Halloween Blue Moons - Obliquity

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Once in a Blue Moon

Halloween Blue Moons

In 2001, there was a Blue Moon on Halloween in the Central, Mountain and Pacific time zones of North America. Many people asked us:

  • How often does a Full Moon fall on Halloween?
  • How often does a Blue Moon fall on Halloween?

Actually, they amount to the same question, because if a Full Moon falls on October 31st, then the previous Full Moon must also have fallen in October.

To find the answer, we calculated the dates and times of all Full Moons between 2999 BC and AD 3000 using the same rigorous methods as those employed by the United States Naval Observatory and by Her Majesty's Nautical Almanac Office.

In that 6000-year period, there are 74211 Full Moons. If the times of the Full Moons are calculated using Greenwich Mean Time, then 211 of them fall on October 31. Thus on average, there will be a Full Moon on Halloween three or four times per century.

The 19-year Metonic cycle is an important factor, since it causes the dates of Full Moons to repeat at intervals of 19 years, or multiples of 19 years. This effect is especially evident in the 21st century, which has an unusually large number of Halloween Full Moons.

Halloween Blue Moons in the 20th and 21st centuries

Year London New York Chicago San Francisco
1906 YES YES YES
1925 YES YES YES YES
1944 YES YES YES YES
1955 YES YES YES
1974 YES
2001 YES YES
2020 YES YES YES YES
2039 YES YES YES YES
2058 YES YES YES YES
2077 YES YES YES YES
2096 YES YES YES YES

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