This Christmas season, the most special thing to see in the sky will not be flying reindeer pulling a sleigh, but rather a rare celestial encounter, a kind of cosmic gift, many lives in the making. On December 21, Jupiter and Saturn will meet in a “grand conjunction,” the closest they could be seen together in the sky for nearly 800 years.
An astronomical conjunction occurs when two celestial bodies appear to cross or meet as seen from Earth. However, to make a “big” one requires an encounter between the two largest planets in our solar system. The orbits of Jupiter and Saturn align to allow the giant worlds to meet approximately every 20 years.
However, some great conjunctions are, well, better than others. The slightly oval shape of the orbits of Jupiter and Saturn, and the inclination of each orbit with respect to the solar equator, causes the proximity of the planets in the sky to fluctuate through their cyclic conjunctions. During some great conjunctions, the two worlds seem to get so close that they practically embrace; during others, they seem to come no closer than arm’s length away. (Of course, the planets are never really close; during their meeting on December 21, they will still be more than 730 million kilometers apart.)
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 the grand conjunction of 2020, which coincides with the December solstice, the shortest day of the year in the northern hemisphere and the longest in the south, the gas giants will appear separated by only 6.1 arc minutes. That’s about the thickness of a dime held at arm’s length.
“If you have a telescope, you can see the rings of Saturn and the Galilean moons of Jupiter together at the same time,” 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 even more poetic: The last time Jupiter and Saturn appeared so close was on July 16, 1623, when Galileo was still alive, just over a decade. after he first used a telescope to discover the four largest moons of Jupiter that now collectively bear his name. However, there is little chance that Galileo or anyone else on Earth managed to witness that grand conjunction, which was virtually impossible to see due to its apparent position close to the sun. The last great conjunction to appear as close and as visible as the next occurred on March 4, 1226. “In perspective, Genghis Khan was still roaming Asia then,” says astronomer Patrick Hartigan of Rice University in Houston.
You can see the next big conjunction in detail with binoculars and telescopes, “but the best part is that we’ll be able to see it with the naked eye,” says Faherty. Find a place where you can watch the sunset with a clear horizon in front of you, free of trees or buildings. About an hour after dark, first Jupiter will appear in the western sky and then Saturn, both bright spots that are distinguished from stars by the fact that they do not twinkle. “They will probably be visible even with light pollution; Jupiter is pretty bright, ”says Hartigan.
Although the grand conjunction will arrive on December 21, “you should be watching Jupiter and Saturn get closer every night until then,” Faherty recommends. Otherwise, “it would be like tuning in to the end of a show without watching all the previous episodes to catch up on what’s going on. By watching them get closer and closer, you can get an idea of how celestial mechanics work in the night sky. “
At times, large conjunctions have led scientists to speculate about their possible links to important events. For example, Johannes Kepler investigated whether the star of Bethlehem, which in the Nativity story of the Gospel of Matthew led the Magi to the birth of Christ, was a great conjunction, calculating that one occurred in 7 BC. C. through ancient stories and see if there could be an astronomical phenomenon behind something captivating that people saw, ”says Faherty. (Hartigan notes that the great ancient conjunction was not especially close or remarkable.)
After this great conjunction ends, stargazers need not wait centuries for the next one. Another encounter where the giant planets are separated by just six arc minutes will arrive on March 15, 2080, says Hartigan. “A young person who goes out and sees this great conjunction now can potentially see the next one close in 2080,” he says. “It would be a good connection between generations, one that makes you think of all those who have seen these conjunctions in the past and those who will glimpse them in the future.”
All in all, the grand conjunction is a reminder of how one can find comfort in the constancy of heavenly cycles over the millennia given the fickleness of modern times, Faherty says. “We get caught up in things that happen during the short period of time that a human life exists, but astronomy encompasses a much longer time frame than that,” he notes. “In the face of everything that is happening, you can find perspective in astronomical time frames.”
.
