Blue Moon and red Mars on Halloween!

The second of two October 2020 full moons falls right on Halloween (October 31, 2020)! By popular acclaim, the second of two full moons in one calendar month is called a Blue Moon. Most often, a calendar month only has one full moon. By the way, that bright red “star” near the moon is no star at all but the planet Mars.

Read more: Full Harvest Moon on October 1, 2020

Around the world, the moon will appear full to the eye for the next couple of nights (October 30 and 31, 2020). Astronomically speaking, however, full moon happens at the instant that it’s directly opposite the sun (180 degrees from the sun in celestial longitude). This full moon moment comes on October 31, at 14:49 UTC (9:49 a.m. EST, 8:49 a.m. CST, 7:49 a.m MDT, 6:49 a.m. PDT, 5:49 a.m. Alaskan Time and 4:49 a.m. Hawaiian Time).

People will also call this full moon a micro-moon because it’s the most distant (and smallest) full moon in 2020 (252,380 miles or 406,166 km). Exactly seven lunar months (seven full moons) previous to this Halloween Blue Moon, we had the closest and biggest full moon of the year on April 8, 2020 (221,851 miles or 357,035 km). And exactly 7 lunar months (7 full moons) after this Halloween Blue Moon, next year’s closest and largest full moon will occur on May 26, 2021 (222,116 miles or 357,461 km).

Full moons at apogee (left) and perigee (right) in 2011. Composite image by EarthSky community member C.B. Devgun in India. Thanks, C.B.!

The next monthly Blue Moon (second full moon in one calendar month) will come on August 31, 2023. This time around, however, it’ll be a Blue Moon supermoon (closest and largest full moon of 2023). This Blue Moon supermoon will be 222,043 miles or 357,344 km distant.

Every 19 years, the phases of the moon recur on (or near) the same calendar dates. Sure enough, looking 19 years into the future, we find the second full moon in October 2039 falling right on Halloween (October 31, 2039).

In this 19-year lunar cycle, there are 235 lunar months (235 returns to full moon) yet only 228 calendar months. Because the number of full moons outnumber the number of calendar months, that means at least seven of these 228 calendar months must harbor two full moons (235 – 228 = 7 extra full moons).

However, if a February within this 19-year period has no full moon at all – as is the case in February 2037 – that means this extra 8th full moon must fall into the lap of another calendar month, as well. Therefore, the year 2037 actually sports two Blue Moons, in January and March of 2037, giving us a total of 8 Blue-Moon months in the upcoming 19-year Metonic cycle:

1. August 31, 2023
2. May 31, 2026
3. December 31, 2028
4. September 30, 2031
5. July 31, 2034
6. January 31, 2037
7. March 31, 2037
8. October 31, 2039

Because calendar months and lunar months are not equal, the discordance between the two creates a calendar oddity known as the Blue Moon. Enjoy the spooky dissonance of this Halloween Blue Moon, as it’ll be the last full moon to fall on Halloween until October 31, 2039!

Bruce McClure