by Scott Levine
A few months ago, we talked about using the sun and moon, along with some simple observation tricks, to find the out solar system’s planets in the night sky. With just a little bit of practice, it’s easy to pick them out from the much more distant stars and paint a picture of what’s going on closer to home. Since then, Jupiter and Saturn have guided us through this difficult year from hundreds of millions of miles away.
As the months have gone on, you might have noticed the two giant planets dancing across the southern sky. They’ve risen a little earlier each night, even have started to creep closer and closer to the western dusk by the time the sun sets.
Everything in space is moving all the time, and as we’ve watched spring become summer, and then autumn, we’ve seen this movement as the subtle changes in the planets’ positions relative to each other and to the stars far behind them. The same way a little bit of water, given enough time, can carve canyons out of stone, little by little the the tiny changes in our point of view on those planets’ orbits have added up, too. With December just about here and autumn starting to feel more and more like winter, we’re in for a great show as the planets’ story for this visit to the night sky comes to an end.
When darkness comes this week, head out and look southwest. Distances across the sky are often measured degrees, as distances around a circle are. On November 24, the pair are separated by about 2 ½ degrees. That’s close enough that you’ll be able to blot them both out with just two fingers held out at the end of you arm. You can pick whichever of your fingers and arms you like.
Together, let’s keep an eye on this as the pair lurch toward the southwest horizon and the last of the leaves fall to our feet. By December 1st, that gap will close to less than two degrees, while they sink deeper and deeper toward the dusk. Keep watching and keep measuring over the next few weeks. When will the two fingers shrink to just one?
On December 16 and 17, the scene will be truly stunning as a thin waxing crescent moon races through the picture. Those nights, the planets will be only about half a degree apart. Then, on the first night of winter, low in the west just after dusk on the 21st, they’ll reach their closest – about one tenth of a degree apart! That’s the closest they’ve been since 1623. Imagine it: the two biggest planets and 161 of our solar system’s moons squeezed so tightly into a tiny patch of sky that we might not be able to tell where one ends and the other begins. When you watch, keep in mind that this is just an optical illusion. The two planets are nearly a half billion miles apart. Then, in just a few days, they’ll be gone from the night sky entirely until next year.
I love moments like these when we get a chance to watch the universe at work. I hope you’ll watch Jupiter and Saturn dance this month. Clear skies, everyone!
Scott Levine (email@example.com) is an astronomy writer and speaker from Croton-on-Hudson. He is also a member of the Westchester Amateur Astronomers, who are dedicated to astronomy outreach in our area. For information about the club including membership, newsletters, upcoming meetings and lectures at Pace University, and star parties at Ward Pound Ridge Reservation, visit westchesterastronomers.org. Events are free and open to the public. Please Note: All in-person club activities are suspended until further notice due to concerns over COVID-19.