Montage of Jupiter (foreground) and Saturn captured by NASA's Juno and Cassini spacecraft, respectively. The Great Convergence of these two planets on Dec. 21, 2020, will not bring them as close as they are shown in this picture, but the two will be a mere tenth of a degree apart, or a fifth the diameter of the full moon. (NASA)
On the evening of Dec. 21, the planets Jupiter and Saturn will appear closer together than in centuries, only a tenth of a degree apart, or one-fifth the width of the full moon. They won’t be this close again until March 2080.
This juxtaposition of giants will shine like few things you’ve seen in the sky, and offers a rare sight through the eyepiece of even a small telescope.
The Great Conjunction
Since before the pandemic began, Jupiter and Saturn have crept closer and closer together, first appearing back in February, rising with the dawn.
Though the two gas giant planets are physically almost half a billion miles apart, their orbital motions periodically bring them close together, from our perspective on Earth.
When Jupiter, orbiting the sun once every 12 years, overtakes the slower-moving Saturn, their visual convergence is called a Great Conjunction. This happens every two decades, though sometimes the pairing appears too close to the sun to be seen. And the two don’t usually get as close as they will on Dec. 21, 2020.
What to Look For
If you’ve been tracking Jupiter and Saturn over the past few weeks, you know where to find them: low over the southwest horizon shortly after sunset -- starting as soon as 5:30 p.m., even before the glow of twilight has faded.
You can’t miss Jupiter, the larger and closer of the duo, a gold-white beacon dominating a patch of sky with no particularly bright stars nearby. After that, Saturn is an easy second, above and to the left of Jupiter.
Just after sunset on Monday, Dec. 21, one thing will change to the casual glance: you may notice only one shining beacon, resting low in the fading glow of dusk near the horizon. With good eyesight you may still see two planets, but they will almost appear to merge.
If you have a small telescope, this meeting of Jupiter and Saturn may be one of the most thrilling things you’ve ever seen through the eyepiece. In a single view you will see not only the two majestic gas giant planets, but Saturn’s iconic system of rings, and several of the largest and most intriguing moons in the solar system, Jupiter’s Callisto, Io, and Europa, and Saturn’s Titan.
The pleasure of witnessing this convergence will be short-lived since Jupiter and Saturn set together around 7 p.m., offering scarcely an hour to enjoy the rare and beautiful spectacle. By 6:30 the pair will be approaching the horizon and any obstructions there may be -- trees, buildings, hills. So don't wait too late!
Juno and Huygens
Currently, there is only one spacecraft, NASA’s Juno, orbiting either of the gas giants. Juno is investigating Jupiter’s previously unexplored polar region, shedding light on an unexpectedly beautiful and mysterious realm of spinning, twisting cloud and storm systems. Juno is also probing Jupiter’s interior, seeking to understand its structure, the processes that create its powerful magnetic field and atmospheric auroras, and what might lie deep in the gas giant’s core.
NASA and the European Space Agency are preparing future missions to Jupiter to investigate the hidden ocean of its moon Europa. NASA’s Europa Clipper and the European JUICE spacecraft are slated to launch early this decade, though it will take several years for the spacecraft to get there.
A billion miles away in the Saturn system, the only human artifact remaining is the tiny and defunct European Huygens probe, which NASA’s Cassini spacecraft dropped onto the surface of Titan in 2005. Cassini was deliberately incinerated in a fiery plunge through Saturn’s atmosphere in 2017.
If you miss this union of giant planets, you’ll only have to wait 60 years for your next chance — so I’d recommend going for it this time.
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