The nice thing is that the spectacle will be shown all over the world without special equipment. All you have to do is look southwest once night falls. And people with an amateur telescope can admire another rarity: with low magnification, the both giant planets can be captured in one and the same field of view. Jupiter, along with its four brightest moons, and Saturn with its striking rings will all be in view at the same time.
But you don’t have to wait until December 21 to admire the night sky. “It’s much, much more fun to look up all month long, rather than just December 21,” says Laura Danly, curator of the Griffith Observatory in Los Angeles. “You see those two lights, Jupiter and Saturn, getting closer and closer, until they are only one-tenth of an arc degree apart on the 21st” – or about one-fifth the diameter of the full moon in the night sky.
That’s a very short distance, considering that you can easily cover the entire lunar disk with a thumb at arm’s length. Yet most night vision goggles will still be able to distinguish the two planets from each other. “If your eyesight is good enough to drive, then you can see that Jupiter and Saturn are two separate objects,” even when they are closest to each other, Danly says.
Near and far
Astronomers call a moment when planets in the sky appear to be right next to each other or even appear to coincide, a ‘conjunction’. Conjunctions of the gas giants Jupiter and Saturn occur about once every twenty years and are called “great conjunctions” because of their rarity. Conjunctions of the inner planets of our solar system, such as Mercury or Venus, with each other or with one of the outer planets are much more common, many times a decade.
The farther a planet is from the sun, the slower it moves across the night sky – and Jupiter in Saturnus are the two planets furthest from the sun and yet visible to the naked eye. (Saturn, further away from the sun and also smaller than Jupiter, is the fainter of the two.) Jupiter takes 12 years to orbit the sun once, while Saturn takes 30 years to do so. Those celestial revolutions cause them to stand right next to each other in the night sky once every twenty years.
But not all conjunctions of Jupiter and Saturn are the same. Their orbits are not in the same disk plane. (If it did, Jupiter would block our view of Saturn completely every twenty years.) Normally, the two giant planets pass each other in the night sky at a short distance, of only a few degrees of arc. But this year, the two gas giants will approach each other even closer from Earth, even though they are actually hundreds of millions of miles apart.
The last conjunction of Jupiter and Saturn so close to each other occurred in 1623, thirteen years after Galileo first turned a telescope into the night sky and discovered Jupiter’s four largest moons. “That conjunction must have been very difficult to see, because both planets would have been close to the sun in the sky,” said Kevin Schindler of the Lowell Observatory in Flagstaff, Arizona. (That was also the case during the last ‘great conjunction’, in 2000.)
“You have to go all the way back to the year 1226 to find a conjunction that was so tight and visible in the night sky at the same time.” The next time such a combination occurs will be in 2080.
Solar system in motion
Throughout history, people have attributed special astrological significance to these types of conjunctions. This is how the English writer Geoffrey Chaucer describes in his poetic epic Troilus and Criseyde the great conjunction of 1385: “Saturn and Jupijn, in [het sterrenbeeld] Cancer came together / That such a rain fell from the heavens. ” Some astrologers predicted global disasters, but of course nothing happened.
Today we know that the planets pull together a little by their gravity, but that their effect on the earth is negligible. So we can relax and enjoy the show – or try to take pictures of it.
To get a good recording of the pair, “you have to practice and experiment a lot, and that with reasonably good equipment,” says Bradley Schaefer, astronomer at Louisiana State University. While your smartphone’s camera may register two bright spots in the night sky, people with a single-lens digital SLR camera, or at least a camera on a tripod, will get better results.
“You should definitely use a tripod and plan ahead of time where you want to take the photos,” says Schaefer. Make sure you can view the night sky in a southwesterly direction without obstacles.
And if it’s cloudy, don’t worry, as a host of observatories, including those at Lowell, Arizona, will watch the event in a livestream to broadcast.
We know that the planets are always in motion, but a conjunction makes us really realize that again. The great conjunction of 2020 is an opportunity to see the solar system as an ever-changing stage on which the planets perform their deed. “When you watch the night sky for several nights in a row, you realize just how dynamic the solar system really is,” says Danly.
Science journalist Dan Falk lives in Toronto. His publications include The Science of Shakespeare in In Search of Time. Follow Dan on Twitter.
This article was originally published in English on NationalGeographic.com