In case you haven’t already heard, we have a Harvest Moon coming our way on Monday night, September 20, 2021 (at precisely 7:55 p.m. EDT – a little over an hour after the sun has set). But if you check your calendar, you’ll notice that fall doesn’t begin until September 22, 2021. How can you have a Harvest Moon arrive before summer has wound down?
Although we associate a Harvest Moon with autumn, this particular Harvest Moon occurs during the confines of the summer season. Traditionally, the Harvest Moon is the full Moon nearest the autumn equinox. And in 2021, the equinox arrives on Wednesday, September 22nd. So, this year’s Harvest Moon will be the last full Moon of the summer instead of the first full Moon of fall—yet it will be still closest to the equinox.
A Season of 4 Full Moons
It is because of this circumstance that the previous full Moon—the full Sturgeon Moon on August 22—was deemed a “Blue Moon.” A seasonal rule states that when four full Moons can be squeezed into a specific season (normally there are just three), then the third full Moon receives the title of “Blue Moon.” We saw full Moons on June 24, July 23, August 22, and now, the fourth “summer” full Moon will come just two days shy of the end of summer, on September 20th.
Why A Harvest Moon?
As the Native Americans prepared to gather their crops each autumn, they looked to this full Moon as a sign that it was time to harvest. For a few days before and after its fullness, it hangs in the sky like a great glowing lantern, prolonging the light well after sunset.
When the winter solstice arrives in December, the daily lag in the rising of the full Moon will be close to an hour, but now it is closer to 23 minutes.
Is It A Rare Occurrence?
Since the autumnal equinox vacillates irregularly from September 22 to 23 from year to year, means that in the overall scheme of things, the Harvest full Moon can come as early in the calendar as September 8 or as late as October 7. So, a summer Harvest Moon, especially one that happens just a couple of days before the equinox, is not really such a rare occurrence.
In fact, about 50% of all Harvest Moons occur during the summer, while the other half occur during the fall.
On the other hand, September is usually the month associated with Harvest Moons; they happen nearly 76% of the time, while October Harvest Moons occur an average of just 24%. If a full Moon falls during the first week of September, that’s much too early to be considered a “harvest” Moon (and it gets the name “Corn Moon”); so, we must wait about a month and bestow that title on either a full Moon at the very end of September or the beginning of October.
September/October Harvest Moons
In 2033 we’ll have an early Harvest Moon—the earliest one possible—on September 8. We had an October Harvest Moon in 2020, on October 1. Our next one will come on October 6, 2025.
The full Moon of October 20 will be designated as the Hunter’s Moon, accompanied by a series of brilliant moonlit nights when, in times past, the hunter and his faithful canine companion spent frosty evenings trailing deer and hoping perhaps to provide some venison on the kitchen table to feed his family.
So when viewing our “Summer Harvest Moon” on September 20th, let’s hope the summer heat dissipates, provides us with clear skies, and sets the stage for the glorious arrival of autumn.
Joe Rao is an esteemed astronomer who writes for Space.com, Sky & Telescope, and Natural History Magazine. Mr. Rao is a regular contributor to the Farmers' Almanac and serves as an associate lecturer for the Hayden Planetarium in New York City.