Full moons are rare

Blue moon

The blue moon is not blue

The second full moon in a month or the fourth full moon in a season is referred to as "blue moon". This full moon is of course not blue (photos that suggest this are fakes). The term comes from the old English "belewe moon", pronounced bil'ju: mu: n, which means something like "lying moon" (note the same stem of "belewe" and "lying"). Because this full moon suggests the next season of the year, where it does not even begin, since there are usually three full moons in one season. Even more direct are references to that full moon, which falsely suggests the Easter full moon and thus the near end of Lent.

Why are two full moons rare in a month?

The moon needs an average of 27 days, 7 hours, 43 minutes and 11.5 seconds to orbit the earth. After this time, the moon is again in the same place in the sky, i.e. with the same stars. Hence the term Sidereal month for this period. Since the lunar orbit is elliptical and is also subject to numerous disturbances, mainly due to the attraction of the sun, this is, as already mentioned, only an average value that is subject to deviations from month to month.

After a sidereal month, the moon is in the same place in the sky again, but not in the same phase. The monphases come about through the constellation of sun, earth and moon and during a sidereal period the earth moves a little around the sun.

The phases of the moon are therefore repeated on average every 29 days, 12 hours, 44 minutes and 2.9 seconds (around 29.5 days). After that period that a Synodic month is called, the moon is again in the same phase.

Since the Gregorian calendar, according to which we live today, is a pure solar calendar (the beginning of the year is linked to the course of the sun), the monphases wander through the months. Since a month - apart from February with 28 or 29 days - is 30 or 31 days long, it can happen that a moon phase occurs twice in a Gregorian calendar month.

If there are two full moons in a month, the second is called one Blue moon. In the English-speaking world, the saying has become "once in a blue moon" established, which means roughly "all holy times". A blue moon is not that rare. On average, it happens every 2.4 years. In a specific month of 31 days there is an average of every 20 years a blue moon, in a specific month of 30 days every 63 years. Only in February there can never be a blue moon, not even in leap years.

If there is a blue moon in January, there can also be one in March, with no full moon in February. This will be the case in 2018 and 2037.

Blue Moons for Vienna in the early 21st century:

The exact definition of the full moon is "opposition of the sun and moon". This takes place at a slightly different time for each place on earth. Therefore, the "geocentric opposition of the sun and moon" is usually referred to as the full moon. The calendar date on which this falls also depends on the respective time zone. Therefore, the "Blue Moon" event does not apply to the whole earth, but only to certain time zones.

Table for the time zone Vienna (Central European Time CET or Central European Summer Time CEST):

monthfirst full moonsecond full moon
November 20011.30.
July 20042.31.
June 20071.30.
December 20092.31.
August 20122.31.
July 20152.31.
January 20182.31.
March 20182.31.
October 20202.31.
August 20231.31.
May 20261.31.
December 20282.31.
September 20311.30.
July 20341.31.
January 20372.31.
March 20372.31.

Attention, the dates of the full moons may differ in other time zones or at a certain location, as the date of the full moon depends on the location. A "blue moon" may not take place everywhere on earth.

By the way, only that used to be called Blue Moon third full moon in a season with four full moons referred to, since normally only three full moons occur in one (three months) (see above; this third full moon incorrectly suggests the upcoming next season). It was not until the middle of the 20th century that the name for the second full moon in a calendar month became widespread.

To interpret anything in this appearance, apart from a calendar curiosity, is just as pointless as to interpret anything else in the course of the stars. The calendar is a man-made, artificial, mathematical structure; there is nothing special in any date. And that the full moon has no effect on people - except for those who let themselves be influenced by artificial scare tactics - should be well known in the 21st century.

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