The Sky This Week, September 26-October 2
“The Sky This Week” appears regularly. It is written by Ian Clarke, Director of the Hatter Planetarium at Gettysburg College. The planetarium offers regular educational presentations about the stars and the skies; there’s something for early elementary through adults. We are accepting field trip requests for the 23-24 academic year. Fall schedule of public shows is available here! Illustration created with Stellarium.
The full moon occurs this week on Friday, Sept. 29. The actual moment of full moon is 5:57 a.m., which means the moon will be just a little more full on the night of 28-29 than it will on 29-30.
As the full moon closest to the autumnal equinox, this full moon is traditionally known as the Harvest Moon. It is also a “supermoon.” “Supermoon” is a fairly recent popular culture term for what’s more properly known as a “perigee full moon.” Our moon reaches its closest point to Earth (perigee) every 27.5 days, and it’s full every 29.5 days. When those two events occur close together, the moon appears a little bigger than average in the sky.
But how much bigger is a supermoon? Not all that much, to be frank. If you could somehow see an average and “super” moon in the sky at the same time, you could surely tell the difference (it’s just over 7%). But by itself? An average full moon is the same size in the sky as a quarter 110 inches away from you (really). Move that quarter 7.5 inches closer, and you have a supermoon moon. That’s it.
Further complicating matters is the “moon illusion,” a poorly understood phenomenon that causes all of us to overestimate the size of distant objects seen near the horizon. If you think this next full moon looks huge when it comes up in the early evening, check again in the middle of the night when it’s high in the sky. It’s all in good fun, though, and a great excuse to get out and look at our beautiful moon.
A few nights later, on October 1, you’ll be sure to notice the moon near the brilliant planet Jupiter. That night the moon will be a 90% illuminated waning gibbous. Jupiter will be just below it and to the right. They may look close together in the sky, but Jupiter is in the background at a distance of 617,000,000 km, while the moon is only 371,000 km away from us.
Ian Clarke is the director of the Hatter Planetarium at Gettysburg College. In addition he has taught introductory astronomy labs and first-year writing there for over 30 years (not necessarily all at the same time). He was educated at Biglerville High School, the University of Virginia, and the University of Iowa. He lives in Gettysburg.