The Corn Moon will be full on the afternoon of Wednesday, Sept. 2, at 1:22 p.m. EDT (1722 GMT), four days before the moon occults Mars, according to NASA's SkyCal site.

In New York City, the moon will be situated in the constellation Aquarius, rising the evening of Sept. 2 at 8:06 p.m. local time. The moon sets the next morning at 7:32 a.m. While observers in North America will miss the actual moment the moon becomes officially full, the difference won't be noticeable later that evening.

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 some five degrees from the plane of the Earth's orbit, so even though the moon is "behind" the Earth, it isn't in the Earth's shadow each time it makes a circuit of 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 lunar eclipse is due on the night of Nov. 29-30).

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See Mars, Jupiter and Saturn on the night of the Full Corn Moon. This sky map shows where these naked-eye planets (and Uranus) will be located at 11 p.m. local time as seen from New York City. (Image credit: SkySafari app

A few days after the full moon in the wee hours of Sunday, Sept. 6, the moon will occult, or pass in front of Mars for observers in South America and western Africa. Other parts of the world will see the moon make a very close pass to Mars — so close in some cases that the two will fit in the same field of view of a pair of binoculars.

For skywatchers in Rio de Janeiro, the occultation begins at 3:15 a.m. local time. Mars and the moon will both be about 44 degrees above the northeastern horizon. The occultation ends some 20 minutes later, at 12:35 a.m. Meanwhile, in La Paz, Bolivia, the occultation begins at 10:39 p.m. local time on Sept. 5 and ends at 11:40 p.m.

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The difference in duration is partly due to one's latitude on the Earth; it's also why the occultation becomes a close pass when one gets farther north or south of a certain region — the position of the moon relative to the background stars and Mars changes slightly with one's latitude. The phenomenon is called a parallax, and you can demonstrate it by the old trick of holding an arm outstretched and sticking up a thumb, then seeing how it seems to "jump" if you observe with first the left eye open and the right closed, and then closing the left eye and opening the right. A complete list of occultation times is available from the International Occultation Timing Organization, which lists locations by country and the times in Universal Time.

For observers in New York City, the moon and Mars make a close pass — the two will be within 1 minute of arc, or one-thirtieth of a lunar diameter. Observers in New York will see this occur at 12:44 a.m. local time, according to skywatching site In-The-Sky.org. The moon will still be almost full and Mars will be 38 degrees above the east-southeastern horizon, but the moon will be close enough to it that finding the two will be easy.

The constellations visible at the start of September are still largely those of Summer; early in the evening the Summer Triangle is still visible near the zenith. Scorpio and Sagittarius will be in the south, and Jupiter and Saturn are both in the latter constellation. As the night of Sept. 2 progresses one will see the summer stars set and the fall stars rise; by midnight Pegasus and Andromeda will both be above the eastern horizon.

The Corn Moon

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. The full moon of September this year wil be the Corn Moon, because the the following full moon on Oct. 1 (which would ordinarily carry the moniker "Hunter's Moon") is closer to the equinox.

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 Ojibwe (Anishinaabe) peoples called the ninth full moon of the year the Corn Moon as well, reflecting the harvest season. 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. 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 (measured from new moon to new moon) as Mahuru: "The Earth has now acquired warmth, as well as vegetation and trees," according to the Encyclopedia of New Zealand. September’s full moon will fall right in the middle of that).

For Abrahamic traditions the full moon of September is important, as for Jewish people it's the one that leads up to the High Holy Days of Rosh Hashanah, which marks the end of the Jewish lunar calendar year (the actual holiday is at the new moon on Sept. 18).

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