Look up in the early morning hours of Wednesday (Sept. 6) to see the Full Corn Moon glowing in the sky. The moon will appear to make a close swing by the distant planet Neptune – and the farther south you are, the closer it will be.
Although the moon will look pretty full for about a day before and after its peak, it will officially be full at 3:02 a.m. EDT (0702 GMT) on Wednesday. Situated in Aquarius, the moon will rise Tuesday evening (Sept. 5) around 8 p.m. local time for viewers in the continental United States.
Less than an hour before moonrise, Neptune will appear over the horizon and will be visible with binoculars and small telescopes, but only the widest-field telescopes will catch both objects in the same frame. The duo will pass within 0.73 degrees of each other around 1 a.m. EDT (0500 GMT), and Neptune will be just to the northwest of the moon. You can find out exactly when and where Neptune and the moon will be visible at your location using this moon calculator and this visible planets chart by timeanddate.com.
For city-bound skywatchers plagued by light pollution, the conjunction should make finding Neptune much easier, though you still won't be able to see it without at least a good pair of binoculars, and preferably a small telescope. Neptune's maximum brightness is about magnitude 7.8, or less than half the brightness a keen-eyed observer can see in a dark-sky location with the unaided eye. [Photos of Neptune, The Mysterious Blue Planet]
Neptune's opposition and occultation
Neptune also happens to reach opposition on the evening of Sept. 5, which means that it will be directly opposite the sun in Earth's skies. This means that it will be the brightest it gets all year long.
Observers lucky enough to be in South America will have a chance to see the moon occult Neptune, meaning that it will pass in front of the planet. For example, in Montevideo, Uruguay, Neptune will disappear behind the moon at 2:02 a.m. local time, and reappear 50 minutes later. In Buenos Aires, the occultation starts about a minute later and Neptune reappears a little sooner at 2:39 a.m. local time. (The National Astronomical observatory of Japan offers a handy calculator for lunar occultations of planets here).
An unusual moon with many names
Traditionally, the full moon of September is called the Harvest Moon, but this year it will be the Corn Moon instead. The name Harvest Moon is given to the full moon that occurs closest to the autumnal equinox, which happens on Sept. 22. October's full moon, which occurs on Oct. 5, is closer to that date this year, and will therefore take the Harvest Moon name. Usually October's full moon is called the Hunter's Moon, but that particular lunar moniker will be skipped for 2017.
For the Native Americans who coined the full moon names hundreds of years ago, September marked the beginning of the harvest season in the Northern Hemisphere. In years when September's full moon fell at the beginning of the month (too early for the typical Harvest Moon), tribes called it the Corn Moon or the Barley Moon, after the two crops that would be ready to harvest at the time, according to the Farmer's Almanac.
Native peoples across 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. The Ojibwe, meanwhile, called it the Falling Leaves Moon to signal the start of autumn. The Cherokee called it the Nut Moon, for when many trees start bearing them. In the Pacific Northwest, the Tlingit called the lunar month in September the Young Animals Moon, while the Haida called it "Cedar Bark for Hats and Baskets."
Given that 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 of August to September as "Mahuru," meaning "The Earth has now acquired warmth, as well as vegetation and trees," according to the Encyclopedia of New Zealand.
In Abrahamic traditions, the full moon of September is important. For Jews, 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. For Muslims, the day of the full moon is Dhul Al Hijjah 15, when the Hajj – the pilgrimage to Mecca – traditionally is done.