Last week in Starwatch, I told you how all of the stars in our sky - including our closest star, the sun - appear to revolve around the North Star Polaris every day. I like to think of Polaris as the "Lynch Pin" of the sky. Alright, alright, I know that's a bad pun, but I couldn't resist. In our sky, Polaris is perched 24/7 halfway between the northern horizon and the overhead zenith. While the Earth rotates on its axis once every 24 hours, it appears to us that the stars are whirling around the North Star in that same period.
That's not the only movement of the stars in our sky. While our Earth orbits the sun every year, the nighttime side of the Earth faces different directions toward space. Because of this, we see different constellations from season to season.
Again, every 24 hours all of the stars appear to circle Polaris because of Earth's rotation on its axis, but at the same time most of the stars also shift a little to the west from night to night while the Earth journeys around the sun. Specifically, they shift one degree west, or about the width of two full moons to the west, every 24 hours. Over the course of a few weeks, you would really start to notice that westward shift of the constellations.
Over the course of a month or two, if you were to observe at the same time every night, you would start seeing new constellations emerge from the eastern horizon and constellations you've been watching over the weeks starting to disappear while they slip below the western horizon earlier and earlier. I love this arrangement because the night sky is constantly freshening up every hour and every season. It's not the same old constellations night after night.
Because of Earth's orbit around the sun combined with its rotation on its axis, the old adage "the early bird gets the worm" comes into play. The early bird also gets a head start when it comes to stargazing. The constellations visible in the pre-morning twilight sky at any given time are the same ones seen in the evening about four months later. At both times, you're facing the same direction of space.
For example, right now in the morning twilight, we see the same arrangement of constellations that we will see in July right after evening twilight. You can get a jump on evening summer stargazing by setting the alarm and getting out under the heavens.
Celestial huggings
Saturday and Sunday night, the new crescent moon will be hanging around Jupiter. Saturday, the thin crescent moon will hanging to the lower right of Jupiter, and on Sunday night, it'll be just to the left of Jupiter. Both objects are wonderful for a small telescope or even a decent pair of binoculars. Check out the craters, mountains and dark volcanic plains of our moon. If your scope has enough power and resolution, you may even see some of the horizontal cloud bands of Jupiter as well as its moons.
(Lynch is an amateur astronomer and author of the book, "Stars, a Month by Month Tour of the Constellations." Contact him at mikewlynch@comcast.net.)