Halloween 2020 not only comes complete with a full moon, but a rare "blue moon" as well. According to the Farmer's Almanac, Saturday, Oct. 31, 2020, will see the blue-tinted full moon rise over spooky celebrants. It will be decades before this confluence of holiday and lunar events comes again.
This year, the Gregorian calendar lined up just right with the lunar cycle so that the seasonal "Harvest Moon" rose on Oct. 1, leaving enough time for another lunar cycle to take its course and send up another full moon before the month was over. When that happens, it is called a blue moon — a phrase that is also generally used to denote a rare event. The full moon may not necessarily look different, but it carries great significance for the Earth's cycle through the solar system, and for those who observe astrology in one form or another.
The fact that all this coincides with Halloween is uncanny for many fans of the supernatural and spooky. The correspondence between the lunar cycle and the solar calendar is such that a full moon on Halloween happens only once every 18 to 19 years. Each time, it is a blue moon, since the previous full moon would have to have fallen at the beginning of October 29 days earlier.
The blue moon can also refer to an unusual number of full moons within a season, not just a month, according to the astronomy site Earth Sky. When it happens that there are two full moons in one month, the second one is called a blue moon. When it happens that there are four full moons in a season rather than the usual three, the third is called a "seasonal blue moon."
Astronomy and astrology enthusiasts are encouraging those who do not unusually pay attention to the sky to get into the hobby this year, given that so many usual Halloween traditions are being disrupted. While it is not safe to go trick-or-treating or gather in large groups, the unique moon overhead can bring people together, creating a common spectacle to observe.
The next time a full moon falls on Halloween night will be in 2039. To many, even that won't compare to this year's haunting holiday, when Halloween falls on a Saturday night for once. If it weren't for the coronavirus pandemic, many people would be out celebrating without fear of getting up for school or work the next morning. Hopefully, next time the stars, planets and moons align this way, things will be different.