In the northern hemisphere, the full moon that occurs closest to the autumn equinox (when day and night are roughly equal around the world) is called the Harvest Moon. For several days in a row, the moon will rise near sunset to provide extra light, which farmers can use to harvest summer crops before winter arrives.

Many cultures — especially those at temperate latitudes — mark the occasion with evening festivities. In China and other Asian countries, the Mid-Autumn Festival is celebrated with paper lanterns and mooncake pastries. In 2021, the Harvest Moon was on September 20.

People often assume the Harvest Moon is bigger and brighter than other full moons, but that's not necessarily true. What makes it so special is that, when it rises above the horizon, an optical illusion makes the rising moon appear enormous.

Moon Basics

The moon travels around the Earth once every ~29.5 days, which is called a lunar month. The Earth travels around the sun once every ~365.25 days, which is called a year. Half the surface of both the Earth and the moon is illuminated by the sun at any one time, while the other half is (nearly) dark. The Earth also spins on its axis, making a full rotation every 24 hours, which gives us day and night. On Earth, the sun rises in the east and sets in the west. The moon also rises in the east and sets in the west, but the time of moonrise and moonset changes dramatically over the course of a lunar month.

When the moon is positioned roughly between the Earth and the sun, a person on Earth can only see the dark side of the moon — the part that is not illuminated by the sun. This is the New Moon, which rises in the morning and sets in the evening. As the moon continues its rotation around Earth, it will rise and set an average of 50 minutes later each day. As this occurs, you can see a larger and larger portion of the illuminated side of the moon. Halfway through the lunar month, the Full Moon rises at sunset and sets in the morning. As the moon continues its rotation, people on Earth will see a smaller and smaller portion of the illuminated side of the moon until it becomes a New Moon and the cycle starts over.

The Moon's Path Across the Sky

The Harvest Moon only occurs in the northern hemisphere. This is because: (1) The Earth is tilted 23.5 degrees on its axis as it travels around the sun (which gives us seasons), and (2) the moon's orbit around the Earth is tilted at a 5-degree angle relative to the equator, as EarthSky notes. As a result of these two tilts, the moon's path across the night sky changes throughout the year and appears different depending on the observer's latitude (distance from the equator).

The path of the Harvest Moon at moonrise is nearly parallel to the horizon, according to The Old Farmer's Almanac. This causes the moon to rise above the horizon at nearly the same time for a few nights in a row. This effect is most pronounced at northern latitudes and becomes less noticeable for observers closer to the equator.

Over the course of a year, the moon rises an average of 50 minutes later each day, but extremes occur during the Harvest Moon. Using information available on Time and Date, anyone with an internet connection can find the moonrise time for any city on a particular date. In the days surrounding the Harvest Moon in 2021, the number of days it took for the time of moonrise to change by 50 minutes was: 10 days in Helsinki, Finland (latitude of 60.2 degrees north), 5 days in London, England (51.5 degrees north), 4 days in New York City, New York, US (40.7 degrees north) and 3 days in Miami, Florida, US (35.8 degrees north). In contrast, on the night after the same full moon in Melbourne, Australia (37.8 degrees south), the moon rose 61 minutes later.

The situation is roughly reversed for the spring equinox, which occurs in late March. In the days surrounding the full moon closest to the spring equinox in 2022, it will take three days for the time of moonrise to change by 50 minutes in Melbourne, Australia (37.8 degrees south). In contrast, the night after the same full moon in Helsinki, Finland (60.2 degrees north), the moon will rise a whopping 94 minutes later.

The Moon Effect Optical Illusion

The Harvest Moon is celebrated for being big and bright, but it's not necessarily bigger or brighter than any other full moon. When the moon rises above the horizon, an optical illusion called the Moon Illusion makes it appear enormous, as explained by NASA.

Essentially, your brain is comparing the size of the moon to things it sees in the horizon, such as oceans, mountains, trees and buildings. Your brain knows that the moon is behind these big things, so the moon must be even bigger. As the Moon rises high in the sky, there are no Earth objects to compare it to, so the brain perceives it to be smaller.

People often describe a moon near the horizon as red or orange, which is not an optical illusion. When the moon is low in the horizon, you are viewing it through the thickest part of the atmosphere. Short wavelength blue light scatters when it travels through the atmosphere, which leaves mostly longer wavelength red and yellow light to reach your eyes.

Comparing Full Moons

The 2021 Harvest Moon was notable because it was the fourth full moon of the summer. Each season usually has three full moons, but because the moon's orbit around the Earth is not synchronized with the Earth's orbit around the sun, there's an "extra" full moon every two-and-a-half years. This lack of synchronization also means that the Harvest Moon can occur up to two weeks before or two weeks after the autumn equinox. Since the autumn equinox is September 22 or 23, the Harvest Moon is usually in September.

The moon follows an elliptical path around the Earth, causing some full moons to be closer to Earth than others. When measured from Earth, the closest full moons can be 14% bigger and slightly brighter than other full moons, according to NASA. This 14% difference pales in comparison to the perceived difference produced by the Moon Illusion.

EarthSky offers a comparison of some of the recent Harvest Moons: In 2021, it was an average-sized full moon. In 2020, it was the second-smallest full moon of the year. In 2019, it was the smallest full moon of the year. In 2015, it was a Super-Harvest Blood Moon, or the largest moon of the year combined with a total lunar eclipse.

With the great variety of ways that the Earth, moon and other celestial bodies travel through space, there is always something interesting to see.

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