As I told you last week in Skywatch, this December has so much going on in the night skies. It’s a good thing, at least for stargazers, that we have some of the longest nights of the year to take it all in.
The absolute premier event is something that hasn’t been seen from this planet since Galileo was alive. It’s the historic conjunction of the bright planets Jupiter and Saturn that culminates on Dec. 21. A conjunction is defined as any close arrangement in the sky between the moon, planets or bright stars. Many conjunctions occur through any given year, but this conjunction between Jupiter and Saturn is super special.
This week you can easily see the bright planets remarkably close together in the low southwest sky from around 5 p.m. in the early evening twilight until shortly after 7 p.m., when they set below the horizon. You can’t miss them, even in a light-polluted sky. Jupiter is much brighter of the two planets and is also is one of the brightest star-like objects in the sky. Just to Jupiter’s upper left is Saturn, not nearly as bright as Jupiter, but easily seen — although you might have to strain your eyes to see it in a heavily lighted area.
At the start of this week, the largest planets of our solar system are less than two degrees apart in the low southwest sky. That is less than the width of two of your fingers held together at arm’s length. By the end of the week they will be less than a degree apart. Over the next couple of weeks they’ll draw even closer to each other from night to night. On Dec. 21, they’ll be “touching” each other, only a tenth of a degree apart! This is the closest conjunction of Jupiter and Saturn since 1623. They won’t be this close to each other again until March of 2080.
Even though Jupiter and Saturn will appear as a “double planet,” they are nowhere near each other physically but are almost precisely in the same line of sight from Earth. On Dec. 21, Jupiter will be just over 550 million miles away from Earth and Saturn will be a little over a billion miles away. They are both orbiting the sun in their individual orbits. Jupiter takes about 12 years to complete one orbit and around 29 years for Saturn. Because of that, they both move at different speeds among the backdrop of very distant stars. Jupiter moves much faster than Saturn. They catch up with each other in the sky every 20 years. They are usually nowhere as close to each other in the sky as they are this month because their orbits around the sun are in slightly different mathematical planes. In much simpler terms, they’ll be in the right place at the right time from our view on Dec. 21.
This monumental event is fantastic with just the naked eye, but seeing it through even a small telescope or even binoculars can make it even sweeter. Both planets can fit in the same field of view. Unfortunately, though, since they’re so close to the horizon, they’ll appear fuzzy. That’s because Earth’s atmospheric layer is much thicker the closer you get to the horizon from our point of view.
Despite any fuzziness, you’ll see the disk of Jupiter with possibly some of its darker cloud bands. For sure, you’ll be able to see up to four of Jupiter’s largest moons that’ll be lined up diagonally on both sides of Jupiter. They constantly change their position relative to Jupiter because they orbit the giant planet in periods of two to 17 days. Some nights one or more of the moons may be hiding behind Jupiter or camouflaged in front of it. You should also be able to see a fuzzy version of Saturn’s ring system and maybe some of its moons, especially Titan, Saturn’s largest moon.
Make sure you glance at the Jupiter-Saturn show every clear night you can between now and Dec. 21. With as lousy as 2020 has been on this planet, it’s great to be able to see a great show being put on by fellow planets in our solar system … and again, you’re seeing something that hasn’t been seen since Galileo saw it in 1623!
This week also keep an eye out for “shooting stars” as the annual Geminid meteor shower gets going, especially seen from around midnight to morning twilight. This is one of the best meteor showers of the year. I’ll have more next weekend in Skywatch.
Mike Lynch is an amateur astronomer and retired broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is the author of “Stars: a Month by Month Tour of the Constellations,” published by Adventure Publications and available at bookstores and adventurepublications.net. Mike is available for private star parties. You can contact him at email@example.com.
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