Here in the Northern Hemisphere, we call the full moon closest to the autumn equinox the Harvest Moon. Depending on your time zone, 2018’s autumn equinox for the Northern Hemisphere comes on September 22 or 23. And the September full moon comes on the night of September 24 for the Americas, and on September 25 for much of the rest of the world. Thus, for the Northern Hemisphere, this upcoming full moon – the full moon closest to our autumn equinox – is our Harvest Moon.
For the Southern Hemisphere, the Harvest Moon always comes in March or April.
Harvest Moon is just a name. In some ways, it’s like any other full moon name. But these autumn full moons do have special characteristics, related to the time of moonrise. Nature is particularly cooperative in giving us full-looking moons near the horizon after sunset, for several evenings in a row, around the time of the Harvest Moon.
What is a Harvest Moon? On average, the moon rises about 50 minutes later each day. But when a full moon happens close to an autumn equinox, the moon rises closer to the time of sunset. For mid-temperate latitudes, it rises only about 30 to 35 minutes later daily for several days before and after the full Harvest moon.
For very high northern latitudes, there’s even less time between successive moonrises.
The difference between 50 minutes and 35 minutes might not seem like much. But it means that, in the nights after a full Harvest Moon, you’ll see the moon ascending in the east relatively soon after sunset. The moon will rise during or near twilight on these nights, making it seem as if there are several full moons – for a few nights in a row – around the time of the Harvest Moon.
Why does this happen? Check out the illustrations below:
Is the Harvest Moon bigger, or brighter or more colorful? Not necessarily.
Because the moon’s orbit around Earth isn’t a perfect circle, the Harvest Moon’s distance from Earth – and apparent size in our sky – is a bit different from year to year. In 2018, the Harvest Moon is a touch smaller than your average-sized full moon, because it’s in a relatively distant part of its orbit around Earth.
Still, in any year, you might think the Harvest Moon looks bigger or brighter or more orange. That’s because the Harvest Moon has such a powerful mystique. Many people look for it shortly after sunset around the time of full moon. After sunset around any full moon, the moon will always be near the horizon. It’ll just have risen. It’s the location of the moon near the horizon that causes the Harvest Moon – or any full moon – to look big and orange in color.
The orange color of a moon near the horizon is a true physical effect. It stems from the fact that – when you look toward the horizon – you’re looking through a greater thickness of Earth’s atmosphere than when you gaze up and overhead.
The bigger-than-usual size of a moon seen near the horizon is something else entirely. It’s a trick that your eyes are playing – an illusion – called the Moon Illusion. You can find many lengthy explanations of the Moon Illusion by doing an online search for those words.
When is the Harvest Moon in 2018? The exact time of this September’s full moon is September 25 at 2:52 Universal Time. At U.S. time zones, that translates to September 24, at 10:52 p.m. EDT, 9:52 p.m. CDT, 8:52 p.m. MDT, 7:52 p.m. PDT, 6:52 p.m. AKDT (Alaska Daylight Time), and 4:52 p.m. HST (Hawaiian Standard Time).
So watch for the Harvest Moon on September 24 or 25 … or any of the nights around then.
By the way, more often than not, the September full moon is the Northern Hemisphere’s Harvest Moon. It’s usually the full moon closest to the autumn equinox. But if the full moon occurs in early October – as it did in 2017 and will again in 2020 – the October full Moon is that year’s Harvest Moon.
How did the Harvest Moon get its name? The shorter-than-usual lag time between moonrises around the full Harvest Moon means no long period of darkness between sunset and moonrise for days in succession.
In the days before tractor lights, the lamp of the Harvest Moon helped farmers to gather their crops, despite the diminishing daylight hours. As the sun’s light faded in the west, the moon would soon rise in the east to illuminate the fields throughout the night.
Who named the Harvest Moon? That name probably sprang to the lips of farmers throughout the Northern Hemisphere, on autumn evenings, as the Harvest Moon aided in bringing in the crops.
The name was popularized in the early 20th century by the song below.
Shine On Harvest Moon
By Nora Bayes and Jack Norworth (1903)
Shine on, shine on harvest moon
Up in the sky,
I ain’t had no lovin’
Since January, February, June or July
Snow time ain’t no time to stay
Outdoors and spoon,
So shine on, shine on harvest moon,
For me and my gal.
And don’t miss this more recent version of the song by Leon Redbone.
Bottom line: According to skylore, the closest full moon to the autumn equinox is the Harvest Moon. In 2018, the autumnal equinox for the Northern Hemisphere comes on September 22 or 23, depending on time zone. So this hemisphere’s Harvest Moon is the full moon on the night of September 24 or 25, 2018.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.