The moon will look huge—large even by Harvest Moon standards—all while the sweet-scented leaves turn yellow, the yellow school buses rumble, and the chill air of fall begins to settle in again this September.
Close to the fall equinox, September’s full moon in 2024 has a special name: Harvest Moon.
On Sept. 17, this full moon will also fall when it is closer to the Earth than usual, making it a supermoon, appearing quite giant.
At exactly 10:34 p.m. EDT, you can find the Harvest Moon rising to the east with the setting sun straight due west—full moons are always directly opposite the sun, as that direct sunlight exposure is the reason for their fullness.
Historically, the Harvest Moon is already famous for looming large, extra low, and ochre-yellow in the night sky. It would seem to linger longer and larger as a centuries-old lantern of tradition for helping farmers to finish their last harvesting as the days diminished, before the frost and winter set in.
The Moon Illusion
A NASA astronomer would say that the Harvest Moon’s apparent gigantism is linked to the horizon. Because the time period close to the equinox is when the moon’s ecliptic plane trends toward the horizon, the moon can often be found there in the fall, from which a trickery of sizes arises.
Typically, Harvest Moons may be no larger than, say, the so-called Strawberry Moon in June or any average full moon. But lingering low, side-by-side with buildings, hills, and trees, the full moon fools the eye. It’s merely psychological. Terrestrial objects seem smaller while it appears to shine larger. From this mere trick of the eye arose the term “moon illusion,” which happens often in autumn.
It Will Be a Supermoon
Only three or four times a year will the moon actually appear to grow. It’s no mere illusion. Full moons will appear up to 7 percent larger at these times, and the moon becomes a supermoon. This phenomenon arises because the moon’s orbit is not a precise circle but is elliptical. So at certain times, it is nearer and at other times farther away from Earth; its nearest point is called the perigee, and furthest point its apogee. When in the same ballpark as the perigee—a technical full moon falling exactly then would be an astronomical improbability yet a close alignment is accepted—the full moon is called a supermoon.
Another Harvest Moon Phenomenon
But wait! The list of special effects behind the Harvest Moon goes on, as for eons, farmers of old noted an oddity whereby low-hanging Harvest Moons seemed to rise strangely—mysteriously popping up earlier than expected to help reaping the fields and provide light after twilight amid the ever-shortening days.This, too, is no illusion. While on average, the moon rises 50 minutes later each day throughout the month as it traverses its eastward orbit (giving rise to moon cycles), it will rise as soon as only 23 minutes later for a short period around the fall equinox. It appears the heavens offered a boon to farmers reaping their corn (thus giving rise to the name Harvest Moon and another name for September’s full moon: Corn Moon).
