Last night, students in my astronomy class and I watched Sirius, the sky’s brightest star, twinkle low in the southwestern sky at dusk. As the April sun spreads its warmth, winter stars flee to the west. Spring stars and planets pushing up from the east seem eager to replace them. Right now, you can watch a delicate balancing act at nightfall involving Jupiter and Sirius. Both stand about 6-7° high around 9:55 p.m. local time: Jupiter in the southwest and Sirius in the southeast. Before that, Jupiter occupies the lower end of the seesaw, and after it stands higher.
You can use the opportunity to watch the planet gain on the star as well as experience the slow westward shift of the stars caused by Earth orbiting around the sun. Because the planet moves, each night, stars rise in the east 4 minutes earlier, while those in the west set 4 minutes earlier. Four minutes isn’t much, but it accumulates. After a week, Jupiter rises almost a half-hour earlier. In two weeks, it’s an hour and in a month, 2 hours. Here’s the fun part: in a 12 months it rises 24 hours earlier — in other words, it’s back in the same spot in the eastern sky. A full circle around the sun, a full cycle of rising, setting and rising again.
OK, I told a white lie there. Stars return to the same position in the sky after a complete revolution of the Earth about the sun, but planets don’t. They travel around the sun just like the Earth. From the ground, we see them move east or to the left across the sky when facing south. Next year on April 25th, Jupiter will have vacated its spot in the constellation Libra and moved east through Scorpius into Ophiuchus. Instead of rising around 10 as it does now, the giant planet will clear the horizon at midnight, well behind Sirius.
These little things we notice about the sky help us better understand the sometimes confusing aspects of celestial and planetary motions. Once you’ve tracked a planet for one year, you’ll always be able to recognize and hold onto it.