The Harvest Moon will be full on Monday (Sept. 20) at 7:55 p.m. EDT (2355 GMT), according to NASA — two days after passing Jupiter and four days before making a close approach to Uranus in the night sky.
From New York City, the moon will be visible just above the eastern horizon when it reaches full moon phase and will be situated in the constellation Pisces. The moon rises in New York the evening of Sept. 20 at 7:13 p.m. and sets the next morning at 7:07 a.m. local time, according to Time and Date.
A full moon occurs when the moon is on the opposite side of the Earth from the sun. Technically, its celestial longitude is 180 degrees away from the sun in the sky. The moon's orbit is tilted about 5 degrees from the plane of the Earth's orbit, so though the moon is "behind" the Earth, it isn't in the Earth's shadow each time it circles our planet. On the occasions when it does pass through the shadow of the Earth we see a lunar eclipse, but that won't happen this time (the next eclipse is due on the night of Nov. 19).
Harvest Moon 2021 explained
The name of the full moon closest to the autumnal equinox, "Harvest Moon," of course reflects that September was the beginning of the harvest season in the Northern Hemisphere, especially in Europe.
Native people in North America had a number of different associations and names for the September lunation and full moon. According to the Ontario Native Literacy project, the Cree peoples called it the Rutting Moon, because elk in September start to rub the velvet off of their antlers ahead of the mating season, while the Ojibwe called it the Falling Leaves Moon. The Cherokee called it the Nut Moon, for when many trees start bearing them. In the Pacific Northwest the Tlingit called the lunation of September the Young Animals Moon, while the Haida called the month "Cedar Bark for Hats and Baskets."
In the Southern Hemisphere September is in the spring, with the days getting warmer, it's no surprise that the Māori of New Zealand described the lunar months in August to September as Mahuru: "The Earth has now acquired warmth, as well as vegetation and trees," according to the Encyclopedia of New Zealand.
For Abrahamic traditions the full moon of September is important, as for Jewish people it's the one that follows the High Holy Day of Yom Kippur (the latter is on Sept. 16) the day of atonement and the start of the Jewish lunar calendar year.
In China, the full moon of September will mark the mid-Autumn Festival, a three-day public holiday (from Sept. 19-21) when families traditionally gather; the festival celebrates the harvest.
Related: Full moon names (and more) for 2021
Harvesting by the light of the moon
Because this month's full moon is the one that arrives closest to the September equinox, we brand it the Harvest Moon. Usually, we associate the Harvest Moon with September, although that is not always the case. Sometimes, when the full moon occurs during the first week of September, we must wait until October for the Harvest Moon. Between 1970 and 2050, there are 18 years when the Harvest Moon comes in October. The last one was in in 2020.
On average, October Harvest Moons come at three-year intervals, although the time frame can be quite variable, and there can be situations where as much as eight years can elapse (the next such example will come between 2020 and 2028).
Many think that the Harvest Moon remains in the night sky longer than any of the other full moons we see during the year, but that is not so. What sets this month's full moon apart from the others is that farmers at the climax of the current harvest season can work late into the night by the moon's light.
So, for several days before and after the full moon, the moon hangs in the sky like a great, glowing lantern and prolongs the light far after sunset. It rises about the time the sun sets, but more importantly, at this time of year, instead of rising at its normal average of 50 minutes later each day, the moon seems to rise at nearly the same time each night.
Observers in mid-northern latitudes will see the moon relatively high in the sky compared to the summertime — from New York City it will be 44 degrees above the southern horizon at 1:05 a.m. Sept. 21, when it crosses the meridian, or reaches its highest point in the sky (think of it as the "local noon" for the moon). That's a full 10 degrees higher than in August. In the Northern Hemisphere, the sun is lower in the sky as the seasons move from summer to autumn, and the moon is correspondingly higher.
On Sept. 24 the moon makes a close pass to Uranus, which isn't usually visible to the naked eye unless one is in a dark-sky location and has keen eyesight. With a good pair of binoculars or even a small telescope it's possible to spot the planet, and in this case one can use the moon to help find it. The moon will be, at its closest, just over 1 degree to the right of Uranus (as seen in the Northern Hemisphere), or about two and a half lunar diameters. The moon will still be quite bright, so it may take a moment for one's eyes to adjust.
On the night of the full moon, the sun sets in New York at 6:56 p.m. and the sky gets fully dark by about 8:30 p.m. (This is just after the end of astronomical twilight, when the sun is 18 degrees below the horizon). At that point looking east (to the right) from the rising moon one will see first Jupiter and then Saturn, both in the constellation Capricornus. The two planets are brighter than the stars in Capricornus, and the moon further washes out this fainter constellation, so they will be obvious.
A little earlier in the evening on Sept. 20 one can still catch Venus in the southwest. It is closer to the horizon than in August, but one can still catch the planet about 10 degrees high about a half hour after sunset, according to the skywatching site Heavens Above. The planet sets at about 8:29 p.m. in New York City.
Mercury and Mars are both too close to the sun to be visible — Mars is moving "behind" the sun from our point of view, while Mercury is moving between the sun and Earth.
The constellations visible in late September are still largely those of summer; early in the evening the Summer Triangle asterism (or star pattern) is still visible near the zenith. Scorpius and Sagittarius will be in the southwest. As the night of Sept. 20 progresses one will see the summer stars set and the fall stars rise; by midnight Scorpius and Sagittarius have set and the Summer Triangle is above the southwestern horizon. Meanwhile high in the sky to the south is Pegasus, the winged horse, Andromeda and Cetus, the whale.
Editor's Note: If you take an amazing photo of the Harvest Moon that you'd like to share for a possible story or image gallery, send images and comments in to email@example.com. Space.com skywatching columnist Joe Rao contributed to this report.
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