On December 21, 2020, the Solar System’s two gas giants will appear closer together in Earth’s night sky than they have been since 1226 CE.
Jupiter and Saturn have been traveling across the sky together all year, but in December, get ready for them to really put on a show. Over the first three weeks of the month, watch each evening as the two planets get closer in the sky than they’ve appeared in two decades. Look for them low in the southwest in the hour after sunset, Image credit: NASA / JPL-Caltech.
On December 21, Jupiter and Saturn will appear just a tenth of a degree apart. This means the two giant planets and their moons will be visible in the same field of view through binoculars or a small telescope.
In fact, Saturn will appear as close to Jupiter as some of Jupiter’s moons. This event is called a ‘great conjunction.’
These occur every 20 years this century as the orbits of Earth, Jupiter, and Saturn periodically align making these two outer planets appear close together in our nighttime sky.
Even so, this is the ‘greatest’ great conjunction between Jupiter and Saturn for the next 60 years, with the two planets not appearing this close in the sky until 2080.
“Alignments between these two planets are rather rare, occurring once every 20 years or so, but this conjunction is exceptionally rare because of how close the planets will appear to one another,” said Professor Patrick Hartigan, an astronomer at Rice University.
“You’d have to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky.”
“On the evening of closest approach on December 21 they will look like a double planet, separated by only 1/5th the diameter of the full Moon,” he added.
“For most telescope viewers, each planet and several of their largest moons will be visible in the same field of view that evening.”
Jupiter and Saturn will appear low in the western sky for about an hour after sunset each evening.
“The further north a viewer is, the less time they’ll have to catch a glimpse of the conjunction before the planets sink below the horizon,” Professor Hartigan said.
“Fortunately, the planets will be bright enough to be viewed in twilight, which may be the best time for many U.S. viewers to observe the conjunction.”
“By the time skies are fully dark in Houston, for example, the conjunction will be just 9 degrees above the horizon.”
“Viewing that would be manageable if the weather cooperates and you have an unobstructed view to the southwest.”
“But an hour after sunset, people looking skyward in New York or London will find the planets even closer to the horizon, about 7.5 degrees and 5.3 degrees respectively.”
“Viewers there, and in similar latitudes, would do well to catch a glimpse of the rare astronomical sight as soon after sunset as possible.”
“Those who prefer to wait and see Jupiter and Saturn this close together and higher in the night sky will need to stick around until March 15, 2080.”
“After that, the pair won’t make such an appearance until sometime after the year 2400.”
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This article is based on text provided by the National Aeronautics and Space Administration and Rice University.
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