The full moon of September, called the Harvest Moon, will grace the skies Sept. 29, just two days after it reaches perigee, its closest point in its orbit to Earth.
The moon becomes officially full for New York City observers on Sept. 29 at 4:47 a.m. Eastern Daylight Time (0957 UT), according to the U.S. Naval Observatory. As it is a bit closer than usual it is also a "supermoon" – a full moon that will appear slightly larger than normal.
A full moon is when the Moon is exactly on the opposite side of the Earth from the Sun. The time between full moons is about 29.5 days, though the time for the moon to complete one orbit is a bit shorter, 27.3 days; the former is called the synodic period and the latter is called the sidereal period. The difference is because after the moon completes a circuit around the Earth, the Earth itself has moved in its orbit around the sun, so to reach a point exactly opposite the sun the moon has to move a bit further.
The timing of lunar phases depends on where the moon is relative to the Earth rather than where one is located on the planet, so the hour of the full moon depends entirely on one's time zone. While the full moon occurs in the predawn hours in Eastern North America and central South America, for example, if one goes to Europe it happens in the morning (9:57 a.m. in London, 10:57 a.m. in Paris). In Asia and Oceania the full moon happens in the evening – 6:57 p.m. local time in Tokyo and 7:57 p.m. local time in Melbourne, Australia.
A supermoon isn't a true astronomical term; it just refers to when the moon is at its closest point to Earth within a couple of days of being full, in this case the moon reaches perigee on Sept. 27 at 8:58 p.m. Eastern Time. Supermoons happen because the lunar orbit is not a perfect circle; it is an ellipse, though one that is only very slightly so. At its closest point, called perigee, the moon is 226,000 miles (363,300 kilometers), and at its farthest it is 253,000 miles (405,000 kilometers). The difference isn't all that much, a typical "supermoon" will appear about 10 to 11 percent larger than an ordinary full moon; but that difference is almost impossible to see unless one is very observant and tracking full moons for many months.
While the timing of lunar phases depends on the position of the Moon, the rising and setting times of the sun and Moon do vary with location. On the night of the full moon, for example, moonrise is at 6:57 p.m. Eastern Time in Miami, a bit later than in New York. Further south, in San Juan, Puerto Rico, moonrise is at 5:58 p.m. Atlantic Standard Time (Puerto Rico doesn't use daylight savings).
Moonrise times are also affected by how far east or west one is in a given time zone; New York City and Pittsburgh are both on Eastern Daylight Time, and less than a degree apart in latitude, but moonrise in Pittsburgh is 7:00 p.m., some 25 minutes after New York. This is an artefact of the Eastern Time Zone extending as far as it does in the east-west direction.
In some countries this produces odd results; China, which is as large as the U.S., is entirely in one time zone, which means moonrise in Beijing is at 6:05 p.m. on Sept. 29, with sunset at 6:01 p.m., which seems perfectly ordinary in a city in the mid-northern latitudes in September. But in Urumqi, in the western part of the country moonrise is at 8:02 p.m. and sunset is at 7:55 p.m. – an hour more suited to midsummer.
On the evening of Sept. 28, at about 8 p.m., the moon will be low in the east (from New York City). Saturn will be above and to the right; about 25 degrees high in the southeast. The moon washes out a lot of fainter stars, and Saturn is in the constellation Aquarius, which is relatively faint; that makes it easy to see even from brightly lit urban areas (if you are looking up from a place with the streetlights on, Saturn will be one of the few "stars" you can see). Saturn sets at 4:00 a.m. Eastern in New York City on Sept. 29.
Jupiter rises at 8:20 p.m. local time in New York City, and by 10 p.m. it is 18 degrees above the eastern horizon, to the left of the moon, which will be 34 degrees high in the southeast. Like Saturn, Jupiter is in a fainter constellation, Aires, the Ram, and as such will be easier to spot, especially with the moon brightening the sky. The planet transits (reaches its highest altitude) at about 3:15 a.m. on Sept. 29, when it will be 64 degrees high in the south.
At about the time Jupiter transits, at 3:17 a.m. in New York, Venus is rising, just north of east, and by 4 a.m. (Sept. 29) it is 7 degrees high, just enough that one can spot it above low buildings. Venus is bright enough that it is often visible until just before sunrise (which is at 6:50 a.m.).
