Shine On, Harvest Moon!
One balmy evening this fall, my friends and I were sitting outside enjoying a bonfire, when the Harvest Moon became the topic of conversation. Everyone has heard about Harvest Moons, but no one had an answer when I asked, “What exactly is a Harvest Moon? Is it larger or brighter than a normal full moon?” — so I decided to check things out.
By most accounts, the official Harvest Moon is the full moon which appears closest to the autumn equinox. This year, the autumn equinox came on Sept. 22. The full moon in September appeared on the 5th and October’s was also on the 5th. The October moon was closer to the equinox, so it was dubbed the Harvest Moon for 2017. A Harvest Moon hadn’t appeared in October for close to a decade, so I hope you got to enjoy it.
Now for one of the other questions, “Is the Harvest Moon bigger than a normal full moon?” The answer to that is “not necessarily.” According to Deborah Byrd in an Oct. 5 post for “Astronomy Essentials” on earthsky.org, because of the elliptical path of the moon in September and October of this year, the full moons in those months weren’t really that close to the earth. I also read that the moon’s size in the sky only changes by a minuscule amount between lunar cycles anyways. So, if those moons looked bigger to you this year, then your eyes were playing tricks on you. It’s called the “moon illusion.”
When I googled “moon illusion,” I was surprised at what I found in an article written by Nadia Drake, a science journalist who writes the National Geographic blog “No Place Like Home.” When we see a huge moon on the horizon, which appears to shrink in size as it climbs in the night sky, the phenomenon is called “moon illusion” and has been perplexing scientists forever. Why it happens has been argued since the fourth century BC. Aristotle thought the Earth’s atmosphere could be responsible for enlarging the moon’s image, much like an object submerged in water appears bigger than it actually is. In the 11th century, Arab mathematician Ibn Al-Haythan came up with a theory that suggested the size difference we see is related to how our brain perceives distance and then adjusts an object’s size to match. Some modern psychologists feel this phenomenon is just a trick of your imagination – things are not always as they seem to be. Drake says, “The truth is, no one yet agrees about what’s going on.” Maybe it’s lunar trickery.
So, if the Harvest Moon isn’t really bigger than normal, other than in our minds, why do we think it might be brighter? Byrd says that “the orange color of a moon near the horizon is a true physical effect.” It has to do with the thickness of the atmosphere and the way it scatters blue light but allows red light pass through. A moon near the horizon takes on a hue which can be yellow, orange or red. Along with that fact, in September and October, the moon rises just shortly after sunset several nights in a row, leaving us with no long stretches of darkness. Because of this, we experience several “nearly” full moons close to the time we are looking for the Harvest Moon, and they all seem brighter. These illuminated nights used to give farmers more time to gather their crops, thus the name “Harvest Moon.”
Hmm, Science and Folk Lore — so intertwined and so fascinating! Can you imagine primitive people trying to make sense out of their world? Here we are, thousands of years later with tons of modern technology, and we are still trying to find answers that elude us. I think I’ll just sit back and enjoy the night, so shine on Harvest Moon! See you on the trails!
Susan M. Songster Weaver is retired teacher, nature lover and longtime CWC volunteer and supporter. The Chautauqua Watershed Conservancy is a local not-for-profit organization that is dedicated to preserving and enhancing the water quality, scenic beauty and ecological health of the lakes, streams, wetlands and watersheds of the Chautauqua region. For more information, call 664-2166 or visit www.chautauquawatershed.org or www.facebook.com/chautauquawatershed.