What, No Harvest Moon This Month?

The Harvest Moon rises with Earth’s shadow, the gray band, over eastern Duluth, Minn. on September 29th, 2012. Click here to get your local moonrise time. Credit: Bob King

Tonight’s the Full Corn Moon, so if you like corn on the cob, salute the rising moon with a buttered ear this evening. We usually know September’s full moon as the Harvest Moon but not this time around. By definition, the Harvest Moon is the full moon that falls closest to the autumnal equinox. I see that fall begins 18 days from now at 3:02 p.m. (Central time) on September 22nd. The next full moon happens on October 5th or just 13 days after the equinox, so it wins and becomes 2017’s Harvest Moon.

Either way, both moons will rise not very long after sunset for several nights in a row.  As the moon orbits the Earth, it moves eastward about one fist held at arm’s length each night and rises about 50 minutes later. You can see its orbital travels for yourself by comparing the moon’s nightly position to a bright star or constellation.

The Full Corn moon rises around sunset in Aquarius tonight (Sept. 5). Many calendars will show the full moon date as September 6, but since the moment of full moon happens at 2 a.m. CDT tomorrow, the moon will appear fuller for observers across the Americas tonight compared to tomorrow night, when it will be some 17 hours past full. Stellarium

As you’re already familiar, a full moon rises around sunset and about an hour later each night after that. But around the time of the Harvest Moon, successive moonrises are only about 30 minutes apart. The extra light afforded by early-rising moon helped farmers harvest crops into the night before electric lighting.

50 minutes is the usual gap between moonrises. But it can vary from 25 minutes to more than an hour depending upon the angle the moon’s path makes to the eastern horizon at rise time. In September around the time of full moon that path runs above the horizon at a shallow angle (see below). As the moon scoots eastward, it also moves northward this time of year.

The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 20-30 minutes apart. Times are shown for illustration purposes and are not the times of moonrise tonight.  Illustration: Bob King

This northward motion isn’t as obvious unless you watch the moon in the coming nights. Then you’ll see it climb to higher and higher or further north each night until reaching its highest point on the 14th. The moon’s northward motion partly cancels out the eastward motion, keeping the September full moon’s path roughly parallel to the horizon with successive rise times only about 30 minutes apart.

Exactly the opposite happened 6 months earlier this spring, when the moon’s path met the horizon at a steep angle. While it traveled the identical distance each night then as now, its tilted path took it much farther below the horizon night to night. The spring full moon moves east and south towards its lowest point in the sky. Seen from the northern hemisphere, that southward travel adds in extra time for the moon to reach the horizon and rise each successive night.

So do something old-fashioned and corny this week, take a long moonlit walk with your beau.