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Celestial News: Jupiter closing in on Saturn

Jimmy Westlake
For Steamboat Pilot & Today
It’s hard to miss Jupiter and Saturn merging closer and closer in the southwest sky at dusk in the stars of the constellation Sagittarius. Jupiter is the brighter of the two. The small quadrilateral to the lower left of the planets is called the Terebellum. On Dec. 21, the planet duo will appear closer together than they have in 400 years. (Photo by Jimmy Westlake, 2020)

Once every 20 years, the solar system’s two giant planets converge in the sky for what is called a Great Conjunction, and the year 2020 culminates with the greatest Great Conjunction of the last 400 years. On the evening of December 21, Jupiter will pass only 0.1 degree from Saturn, the closest Great Conjunction since July 16, 1623.

Although Jupiter and Saturn were slightly closer for the Great Conjunction of 1623, the planets were too close to the Sun to be observed, much to the chagrin of Renaissance astronomers like Galileo Galilei and Johannes Kepler.

Great Conjunctions have fascinated astronomers for centuries. It has been suggested that the Great Conjunction of Jupiter and Saturn in the spring of 7 BC is what the Magi observed and interpreted as the Star of Bethlehem.



Jupiter and Saturn are in a slow-motion race around the sun. Jupiter requires 12 years to circle the sun, and slow-moving Saturn requires almost 30 years. As a result, Jupiter gains a lap on and passes Saturn every 20 years, giving us a Great Conjunction.

The most recent Great Conjunction was on May 30, 2000, but this one occurred when the planets were close to the sun in the sky and were very difficult to observe. You have to go back to July 30, 1981, for the last easy-to-observe Great Conjunction.



To observe the Great Conjunction of 2020 on the night of closest approach, find a location where you have a clear view to the horizon toward the southwest. Start looking for the planets soon after the sun sets at 4:44 p.m., about a single handspan above the horizon — binoculars will help. By 5:44 p.m., the planets should be easy to see in the fading twilight, but now only a fist above the horizon.

At first, the object you spot might look like a single planet, but look more closely and you’ll see that it is two objects very close together. Fainter Saturn will appear at the 2 o’clock position, relative to Jupiter. You’ll have about two hours after sunset to view the planets before they set together around 7 p.m.

As stunning as the sight of two planets almost touching will be to the naked eye, the view through a small telescope will be even better. Not only will both of the giant planets be visible at the same time in the eyepiece, but you might also see three of Jupiter’s giant moons — Io, Europa and Callisto — and Saturn’s single giant moon, Titan. The rings of Saturn and the dark cloud stripes of Jupiter should be visible, too. It’s a view that comes around once in a lifetime, if you are very, very lucky. And, we are.

Don’t just wait for the main event on Dec. 21. Jupiter and Saturn will be sky companions all year, inching closer and closer toward the Great Conjunction. The two planets will be within 1 degree of each other from Dec. 12 until Dec. 30, shining through the evening twilight.

You will have to wait until March 15, 2080, to see Jupiter and Saturn this close together again.

Jimmy Westlake is the former fulltime professor of physical xciences at Colorado Mountain College Steamboat Springs and former director of the Rollins Planetarium at Young Harris College in Georgia and the St. Charles Parish Planetarium in Louisiana. His “Celestial News” column appears monthly in the Steamboat Pilot & Today. Check out Jimmy’s new “2020 Cosmic Calendar” of sky events on his website at http://www.jwestlake.com. It features 12 of his best astro-photos and a day-by-day listing of cool celestial events that you and your family can enjoy watching all year.

 


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