The start of fall arrives with the autumnal equinox next Wednesday, September 22 at 3:21 p.m. That’s when the midday Sun hovers directly over the equator so that neither of Earth’s hemispheres gets more sunlight than the other.
The Harvest Moon is defined as the nearest full Moon to that equinox and that occurs just two days earlier, this Monday, September 20. This close proximity in time between 2021’s September full Moon and the fall equinox produces an effect we’ll get to in a moment. But first some basics. Although it’s the year’s most famous full Moon, a Harvest Moon never looks any different from any other full Moon. It’s not bigger, redder, lower, higher or anything else. Yet it isn’t merely a meaningless label for the September full Moon, either. Hmm, not just a name and yet it doesn’t look special. What’s left?
Behavior, that’s what. The Harvest Moon ACTS differently from all other full Moons.
The Moon normally comes up about an hour later each night. But the Harvest moon rises just 20 or 25 minutes later on successive evenings.
Result? From all this weekend right through the equinox on Wednesday, we›ll see a series of full or nearly-full Moons rising right around dusk again and again.
Tying this into the traditional folklore is easy. Farmers trying to finish harvesting work by nightfall are given a helping hand with the light of the full or nearly-full Moon conveniently appearing at sunset for several evenings in a row. (Of course, alternatively, they could simply turn on their tractor lights).
This year’s close match-up between the dates of September full Moon and equinox places that Moon almost exactly opposite the Sun. Which means that right around when the Sun sets precisely due West, which it does only on an equinox, the full Moon is rising due east, or actually a bit to the right of that spot. This will let photographers know exactly where to expect the rising full Moon, and those with telephoto lenses (at least 200 mm is desirable) can capture the full, low Harvest Moon against a still-beautifully illuminated foreground like the river or a distant mountain range. As a bonus, that’s exactly when the “Moon Illusion” kicks in to make it seem enormous even before the telephoto does its magnifying.
It’s all coming up this Monday through Wednesday.