This week South Carolinians will be able to view an astronomical event that hasn't been seen in 800 years. Jupiter and Saturn, our solar system's largest planets, will appear to come together in what astronomers call "the Great Conjunction."
"As Jupiter, Saturn and Earth go through their orbits around the sun, every 20 years Jupiter passes Saturn from Earth's point of view. But usually they're much farther apart than they will be this year," explained Martin Bowers, director of the University of South Carolina's Melton Observatory. This year, "they will be very close together when that pass occurs" - relatively speaking, of course, as they're still actually millions of miles apart. "They will look almost indistinguishable, almost a double star."
South Carolina State Museum Planetarium Manager Liz Klimek compared the planets' orbits to a NASCAR race. "If you kind of imagine cars going around a race track and Earth, Saturn and Jupiter are race cars, Jupiter is going to lap Saturn. So if you're standing in the center of that race track, one's just gonna appear to pass the other one."
Klimek gave advice on how to view the event. "Maybe a half-hour past sunset you'll see a starlike object in the Southwest that appears to have kind of a partner not too far away. There's really no other bright stars around, so you'll definitely see them first. They'll be pretty hard to miss." The planets will be visible roughly from 6 to 7 p.m. before sinking beneath the horizon.
Despite the eight-century gap (actually, they were nearly this close 400 years ago, said Bowers, but they were so close to the sun at that time that no one could have seen them), the planets' odd orbital patterns will bring them back to another great conjunction in only 60 years.
Bowers said it's just a coincidence that the conjunction is happening on Dec. 21 - the winter solstice - but he added that scientists at least as far back as legendary 17th-century astronomer Johannes Kepler have speculated that this phenomenon may have been the famous Star of Bethlehem. "Kepler was one of the first ones to speculate that perhaps a planetary conjunction might have been the star that the Magi saw," he said. There are various conjunctions of Jupiter and Saturn, but also of Jupiter and Venus, that happened in that time frame within a few years BC, that have been put forward as candidates for that. Perhaps that was the star, an unsual event that caught the attention of some astrologers at the time, that they saw this coming together of planets and said 'that's really unusual.'"
Klimek echoed the Christmas star hypothesis. "There were other things thrown around, things like comets and meteors, but...this is the closest thing that fits the description, and so it's the best explanation." But she said it can't be definitive because "you need an exact date. Without pinning down that date, it's impossible to say for sure."
To make sure people have a good view, Klimek advises finding a spot with a clear view of the Southwest - no trees or buildings, as it will be low on the horizon - and practicing watching the planets. "Just go out every few nights and watch them get closer and closer together, so you can kinda watch the process as Jupiter seems to approach Saturn in the sky. You can watch them by night between now and then. You don't have to wait until the 21st."
The planets can be seen the rest of December as they approach and then recede from each other, and can be seen with the naked eye, though binoculars or a telescope will make the sight even more spectacular.
Of course, the night must be clear of clouds, which is another reason to watch the planets before and/or after the 21st in case it's cloudy on the solstice.