Harvest Moon and Autumn Equinox
Here in the Northern Hemisphere, we call the full moon closest to the autumn equinox the Harvest Moon. In 2021, the Northern Hemisphere autumn equinox comes on September 22. The full moon falls less than two days earlier, on September 20. Thus, for the Northern Hemisphere, this upcoming full moon – the full moon closest to our autumn equinox – is our Harvest Moon.
Depending on the year, the full Harvest Moon can occur anywhere from two weeks before the autumn equinox to two weeks after. The Harvest Moon is either the last full moon of the summer season, or the first full moon in autumn. This year, the September full moon is the fourth of the season’s four full moons. That’s the fourth moon of summer for the Northern Hemisphere, and the fourth winter full moon for the Southern Hemisphere.
For the Southern Hemisphere, the Harvest Moon always comes in March or early April. It last took place on March 28, 2021, and will next happen March 18, 2022.
Most often, a season has three full moons. This time around, however, four full moons take place in one season, or in between the June solstice and September equinox. Four full moons in one season is quite atypical. Therefore, some people refer to the third of these four full moons as a seasonal Blue Moon.
Actually, a Blue Moon by the definition of the term preceded the better known definition. By popular acclaim, most regard a Blue Moon as the second of two full moons in one calendar month.
What is the Harvest moon?
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 dusk-till-dawn moonlight, for several evenings in a row, around the time of the Harvest Moon.
On average, the full moon rises around sunset, and rises about 50 minutes later each day. But when a full moon happens close to an autumn equinox, the moon on the following nights rises closer to the time of sunset. For mid-temperate latitudes, it rises only about 20 to 25 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 farther north you live, the greater the Harvest moon effect. For instance, at Anchorage, Alaska (61 degrees north latitude) the moon will rise at nearly the same time for a week!
The difference between 50 minutes and 25 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, presenting dusk-till-dawn moonlight for several 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?
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 – differs from year to year. Last year, in 2020, the Harvest Moon was the second-smallest full moon of 2020. In 2019, the Harvest Moon was actually a micro-moon or mini-moon: the most distant and smallest full moon of the year 2019. But five years ago – September 28, 2015 – the Harvest Moon was the year’s closest and biggest supermoon.
No, the Harvest full moon is not necessarily closer than any other full moon. It all depends on the year. In 2021, it’s pretty much an average-sized full moon.
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 the time of any full moon, the moon will always be near the horizon. It’ll have just 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.
Orange color due to thickness of Earth’s atmosphere near horizon
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 2021?
The exact time of the full Harvest Moon is September 20 at 23:54 Universal Time. At U.S. time zones, that translates to 8:54 p.m. ADT, 7:54 p.m. EDT, 6:54 p.m. CDT, 5:54 p.m. MDT, 4:54 p.m. PDT, 3:54 p.m Alaskan Time and 1:54 p.m. Hawaiian Time.
So watch for the Harvest Moon in late September and early October … 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. But if the full moon occurs in early October – as it did in 2020 – the October full moon is that year’s Harvest Moon.
Bottom line: The closest full moon to the autumn equinox is the Harvest Moon. In 2021, the autumnal equinox for the Northern Hemisphere comes on September 22. So this hemisphere’s Harvest Moon comes on September 20.