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Ephemeris

Twin "Blue Moons" in 1999

Bob Garfinkle, F.R.A.S.


You have probably heard the old saying that some rare event occurs "Once in a blue moon." The moon does not actually turn blue, but a blue moon occurs when there are two full moons in a single calendar month. Blue moons usually occur once in cycles that can be about two to three years apart or about five to six years. Sometimes, one blue moon will occur approximately two years after we have had one, and sometimes the next blue moon will not be seen for up to about six years later. Prior to the blue moon on January 31, 1999, the last one was on July 30, 1996. The blue moon on March 31 is the last one for the twentieth century.

For only the third time this century, we will have two blue moons in one calendar year. This month's blue moon on the 31st is the first time since 1961 in which two blue moons occur in one calendar year. How rare? The last two before 1961 were 1915 and 1885.

This year began with a full moon on January 2 (2:49 Universal Time at Greenwich, England), followed by a blue moon and a very shallow penumbral lunar eclipse on the last night in January. There was no full moon in February; another rarity is for a month to have no full moon at all. Being shorter than the moon's 29-1/2-day-long mean synodic month, February is the only month where it is possible for no full moon to occur. Only four years this century had a February without a full moon (1915, 1934, 1961, and 1999).

Though the moment of new moon is not occurring at exactly at the same time of day, the blue moon pattern of this year is nearly an identical repeat performance of the full moon pattern seen 84 years ago (4.4 metonic cycles). There were blue moons on January 1 and 31, and on March 1 and 31, 1915.

The last near twin blue moon year was 1961, with full moons on January 1 and 31 and April 1 and 30. We had a near miss in 1934, when two full moons occurred in December 1933, a single full moon in January 1934, no full moon in February, and two full moons in March 1934.


Bob Garfinkle; last updated: October 04, 2007 Prev Next