“The Sky This Week” appears every Tuesday. 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. Field trip requests are welcome. NOTE: field trip request form for Fall 2022 is now live, and the schedule of free public shows has been posted.
This week brings us a full moon on September 10. On its journey toward fullness the moon moves eastward about a fist-width held at arm’s length each night. On September 7 and 8 the waxing moon will be passing by the planet Saturn and on the 10th and 11th it will be near Jupiter.
Since the moon will be dominating the night sky, here are a few facts about the moon’s appearance to ponder. You may know that the moon always keeps the same side toward the earth. Astronomers say it’s “tidally locked,” and the same is true of many other planetary satellites. However, due to subtle shifts in the earth’s perspective (both east-west and north-south) we actually get to see 59% of the moon’s surface over time.
Next fact: the “supermoon” isn’t really that super. The moon’s orbit is elliptical; at its closest the moon is 225,700 miles from the earth and at its farthest it is 251,900 miles. A “supermoon” (more properly called a perigee full moon) is when the full moon happens near that closest point. For some perspective, however, note that an average full moon is the same apparent size as a quarter held 103 inches away from you. If the quarter were 98 inches away instead, that would be a “supermoon.”
When looking for a big moon, the “moon illusion” may make a bigger impression. It is a poorly understood optical illusion that causes us to overestimate the size of sky objects near the horizon.
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.