What Is the Harvest Moon? Meaning of September 2021 Full Moon Rising Before Fall Equinox

Tonight, a full "Harvest Moon" will grace the skies, less than two days before the Northern Hemisphere's autumn equinox.

The term "full moon" refers to the moment when the Earth is located directly in between the sun and our natural satellite.

At these times, the face of the moon that we can see from Earth is fully illuminated, appearing like a perfect circle.

Technically, the moon is full for just a single moment. For the upcoming Harvest Moon, for example, this moment occurs at 7:54 p.m. EDT on September 20.

But to most casual observers, the moon will appear fully illuminated in the night sky from Sunday through Tuesday.

In the Northern Hemisphere, the full moon that appears closest to the fall equinox is traditionally referred to as the "Harvest Moon."

The term "Harvest Moon" is different from other traditional full moon names, which are each associated with a specific month.

While most Harvest moons tend to occur in September, on some occasions, this name is given to a full moon in October, depending on how the lunar cycle lines up with the Gregorian calendar.

During the Northern Hemisphere's fall equinox, the Earth's axis, which is tilted at an angle of around 23.4 degrees relative to the plane of its orbit around the sun, is pointed neither towards nor away from the sun.

At this time, the sun shines directly over Earth's equator, and the length of day and night is roughly equal. In the Northern Hemisphere, the September equinox marks the beginning of fall in astronomical terms.

The fall equinox usually falls between September 21 and 24 each year. In 2021, the autumn equinox occurs on September 22.

The names given to full moons originate from a number of places and historical periods, including Native American, colonial American and European sources.

According to the Old Farmer's Almanac, the name "Harvest Moon" may come from the fact that, traditionally, it was an aide to farmers during the fall harvest season.

Around this time of year, the moon rises very soon after sunset, resulting in bright moonlight early in the evening. This would have been helpful to farmers in the days before artificial lighting.

Throughout the year, the moon rises an average of around 50 minutes later each day. But for the few nights around the Harvest Moon, this difference is smaller. Across the northern U.S., for example, the moon rises only 25 to 30 minutes later each day at this time of year.

Several eastern and southeastern Asian cultures hold spiritually significant celebrations around the time of the harvest moon. Chinese people, for example, celebrate the Mid-Autumn Festival, which involves family reunions, moon gazing and the sharing of "moon cakes."

In Korea, a similar festival, known as Chuseok, is celebrated around the time of the Harvest Moon. People travel to their ancestral and parental homes, where they eat traditional sweets, play games and hold ceremonies in honor of their ancestors.

A Harvest Moon above New York City
A Harvest Moon sets behind the Statue of Liberty before sunrise on September 20, 2021 in New York City. Gary Hershorn/Getty Images