendar dates because 19 solar years are very close to 235 lunar months (235 returns of the full moon).  There are 235 full moons yet only 228 calendar months (19 x 12 = 228) in this period, known as the Metonic cycle.  Therefore, 7 of these 228 months contain an extra full moon.

This nice pattern will be broken in 2056 because the slight discrepancy between 19 years and 235 moons will result in a full moon on 1st February.  The same will happen in 2075 - there will be only one Blue Moon in 2056 (March 31st) and one Blue Moon in 2075 (April 30th).

57 years (3 times 19) after 2037 – in 2094 – there will again be a February with no full moon and a year with two Blue Moons (January 31st and April 30th).