This Halloween, Look for the Hunter’s Blue Moon
The second full moon of the month gives Halloween an extra spooky atmosphere

Halloween features a full moon every 19 years, and it's always a blue moon. (NASA's Scientific Visualization Studio)

By Theresa Machemer

SMITHSONIANMAG.COM

OCTOBER 26, 2020 

This October is flanked by full moons. October 1 featured an unusually late Harvest Moon, and on October 31, we’ll get an encore—a Hunter’s Blue Moon, Ashley Strickland reports for CNN.

The name “blue moon” doesn’t mean that the full moon will literally take on a blue hue, but that it is the second full moon in a single month. Blue moons only happen about every two and a half years. The last one rose on March 31, 2018, Tim Sharp wrote for Space.com in 2018. As the full moon following the Harvest Moon, the full moon that will rise on Halloween is also known as the Hunter’s Moon. And because the moon is within a day of reaching the furthest point from Earth along its orbit, the Halloween full moon will also appear about 14 percent smaller than the supermoon that appeared this April, Joe Rao explains for Space.com this month.

Halloween’s full moon will reach peak illumination at 10:51 a.m. on the east coast of the United States, according to the Old Farmer’s Almanac. With some variation, in the U.S. the full moon will rise between 6 and 7 p.m. local time and set around 7 a.m. the next morning.

Because full moons occur about every 29.5 days, there is usually only one full moon per month, or 12 full moons in a year. But sometimes the lunar cycle will line up just right so that there are 13 full moons in a year, with one month (but never February) doubling up.

This definition is a relatively new way of describing a blue moon, though. Per Sharp at Space.com, a 1937 issue of the Maine Farmer’s Almanac relied on seasons, bounded by equinoxes and solstices, to identify the extra full moon. Usually, each season gets three full moons. But in the odd season with four full moons, the third was considered the blue moon.

The seasonal definition of blue moons doesn’t line up with the monthly definition, so the last time Earth saw a seasonal blue moon was in May 2019.

Whenever there’s a full moon on Halloween, it has to be a blue moon because of the 29.5-day-long lunar cycle, according to NASA. And while blue moons pop up every two and a half years, full moons only occur on Halloween every 19 years, reports CNN. That’s because the lunar calendar repeats itself every 19 years, a pattern known as the Metonic cycle because it was identified in 432 B.C. by Athenian astronomer Meton.

Halloween’s full moon is also called a Hunter’s moon. The Hunter’s moon always follows the Harvest moon, which is the full moon closest to the autumnal equinox on September 22. The Harvest moon may have gotten its name because farmers could use the light of the moon to extend their workday even as daylight hours shortened, and so they could finish harvesting fall crops before the first frost. The Hunter’s moon is a reference to the way that humans and animals can use this time to stock up on food for the winter ahead, per the Old Farmer’s Almanac.

Finally, the Halloween full moon appears within a day after the moon reaches the furthest point on its orbit around Earth, called apogee, per EarthSky’s Bruce McClure. The moon had its furthest apogee of the year in March. On October 30, the moon will be 252,522 miles away from Earth, and will look about 14 percent smaller than April’s supermoon. That makes it the opposite of a supermoon, sometimes called a “minimoon.”

Theresa Machemer is a freelance writer based in Washington DC. Her work has also appeared in National Geographic and SciShow. Website: tkmach.com