By Charles Q. Choi on
This holiday season, the most special thing to see in the sky won’t be flying reindeer pulling a sleigh, but rather a rare celestial rendezvous—a cosmic gift of sorts, many lifetimes in the making. On December 21, Jupiter and Saturn will meet in a “great conjunction,” the closest they could be seen in the sky together for nearly 800 years.
An astronomical conjunction occurs when any two heavenly bodies appear to pass or meet each other as seen from Earth. To make one “great,” though, requires an encounter between our solar system’s two largest planets. The orbits of Jupiter and Saturn align to allow the giant worlds to seemingly convene roughly every 20 years.
However, some great conjunctions are, well, greater than others. The slightly oval shape of Jupiter and Saturn’s orbits, and how inclined each orbit is with respect to the sun’s equator, causes the planets’ closeness in the sky to fluctuate across their cyclic conjunctions. During some great conjunctions, the two worlds appear to come so close as to practically hug each other; during others, they seem to approach no nearer than arm’s length. (Of course, the planets are never actually close at all; during their December 21 encounter, they will still be separated by more than 730 million kilometers.)
For the last great conjunction, on May 28, 2000, the apparent distance between Jupiter and Saturn in the sky was 68.9 arc minutes, or more than twice the diameter of the full moon. In contrast, with 2020’s great conjunction—which coincides with the December solstice, the shortest day of the year in the northern hemisphere and the longest in the southern—the gas giants will appear separated by just 6.1 arc minutes. That is roughly the thickness of a dime held at arm’s length.
“If you have a telescope, you’ll be able to see both the rings of Saturn and the Galilean moons of Jupiter close together at the same moment,” says astronomer Jackie Faherty at the American Museum of Natural History in New York.
In a way, that particular detail makes this year’s astronomical spectacle all the more poetic: The last time Jupiter and Saturn appeared so close was July 16, 1623, back when Galileo was still alive, a little more than a decade after he first used a telescope to discover Jupiter’s four largest moons that now collectively bear his name. The odds are low, however, that Galileo or anyone else on Earth managed to witness that great conjunction, which was virtually impossible to see because of its apparent position near the sun. The last great conjunction to appear as close and as visible as the upcoming one occurred on March 4, 1226. “For perspective, Genghis Khan was still roaming Asia then,” says astronomer Patrick Hartigan at Rice University in Houston.
You can see the upcoming great conjunction in detail with binoculars and telescopes, “but the best part about it is we’ll be able to watch it with the naked eye,” Faherty says. Find a spot where you can watch the sunset with a clear horizon in front of you, free of trees or buildings. In the hour or so after nightfall, first Jupiter will appear in the western sky, and then Saturn, both shining dots distinguishable from the stars by the fact they do not twinkle. “They will likely be visible even with light pollution—Jupiter is pretty bright,” Hartigan says.
Although the great conjunction will arrive on December 21, “you should be watching Jupiter and Saturn draw close every night until then,” Faherty recommends. Otherwise, “it’d be like tuning into the finale of a show without seeing all the episodes before it to get you caught up on what’s going on. By watching them get closer and closer, you can get a sense how celestial mechanics works in the nighttime sky.”
Great conjunctions have at times drawn scientists to speculate over their possible links with major events. For instance, Johannes Kepler investigated whether the Star of Bethlehem, which in the nativity story of the Gospel of Matthew guided the Three Wise Men to Christ’s birth, was a great conjunction, calculating that one did occur in 7 B.C. “Often astronomers like to look through stories from ancient times and see if there might be an astronomical phenomenon behind something captivating that people saw,” Faherty says. (Hartigan notes that ancient great conjunction was not an especially close or remarkable one.)
After this great conjunction ends, stargazers need not wait centuries for the next close one. Another rendezvous where the giant planets are separated by just six arc minutes will arrive on March 15, 2080, Hartigan says. “A young person who goes out and sees this great conjunction now can potentially see the next close one in 2080,” he says. “It’d be a nice connection between generations, one that makes you think about all those who have seen these conjunctions in the past—and those who will glimpse it in the future.”
All in all, the great conjunction is a reminder of how one can find solace in the constancy of heavenly cycles over the millennia given the inconstancy of modern times, Faherty says. “We get caught up in things that happen over the small given amount of time that a human life exists under, but astronomy encompasses a timeframe so much more than that,” she notes. “In the face of everything that is going on, you can find perspective in astronomical timeframes.”