The Sky This Week, September 19-25
The Sky This Week” 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! Illustration created with Stellarium.
The autumnal equinox, the first day of fall in the northern hemisphere, arrives at 2:50 a.m. on September 23 this year. That is the moment the sun crosses an imaginary line in our sky (the celestial equator) from the northern half of the sky to the southern half. Each day from then until December 21, the sun will be lower in the sky at noon, and the days will be getting shorter.
Let’s have a look at the predawn sky this week. Mercury reaches its greatest elongation on September 22, which means it’s as far from the sun in the sky as it can get. That’s still not very far, so can only view Mercury shortly before sunrise. This illustration shows the eastern sky this week about a half hour before the sun comes up. Brilliant Venus will be the first thing you spot; then scan for fainter Mercury below it. Binoculars will help. Nearby you’ll find the stars Regulus, Procyon, and Sirius. The appearance of Sirius in the predawn sky was historically a signal of the end of the “dog days” of summer.
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.