When Is the Next Full Moon September 2021, and What Is It Called?

This month's full moon falls on September 20 at 7:54 p.m. EDT and is known as the Harvest Moon. A Harvest Moon is the name given to a full moon that occurs closest to the autumnal equinox, the moment when the Sun is exactly above the equator and Earth's day and night are of equal length.

The autumnal equinox, also known as the September equinox, occurs this year on September 22, meaning that the Harvest moon is happening two days prior to it. Harvest Moons can happen as early as two weeks before this point—which is considered the start of the Fall or Autumn season in the Northern Hemisphere—or follow it by as much as two weeks.

The Harvest Moon for the Southern Hemisphere occurs in March or early April, last happening on March 29, 2021, and will next fall on March 18, 2022.

The fact that it occurs anywhere within that four-week period means that the Harvest Moon is either the last full moon of the Summer season or the first full moon of the Autumn season. This year's Harvest Moon is the fourth of the Summer season, which is fairly unusual as seasons normally only have three full moons.

What is a Harvest Moon?

The Harvest Moon is just a name given to this particular full moon, but it does have some characteristics that make it a significant event. Usually a full moon rises at sunset and then each night after that, the moon rises around 50 minutes later.

When a full moon happens close to the equinox in mid-temperature altitudes, moon rises happen only 20 to 25 minutes later than sunset for a few days before and after. Further north, this time difference is even shorter.

That means that the Harvest Moon period is marked by moonlight from dusk-till-dawn for several days and there is no long period of darkness between sunset and moonrise for days.

Before the advent of electricity, farmers depended on moonlight to harvest crops. The moon was therefore of massive importance in the fall as this was traditionally the time of large crop harvests.

It is claimed that the full moon is larger, brighter, and of a more orange hue during the Harvest Moon, but this generally is not the case and likely arises from the cultural influence the Harvest moon has had, and the fact that that people look for it around sunset when the moon is on the horizon, according to EarthSky.org.

The moon can take an orange hue and appear larger when it is closer to the horizon because we are looking through a thicker point in the atmosphere to see it. That means that the atmosphere has a greater effect on the light reflected by the moon, scattering more blue light and allowing more red light to pass through and hit our eyes.

This year's Harvest Moon will be an average full moon but will be larger than last year's which was the second smallest full moon of 2020, and 2019 when the Harvest Moon was a so-called micro or mini-moon and was the smallest full moon of that twelve-month period.

Harvest Moon
A stock image showing the moon. The next Harvest Moon falls on September 20. Franz Schallmeiner/Getty