The full moon of October 2020, called the Harvest Moon, will grace the skies this week on Thursday (Oct. 1).
That's the same day that the planet Mercury reaches its greatest eastern elongation, or the furthest distance east of the sun. That means on the night of the full moon Mercury will be visible after sunset, though still close to the horizon.
The moon becomes officially full on Oct.1 at 5:05 p.m. EDT (2108 UTC), according to NASA's SkyCal site. For New York City observers, the moon will rise on that day at 6:57 p.m. local time and set the next morning at 7:23 a.m. Rising about 20 minutes after sunset (which will happen at 6:37 p.m., per timeanddate.com) the moon will be in the constellation Cetus, the whale, a faint group of stars that will be mostly overwhelmed by the moon itself.
Given that it is autumn in the Northern Hemisphere, the moon will reach a maximal altitude of about 47 degrees at midnight in New York. As one moves south the moon's altitude will increase, and the reverse is true as one moves north. For Southern Hemisphere skywatchers in Melbourne, Australia, the full moon will occur at 7:05 a.m. local time on October 2, rising at 6:41 p.m., and reaching a maximum altitude of about 50 degrees.
Harvest Moon 2020 explained
The October full moon is often called the Hunter's Moon, according to the Old Farmer's Almanac, because that moon occurs when the season for hunting many game animals begins. This year, the Oct. 1 full moon is also the Harvest Moon of 2020, as it falls closer to the autumnal equinox on Sept. 22 than the September full moon on Sept. 2. September's moon was known as the full Corn moon.
Traditional names for the full moon often reflect local environment and history; according to the Ontario Native Literacy Coalition, the Ojibwe people called October's full moon the "Mskawji Giizis," or the Freezing Moon, because October is when the first frosts occur in their traditional territory in the Great Lakes region. The Cree people called it "Pimahamowipisim" (Migrating Moon), as in North America, many bird species start migrating south for the winter in mid-autumn.
In the Pacific Northwest, the Tlingit called the October full moon "Dís Tlein" (Big Moon), while the Haida called the moon "Kalk Kungaay," or the Ice Moon, according to the "Tlingit Moon and Tide Teaching Resource" published by the University of Alaska Fairbanks.
Traditional Chinese calendars call the eighth full moon as Guìyuè, or Osmanthus Month for the eponymous flower, marking the start of the Mid-Autumn Festival. The Festival is also called the "Mooncake Festival" for the food of the same name. One of the more important holidays in China, sometimes called "Chinese Thanksgiving," this year also happens to fall on China's National Day, which commemorates the founding of the People's Republic
Related: Full Moon Names (and More) for 2020
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. While the last one was in in 2019, the one prior was two years earlier in 2017.
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.
Mercury will reach greatest eastern elongation at 11:59 p.m., according to NASA, which will be well after it sets in New York, which will be at 7:20 p.m., according to heavens-above.com calculations. But the planet will still be about 8 degrees above the southwestern horizon at sunset, in the constellation Virgo. Finding it will still be tricky because from mid-northern latitudes it will be so close to the horizon.
The planet itself will shine magnitude 0.0, about as bright as Vega. The sun reaches about 6 degrees below the horizon – called nautical twilight because the brightest stars become visible, making it possible to navigate at sea – at about 7:04 p.m. in New York, and Mercury will be only 2 to 3 degrees in altitude. Seeing it will require some patience and the ability to pick out the planet against a still-light sky.
The situation for seeing Mercury improves as one moves south. In Miami, the sun sets at 7:06 p.m. local time, and Mercury sets at 8:10 p.m., and is at an altitude of about 13 degrees at sunset, and the planet will be at about 8 degrees when nautical twilight begins at 7:29 p.m.
Further south, as one gets to the Southern Hemisphere, Mercury is higher because the angle the plane of the Earth's orbit, called the ecliptic, makes with the horizon is steeper. The ecliptic also marks the yearly path of the sun across the sky, and in fall and winter, when the sun is low, that path is closer to the horizon, whereas in the summer it is higher (and the sun appears higher in the sky).
Southern Hemisphere seasons are reversed, so observers located there are entering the summer months. The result is that Mercury, from the point of view of a resident of Melbourne, reaches greatest elongation at 1:59 a.m. on Oct. 2, and at sunset on that day, which occurs at 6:25 p.m., the planet is a full 24 degrees above the northwestern horizon.
Venus will still be a "morning star" rising at 3:31 a.m. local time in New York on Oct. 1. Sunrise is not until 6:53 a.m. local time, and the planet will be 38 degrees in altitude by then. Venus is bright enough that it will stay visible even as the sky becomes light. Venus will be in the constellation Leo, making a close pair with the star Regulus which will be just below and to its left. Venus will be the brighter of the two, appearing bright white to Regulus' more yellowish hue.
On the night of the full moon, Mars will follow our natural satellite, rising at 7:26 p.m. local time, appearing to the left of the moon as they travel together across the sky. The moon moves approximately one of its own diameters per hour relative to the background stars, which means that over the course of the night the two will appear to approach each other very slowly. By the next night, Oct. 2, the two will be in conjunction, or sharing the same celestial longitude, at 11:25 p.m. local time. The two will be separated by 43 minutes of arc, or a bit more than a lunar diameter. Both will be in the constellation Pisces.
Jupiter and Saturn will both be in the constellation Sagittarius, and be in the western half of the sky by 8:00 p.m. Eastern Time. From New York City they will be respectively about 26 and 28 degrees in altitude. Saturn will be to Jupiter's left, with the "teapot" shape of Sagittarius to the right of Jupiter.
Looking directly up from the two giant planets observers will see Altair and then Vega, which form part of the Summer Triangle. In the Northern Hemisphere the Summer Triangle stars are fading from view; after midnight in October they are low in the western sky. Turning northward form Jupiter and Saturn one will see Arcturus, in the constellation Boötes, which in mid-northern latitudes will be in the northwest.
As the full moon rises in the east, one can trace a path "above" the moon and see the Great Square asterism which marks the constellations Pegasus and Andromeda. Both constellations are not especially bright, and one might have to let one's eyes adjust by not looking directly at the very bright moon for a few seconds. Andromeda contains the Andromeda Galaxy, M31, but with a full moon that's a more challenging object to see.
Some of the bright winter constellations start to become prominent in October. Orion and Taurus are above the horizon in the east by midnight.
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 firstname.lastname@example.org. Space.com skywatching columnist Joe Rao contributed to this report.
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