What Is a Harvest Moon?

For many, the words “harvest moon” conjure up thoughts of flaxen-colored wheat and corn fields, and for good reason: It’s the name of the full moon that occurs closest to the autumnal (fall) equinox, or the beginning of astronomical fall.

The Harvest Moon

The harvest moon can be the last full moon of summer or the first full moon of fall. The exact date depends on the moon phase calendar and fall equinox date match-up in a given year. For example, September 2021's full moon occurred on September 20—two days before the autumnal equinox, which fell on September 22.

What was noteworthy was that the 2021 harvest moon was the fourth full moon of the season. Ordinarily, each season of the year has three full moons (one full moon per month, and three months per season). However, an extra full moon appears in a calendar year every three years or so. The last time a harvest moon coincided with one of these additional full moons was in 2013.

Why Is It Called the 'Harvest' Moon?

According to American folklore, each month’s full moon has a unique name. These names weren’t chosen by NASA, as one might initially expect, but rather by the Indigenous Algonquin and Iroquois tribes of North America, who marked the passing seasons by relating changes in the land to those in the sky. It’s believed these full moon names were then adopted by colonial Americans who referred to them in various texts (their first recorded use can be traced back to 1706). While the now out-of-print Maine Farmers’ Almanac is the first American almanac credited with publishing Indigenous full moon names in the 1930s, today’s almanacs, including The Farmers’ Almanac and The Old Farmer’s Almanac, still carry on the tradition.

Out of all the full moons, the one closest to the start of autumn was dubbed “harvest” because it aligns with the time when summer-grown crops, such as corn, are ready to be harvested.

The Harvest Moon's Extra Moonlight

Not only does the harvest moon serve as an astronomical reminder that it is time to harvest crops, but its unique moonlight actually aids farmers in completing this task.

While all full moons rise into the evening sky around sunset, only the full harvest moon and the waning moon over the next few nights floods landscapes with moonlight as soon as twilight ends. In other words, the moon shines brightly in the early evening for several consecutive days this time of year, gifting harvesters with extra light to glean their crops by. (Ordinarily after a full moon, the moons on the nights that follow rise about 50 minutes later and later each night, yielding a period of darkness between sundown and moonrise.)

Why do harvest moons, in particular, offer extra moonlight? It has to do with the seasonal “ecliptic," or the path the Moon travels across the sky as it moves around the Earth in its monthly orbit.

In the Northern Hemisphere, the Moon moves eastward along the ecliptic as it orbits Earth, traveling the equivalent of what looks to be one fist held at arm's length each night. Its path cuts across the eastern horizon, creating an angle with the ground.

This angle varies throughout the year. In spring, the moon's path steeply intersects the horizon, which leads moonrise times to vary significantly from one night to the next. However, in the weeks before and after the autumnal equinox, the ecliptic meets the horizon at an angle so shallow it's nearly parallel to the horizon. Because of this, the moon's position above the horizon changes the least from day to day (rather than being 50 to 75 minutes apart, a 20 to 30 minute lag in successive moonrises exists) and earth is simultaneously bathed in twilight and moonlight for several nights in a row.

Is It Bigger, Brighter, or More Golden?

Contrary to popular belief, harvest moons aren’t any bigger, brighter, or more honey-hued than any other full moon—at least, not unless they happen on the same date as a “supermoon” or other extraordinary lunar phenomena.

If a harvest moon does appear larger-than-usual or more golden to your eyes, it’s likely because you’re catching a glimpse of it right as it’s rising into the evening sky. At moonrise, any full moon will appear larger and more creamy-colored, since this is when the moon is nearest to Earth’s horizon. (When the moon sits along the horizon, it appears bigger as a result of an optical illusion. Similarly, the atmosphere is thicker near the horizon than it is higher up in the sky, so as moonlight travels through more air, more blue light waves are scattered, leaving mostly red and yellow light to reach our eyes.)

Historic Harvest Moons

Harvest moons may occur once a year, every year, but that doesn't mean they're all ho-hum. The following harvest moon events remain some of the most memorable in the minds of North American stargazers.

The 2010 Harvest Moon

Although it’s rare, harvest moons sometimes occur on the night of the autumnal equinox itself. This most recently occurred on September 22, 2010. Before that, there hadn’t been a full moon on the first day of fall in nearly 20 years. It won’t happen again until 2029.

The October Harvest Moon of 1987

Traditionally, the harvest moon is the name given to September’s full moon, but every so often, it has occurred in October. For example, in 1987, it didn’t roll around until October 7, which is the latest-known October harvest moon. According to The Farmers’ Almanac, October’s full moon will only be considered a harvest moon 18 times between 1970 and 2050.

The Super Harvest Blood Moon of 2015

In 2015, the harvest moon was also a supermoon; it appeared 14% larger than a typical full moon as a result of it travelling its closest to Earth. What's more, a total lunar eclipse, or "blood moon" also occurred that same evening. According to National Geographic, this confluence of events has only ever happened five times since 1900. Stargazers will have to wait until 2033 for a repeat.