Fall sneaks in at 2:50 a.m. (Central Time) on Monday September 23. We’ll literally wake up to a new season that morning. But before that happens, watch for the Harvest Moon this Friday night September 13. The big yellow moon will rise around sunset and shine all night. The Harvest Moon is the full moon that occurs closest to the autumnal equinox or first day of fall. Often that happens in September, but occasionally in early October.
What makes the Harvest Moon special is that it rises at nearly the same time several nights in a row. Normally, the moon rises about 50 minutes later and moves about a fist east each night, but during the Harvest Moon successive moonrises are only about 20-25 minutes apart! This can make it seem like we’re getting one full moon after another.
There’s a reason for this of course. In September the path the moon takes across the sky meets the horizon at a shallow angle around the time of full moon. As the Moon slides east each night, it also moves steadily northward this time of year. The northward motion partly cancels out the eastward motion, making the path of September’s full Moon very shallow (closer to parallel) with respect to the eastern horizon. Later this month, when the Moon shrinks to a thin crescent in the morning sky, its path will cut a much steeper angle to the horizon. Although it still moves the same distance to the east (left in the northern hemisphere) each night, its path cuts deeply south, with rising times separated by up to 75 minutes.
The key to understanding the diagram is to remember that the farther below the horizon the moon is, the more time Earth has to rotate to bring it to the rising position. It rotates only a little to bring the Harvest Moon to moonrise, but in spring, the planet must spin for a much longer time to bring the full moon into view.
Those successive early moonrises of September and sometimes October were a boon to farmers in the time before electric lights. For most of humanity’s farming past, our ancestors had to rely on whatever natural light they could get to complete outdoor tasks like the fall harvest. Several bright “harvest” moons in a row allowed farmers to get an early jump on harvesting during the evening hours. As long as the sky was clear I guess!
The Harvest moon effect becomes more pronounced the further north you live and less so for those who live closer to the equator. In the far north, the moon’s path is almost exactly parallel with the eastern horizon for a few nights. In Nome, Alaska, successive moonrises are only 5-7 minutes apart.
However you like your full moons, you’ll probably want to know when the Harvest Moon rises for your town. And to check to see how those rise times change night to night. Click the Moonrise Moonset Calculator to find out.
**** Is there really a lunar base on the far side of the moon? You can find out in my new book titled Urban Legends from Space from Amazon, Barnes & Noble and Indiebound currently available for pre-sale at a nice discount.