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On December 21, 2020, the two gas giants of the Solar System will appear closer together in Earth’s night sky than they have been since 1226 CE.
Jupiter and Saturn have been traveling the sky together all year, but in December, get ready for them to put on a show. During the first three weeks of the month, watch each night as the two planets get closer in the sky than they have appeared in two decades. Look for them in the southwest in the hour after sunset, Image Credit: NASA / JPL-Caltech.
On December 21, Jupiter and Saturn will appear one tenth of a degree apart. This means that 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 ‘grand conjunction’.
These occur every 20 years in this century as the orbits of Earth, Jupiter, and Saturn periodically align, causing these two outer planets to appear together in our night sky.
Still, this is the ‘biggest’ grand conjunction between Jupiter and Saturn for the next 60 years, and the two planets won’t appear that close in the sky until 2080.
“The alignments between these two planets are quite rare, they occur once every 20 years or so, but this conjunction is exceptionally rare because of how closely the planets will be seen,” said Professor Patrick Hartigan, an astronomer at Rice University.
“You would have to go back to just before sunrise on March 4, 1226, to see a closer alignment between these visible objects in the night sky.”
“On the night of closest approach on December 21, they will look like a double planet, separated by only 1/5 the diameter of the full Moon,” he added.
“For most telescopes, each planet and several of its largest moons will be visible in the same field of view that night.”
Jupiter and Saturn will appear low in the western sky for about an hour after sunset each night.
“The further north a viewer is, the less time they have to glimpse the conjunction before the planets sink below the horizon,” said Professor Hartigan.
“Fortunately, the planets will be bright enough to be seen at twilight, which may be the best time for many American viewers to observe the conjunction.”
“By the time the skies are completely dark in Houston, for example, the conjunction will be only 9 degrees above the horizon.”
“Seeing that would be manageable if the weather cooperates and you have a clear view to the southwest.”
“But an hour after sunset, people looking up at the sky in New York or London will find the planets even closer to the horizon, at about 7.5 degrees and 5.3 degrees respectively.”
“Viewers there, and at similar latitudes, would do well to take a look at the rare astronomical sight as soon as possible after sunset.”
“Those who prefer to wait and see Jupiter and Saturn so close together and higher in the night sky should stay until March 15, 2080.”
“After that, the couple will not 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.
