Sky ArchiveTonight

Harvest Moon on September 5-6?

Bettina Berg in Las Vegas contributed the image above of 2016’s Harvest Moon.

Tonight – September 5, 2017 – look for the full-looking moon to beam in the east at dusk. It’ll climb highest up for the night around midnight and sit low in west at dawn September 6. For us in the Americas, the moon turns full on this night. In other time zones, full moon falls closer to the night of September 6. Either way, some will call this September 2017 full moon the Northern Hemisphere’s Harvest Moon. Others will say the 2017 Harvest Moon comes in October. More about this issue later on in this post.

First let’s talk about the date and time of full moon. The moon turns full on September 6, 2017, at 7:03 UTC. Although the full moon happens at the same instant worldwide, the clock reads differently by time zone. Here, in the contiguous United States, the moon turns precisely full on September 6, at 3:03 a.m. EDT, 2:03 a.m. CDT, 1:03 a.m. MDT and 12:03 PDT. That’s why we say the full moon falls on the night of September 5, for the Americas.

If we include the states Alaska and Hawaii, the full moon really does happen on September 5. The time is 11:03 p.m. on September 5 in Alaska and 9:03 p.m. on September 5 in Hawaii.

But we’re talking technicalities here. Technically speaking, the moon is full at the instant that it’s 180o from the sun in ecliptic or celestial longitude. Realistically speaking, for a day or two around the exact time of full moon, the moon looks and acts full. That is, it stays more or less opposite the sun throughout the night – rising in the east at dusk, highest up around midnight and in the west at dawn – for all of us around the globe.

Worldwide map via the US Naval Observatory. Day and night sides of Earth at the instant of the full moon (2017 September 6 at 7:03 UTC). The shadow line passing to the left of Africa depicts sunrise and the shadow line to the right of Australia represents sunset.

Is the September 5-6 full moon the Harvest Moon? More often than not, the September full moon is the Northern Hemisphere’s Harvest Moon. The Harvest Moon is usually defined as the full moon closest to the autumn equinox, which – in the Northern Hemisphere – comes on or near September 22 each year.

Last year’s Harvest Moon fell in September. Next year’s Harvest Moon will, too.

But, in 2017, the September 6 full moon comes too early to be the Northern Hemisphere’s official Harvest Moon, according to the most widely accepted definition of the term. That’s because the full moon of October 5, 2017, will fall closer to this year’s September 22 equinox. The October 2017 full moon will be this year’s Harvest Moon, while the September 5-6 full moon will carry its ordinary monthly full moon name of Corn Moon or Fruit Moon in the Northern Hemisphere (and Worm Moon, Lenten Moon, Crow Moon, Sugar Moon, Chaste Moon or Sap Moon in the Southern Hemisphere). Read more about full moon names.

However, in most respects, the September 2017 and October 2017 full moons can be regarded as Harvest Moon co-stars. By that we mean that both have the characteristics of a Harvest Moon. The primary Harvest Moon characteristic has to do with the moonrise. On the average, the moon rises some 50 minutes later with each passing day. Around the time of the full Harvest Moon, the lag time between successive moonrises is reduced to a yearly low.

In 2017, there’s no appreciable difference between the lag in moonrise times associated with September and October full moons. In both of these months, the moon rises a shorter-than-usual time after sunset for several evenings in a row, following the date of full moon.

As the sun sets at and near the autumn equinox, the angle of the ecliptic – or sun and moon’s path – makes a narrow angle with the horizon. Image via
The narrow angle of the ecliptic means the moon rises noticeably farther north (left) on the horizon. For a few to several days after the full Harvest Moon, there is no long period of darkness between sunset and moonrise. Image via

For instance, at and near 40o north latitude (the latitude of Denver, CO and Philadelphia, PA), the moon will rise about 35 (not 50) minutes later for the next several days after September 5. That’s virtually the same lag time that accompanies the October 2017 full Harvest Moon.

Take another example. Farther north, at Fairbanks, Alaska (65o north latitude), the moon will rise about 10 (not 50) minutes later for the next several days after tonight (September 5). Again, that’s essentially equal to the lag time accompanying the October 2017 full Harvest Moon.

Click here to find out the moon’s rising time, remembering to check the moonrise and moonset box.

In any year when the October full moon is the Harvest Moon, the September full moon displays the characteristics of a Harvest Moon, too.

So this year we can enjoy the double feature, whereby two Harvest Moons extend the daylight hours in the season of diminishing daylight.

Bottom line: Tonight- on September 5, 2017 – watch for the full moon to light up the nighttime from dusk till dawn.

September 5, 2017
Sky Archive

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