Harvest Moon: the full moon nearest the September equinox

How the Harvest Moon is defined

Unlike most full moon names, “Harvest Moon” follows an astronomical rule. The Old Farmer’s Almanac and NASA both define it as the full moon nearest in time to the September equinox, the start of autumn in the Northern Hemisphere. The Farmers’ Almanac, a separate publication based in Maine, stresses that the date is set by the equinox and not by the month. In most years the Harvest Moon falls in September. The Old Farmer’s Almanac says that about every three years it falls in October instead, and September’s full moon is then called the Corn Moon. The dates above follow that rule.

Where the name comes from

The name comes from English usage. NASA’s former full moon guide cites the Oxford English Dictionary for the first known written use of “Harvest Moon” in English, in 1706, and EarthSky suggests the term may be European in origin. The idea behind the name is practical. Before electric light, farmers bringing in crops could keep working after sunset by the light of a bright Moon that rose early in the evening, night after night.

NASA’s guide also gives the European names Fruit Moon and Barley Moon for this time of year. It adds that the Maine Farmers’ Almanac, in the 1930s, attributed the name Corn Moon to the Algonquin tribes of the Northeast. The Old Farmer’s Almanac lists “Harvest Moon” as a Dakota name for August’s full moon. That is a different use of the same English phrase from the equinox rule. For the other autumn names attributed to specific nations, see the Corn Moon page and the full moon names overview.

Why the Harvest Moon rises early night after night

The Moon moves eastward around the sky by about its own width every hour, or roughly 12 to 13 degrees a day. Each night, Earth has to turn a bit further before the Moon comes up. NASA’s Skywatching FAQ puts the average delay at about 50 minutes a day.

That figure is only an average. How long the delay is depends on the angle at which the Moon’s path meets the eastern horizon when it rises. The Moon travels close to the ecliptic, the Sun’s yearly path among the stars.

• When the path climbs steeply out of the horizon, each day’s eastward step carries the Moon well below the horizon. Earth needs a long extra turn to bring it up, and moonrise comes much later the next night.
• When the path lies at a shallow angle, much of each day’s step runs sideways along the horizon instead of down below it. EarthSky describes the Moon as rising noticeably farther north along the horizon each evening. Then only a small extra turn is needed.

Around the September equinox, the full Moon sits in the part of the sky where its path meets the evening horizon at the shallowest angle of the year for northern observers. NASA’s Harvest Moon guide gives the result. Around this full moon, the delay shrinks to about 25 minutes a night at the latitude of Washington, DC, and to just 10 to 20 minutes farther north, in Canada and Europe. The Moon looks full for about three days around the exact instant. So for several evenings in a row, a bright, nearly full Moon comes up in the east soon after sunset.

The effect changes from year to year. EarthSky explains that the tilt of the Moon’s orbit adds to or subtracts from it over an 18.6-year cycle. In “major lunar standstill” years, such as 2006 and 2025, the Moon swings farther north and south and the Harvest Moon effect is strongest. Around minor standstills, such as 2015, it is weaker.

How to watch the Harvest Moon

Plan for two or three evenings, not just one. The table above lists moonrise on the best evening for major cities. On the next evening or two, the Moon will rise only a little later and a little farther north. Pick a place with a clear, low eastern horizon, and be there at sunset.

The rising Harvest Moon is famous for looking huge and orange. The orange is real: near the horizon, its light passes through much more air, which scatters away blue light. The size is the Moon illusion. NASA notes that photos show the Moon is the same width on the horizon as overhead, and no single explanation of the illusion is fully settled. To test it, compare the Moon with your index fingernail at arm’s length at moonrise and again two hours later.

Check our city sky pages for your local moonrise times on the evenings after the date shown.