Finally Mercury, the innermost planet, rises at 5:31 a.m. EDT, and will be about 5 degrees above the horizon by 6:00 a.m. The planet is always a challenge to observe as it is so close to the sun – be careful when trying to see it. Over the next few days it will appear closer and closer to the sun until it is lost in the solar glare; after that the planet emerges in October and by early December reaches its highest point in the evening sky.
For those in the Southern Hemisphere, the planets will rise in the same order, but they appear towards the northern half of the sky. In Melbourne, Australia, where the full moon is on Sept. 29 in the evening, the sun sets at 6:22 p.m. Australian Eastern Standard Time. By 7 p.m. the sky is dark enough that one can see Saturn, which will be almost directly above the moon and to the left, high (about 41 degrees) in the east-southeast. The planet transits at 10:05 p.m. local time, when it hits an altitude of about 64 degrees, and sets at 4:50 a.m. Sept. 30. As the night progresses Saturn will appear to move from above the moon to the left and then below it, but this is just because of the two bodies' orientation against the sky and the changes in the angle one views it from.
Jupiter will rise at 9:23 p.m. in Melbourne, and transits at 2:37 a.m. Sept. 30., reaching an altitude of 37 degrees above the northern horizon. The planet sets at 7:56 a.m. on Sept. 30, well after sunrise (which is at 5:58 a.m.) Venus rises at 3:55 a.m. local time, and will stay visible until just before sunrise. In mid-southern latitudes Mercury will be harder to see than in the Northern Hemisphere; at the latitude of Melbourne it rises at 5:30 a.m. local time and is very nearly lost in the glare before sunrise, when its altitude is only 5 degrees.
The full moon tends to overwhelm the fainter stars. But in the late September sky there are still constellations that can be seen despite the brightness of both the moon and city lights. For example, as the moon gets higher the brighter winter constellations such as Taurus and Orion appear in the eastern sky; by midnight those two will have risen in the east and cleared most buildings.
In the Southern Hemisphere, looking towards the southwest at about 10 p.m. local time, one can see Centaurus and the the Southern Cross; if one looks to the southeast one will see Eridanus the River, winding upwards until it reaches the star Achernar. From mid-southern latitudes low in the southeast one can also see Canopus, the brightest star in Carina, the Keel.
The September full moon is often called the Harvest moon, according to the Old Farmer's Almanac, because that was when the corn harvests traditionally started. Sometimes the September moon is further away from the autumn equinox than the October full moon, and in those situations it is called the Corn Moon, as happened in 2017. This time, though, the September full moon is closer to the equinox.
Besides being a marker for North American harvests, the full moon of September marks the Jewish holiday of Sukkot, also called the feast of Tabernacles. Observant Jews will construct a sukkah, a small structure with a roof of natural material, and spend time inside it. The holiday commemorates the time in the wilderness after the exodus from Egypt described in the Torah. For some Muslims, the 12th day of the third month of the calendar – which falls on Sept. 27, two days before the full moon – is Mawlid al-Nabi, the birthday of the prophet Muhammad.
While European settlers adopted some Native imagery for full moon names, the various peoples in the Americas gave the full moons names that reflected their local environment. The Anishinaabe (or Ojibwe) who lived near the Great Lakes, referred to the ninth moon of the year as the Falling Leaves Moon, which reflects that September to October tends to be when the autumn leaves start to come off the trees.
The Cree of Ontario called the September full moon Rutting Moon because it was when moose would start scraping the velvet from their antlers ahead of the mating season. In the Pacific Northwest, the Haida called it the Cedar Bark moon (Kíit'aas Kungáay) as they use the bark in weaving.
In China September in the western calendar is in the seventh and eighth lunar month. That eighth month is called Guìyuè, or Osmanthus Month, for when the eponymous flowers bloom, and the full moon occurs on the 15th day, halfway through. The day of the full moon in the eighth month is called Mid-Autumn Day, and is an important festival in China; there are similar holidays in other east and southeast Asian countries. In China one part of the celebration is making mooncakes, as well as lighting "sky lanterns" – essentially small paper hot air balloons.
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