“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 2022-23 is now live. The spring 2023 schedule of free shows is available here. Images created with Stellarium.
We have almost reached the first day of spring! The vernal equinox this year occurs at 5:24 p.m. on Monday, March 20. That’s the moment the sun crosses the celestial equator, the imaginary line in our sky that divides the sky into northern and southern halves. The farther north in the sky the sun is, the higher it gets at noon and the longer our days are. We see the sun moving from south to north on the celestial sphere due to the 23.4-degree tilt of the earth’s axis with respect to its orbit around the sun. Imagine for a moment that this tilt did not exist. For an observer in one location, the sun would take the same path across the sky every day, and we would not have appreciable seasons. The weather would simply get colder the farther you were from the equator. In order to recreate the feeling of seasons, you would need to travel in a yearly loop covering 46.8 degrees of latitude. That’s about 3,200 miles! Think of the seasons as free travel.
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.