On the evening of December 21, the planets Jupiter and Saturn will appear closer together than in almost 800 years, only a tenth of a degree apart, or one-fifth the width of the full moon. They wonât be this close again until March of 2080.
This juxtaposition of giants will shine like few things youâve 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 donât usually get as close as they will on December 21, 2020.
What to Look For
If youâve been tracking Jupiter and Saturn over the past few weeks, you know where to find them: above the southwest horizon shortly after sunset. You canât 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.
On December 21, 2020, Jupiter and Saturn will appear close enough to each other to be seen simultaneously through the eyepiece of even a low-powered telescope. Several of the solar system’s largest and most fascinating moons will also be seen in the view, including volcanic Io, the ocean-harboring Europa, and Saturn’s Titan, the only moon to possess a thick atmosphere. (Graphic created using Stellarium)
Just after sunset on Monday, December 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 youâve ever seen through the eyepiece. In a single view you will see not only the two majestic gas giant planets, but Saturnâs iconic system of rings, and several of the largest and most intriguing moons in the solar system, Jupiterâs Callisto, Io, and Europa, and Saturnâs Titan.
Montage showing Jupiter’s moon Europa set before the gas giant planet’s mighty face. Europa hides a massive ocean of liquid water beneath its icy crust. (NASA/JPL-Caltech)
The pleasure of witnessing this convergence will be short-lived since Jupiter and Saturn set together around 7:00 p.m., offering scarcely an hour to enjoy the rare and beautiful spectacle.
Juno and Huygens
Currently there is only one spacecraft, NASAâs Juno, orbiting either of the gas giants. Juno is investigating Jupiterâs previously unexplored polar region, shedding light on an unexpectedly beautiful and mysterious realm of spinning, twisting cloud and storm systems. Juno is also probing Jupiterâs 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 giantâs core.
NASA and the European Space Agency are preparing future missions to Jupiter to investigate the hidden ocean of its moon Europa. NASAâs 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.
Artist rendering of the European Huygens probe, which landed on the surface of Saturn’s moon Titan in 2005. The now derelict lander is the only piece of human technology anywhere in the Saturn system, after the deliberate incineration of mothership Cassini several years ago. (NASA)
A billion miles away in the Saturn system, the only human artifact remaining is the tiny and defunct European Huygens probe, which NASAâs Cassini spacecraft dropped onto the surface of Titan in 2005. Cassini was deliberately incinerated in a fiery plunge through Saturnâs atmosphere in 2017.
If you miss this union of giant planets, youâll only have to wait sixty years for your next chance — so Iâd recommend going for it this time.Â
Copyright 2020 KQED