“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. We are accepting field trip requests for the 23-24 academic year. The fall schedule of public shows is available here!
The Sky This Week, September 12-18
This week brings us a new moon. That’s something that occurs every 29.5 days, but I thought I’d take a few moments to talk about this phase and its aftermath. You probably know it as the phase when you can’t see the moon because the three bodies are lined up EARTH →MOON→→ SUN. That’s true, though the three rarely line up exactly, or else we would have eclipses every month. This week the new moon occurs at 9:40 p.m. on September 14. (If your calendar lists it as September 15, that’s because it is using Universal Time, which is four hours ahead of EDT.) Once the moon has passed the new phase, it becomes a waxing crescent in the evening sky. Unlike the primary phases (New, First Quarter, Full Moon, Third Quarter), which occur on specific dates, the crescent and gibbous phases cover a range of dates. “waxing” just means getting bigger, and every phase of the moon between new and first quarter is a waxing crescent. A waxing crescent moon begins as a very slim sliver close to the sun, and as the days go by, it gets higher and thicker. The first few days of a waxing crescent moon are an excellent time to spot earthshine, that beautiful effect that allows you to faintly see the full disk of the moon. The bright crescent is lit by the sun directly, but the fainter part is illuminated by sunlight reflecting off the earth. That earthshine is always present, believe it or not, but as the sunlit portion grows and brightens, the eye can no longer detect the fainter portion.
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.