September will be a wonderful month to spot the two largest planets of our solar system. Back at the Winter Solstice Great Conjunction, so many of us were wowed to see Jupiter and Saturn just a 10th of a degree apart (only a quarter of the width of the moon). Jupiter orbits the sun at more than twice the speed of Saturn, so the distance between them has now grown to 17 degrees. If you hold out your arm and flash the “rock on” sign, that’s the distance between your pinkie and pointer. And they are easy to spot. They will be the first lights you see in the southeast as the sky darkens. Jupiter is about 15 times brighter, so you’ll see it first. Then Saturn will emerge, just one “rock on” to the right of Jupiter. Both will be about a third of the way between horizon and zenith in the evening sky.
The two gas giants were at their closest to Earth during their oppositions last month, so they are still at their brightest and visible for most of the night. But close is relative for planet watchers. Jupiter is 375 million miles away, and it takes the sunlight reflected by its clouds 33 minutes to reach us. Saturn is over twice as far at 842 million miles or 75 light-minutes. If you are able to view through a small telescope, you can be amazed by the four largest moons of Jupiter and the rings of Saturn. As you watch them throughout the month, be sure how to notice that they are rising two hours earlier and appear more in the south as the sky darkens. The waxing gibbous moon makes it even easier to spot both worlds. It will be below Saturn on Sept. 16 and below Jupiter on Sept. 17. If you’re looking up on a moonless night, watch for our Milky Way in the south to the right of Jupiter and Saturn and then streaming high overhead in the dark skies of the Driftless.
You may want to include Jupiter and Saturn in your Autumnal Equinox celebration. We pass from summer into fall the afternoon of Sept. 22 at 2:21 p.m. The first sunset of the new season will be due west at 7 p.m. The Full Harvest Moon was two days earlier, so the waning gibbous moon will rise that evening a little after 8 p.m. Don’t miss brilliant Venus in the southwest before it sets at 8:30. Then look southeast to spot Jupiter and Saturn. And enjoy the view of other worlds as ours heads into autumn.
John Heasley is an astronomy educator and stargazer who enjoys connecting people with the cosmos. He volunteers with NASA/JPL as a Solar System Ambassador, with the IAU as a Dark Skies Ambassador, and with International Dark-Sky Association as an Advocate. For more information about stargazing in southwestern Wisconsin, like Driftless Stargazing LLC on Facebook and find out whenever there’s something awesome happening in the skies.