Saturday will see a once-in-a-lifetime rare astronomical event occur – a full moon on Halloween.
The last time a full moon occurred on October 31st was back in 1944, 76-years ago.
Saturday’s moon is also a ‘blue moon’.ย This doesn’t mean it is actually blue but is a term given to the second full moon in the same month. This only occurs once every 2.5 years or so.
The full moon on 1st October was the Harvest Moon because it occurs closest to the autumnal equinox. The 31st October full moon is the Hunter’s Moon, traditionally appearing in October, the month when game was fattened and preparations for winter begin.
According to NASA, the next time we will see a full moon on Halloween will be 2039.
The term “Blue Moon” historically referred to the third full moon of the astronomical season which is bounded by an equinox or solstice, not calendar dates. Every 2 or 3 years there is a thirteenth full moon in a calendar year (this happens 7 times in a 19-year cycle).
For centuries, full moons have been given names that are appropriate to the occurrences around their timing: ‘Hunter Moon’, Lenten Moon’, Harvest Moon’, etc. Since the occurrence of an extra full moon would confuse the name, the early full moon was called a blue moon so the names of the remaining moons of the year would remain meaningful.
The application of the term to the second full moon of the month initiated as a misinterpretation of the use by a single author who had a fairly wide audience but not enough patience for research.