Great Conjunction: Jupiter and Saturn appear to pass each other nearly once every 20 years, but what is so extraordinary about tomorrow’s celestial extravaganza?
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.
Great Conjunction of Jupiter And Saturn: Is it the Christmas Star?
The Christmas Star, symbolizing the year-end festive spirit and the New Year celebrations, has just got more exciting. The ‘Great Conjunction’ of our two planetary giants, Jupiter and Saturn, that will take place on Monday is being thought by many to be the ‘Christmas Star’ or the ‘Star of Bethlehem'”. According to space.com; it is being suggested that the “two planets might be a replica of the legendary Star of Bethlehem”. Many astronomers and scientists have tried to determine whether the ‘Star of Bethlehem’ really ever existed. One of the theories claim that in the year 7 B.C., Jupiter and Saturn had three conjunctions in the same constellation, and that could possibly explain it.
What makes the Jupiter-Saturn ‘Great Conjunction’ once in a lifetime event?
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 hug each other practically; they seem to approach no nearer than arm’s length during others. (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.)
The NASA (National Aeronautics and Space Administration) says, “It’s been nearly 400 years since the planets passed this close to each other in the sky, and nearly 800 years since the alignment of Saturn and Jupiter occurred at night, as it will for 2020, allowing nearly everyone around the world to witness this ‘Great conjunction'”.
The Jupiter and Saturn will be separated by just “0.1 degrees or about one-fifth the apparent width of the Moon”.
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 full moon’s diameter. 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. However, the odds are low 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 appears 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 of how celestial mechanics works in the nighttime sky.”
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.”