When gigantic Jupiter overtakes gorgeous Saturn in the night sky, astronomers call it a Great Conjunction. In the coming days, starting at about 45 minutes after sunset, you should be able to catch the action in the southwestern sky as the two largest planets come within kissing distance.
They won’t quite merge into a single Star of Bethlehem redux, as you may have heard. But they will come close enough together to shine like a gift at the end of an ungodly hard year.
“The inner planets move faster and the outer planets move slower,” said Stan Seeberg of the Vancouver Sidewalk Astronomers club. “Every once in a while, two planets appear close together along our line of sight. Most of the time it’s not anything to write home about, but this one is extremely unusual.”
Regular conjunctions are a dime a dozen as objects overhead appear to cross paths, but there’s only one Great Conjunction between our solar system’s planetary heavyweights. Saturn’s orbit propels it around the sun once every 29.5 years while Jupiter’s pace is every 12 years. Once every 20 years, the two seem nearly to merge and then separate again as Jupiter laps Saturn in the sky.
Every 20 years may not sound all that rare, but many Great Conjunctions end up not so great, just fair to middling. Sometimes the planets don’t squeeze all that tightly together; other times they follow the sun too closely to be visible to us. That’s what happened in the year 2000 as well as the year 1623, when sun glare prevented telescope inventor Galileo (and everybody else) from observing that year’s Great Conjunction.
This year’s conjunction is shaping up to be truly great, according to space science director Jim Todd of the Oregon Museum of Science and Industry. On Dec. 21, the winter solstice, Jupiter will shine brightly one-third of the way up the southwestern sky — that’s fairly low — while fainter Saturn hangs out startlingly close, just above it.
Maybe it’s a hopeful sign at the end of a terrible year. Incredibly, the last time a Great Conjunction was both so close and so visible was the year 1226, Todd said. The next time won’t be until 2080.
How to watch
You can enjoy sweet viewing with the unaided eye, but high-powered binoculars or an entry-level telescope will reveal real wonders.
The major wonder is seeing the solar system’s two biggest, prettiest planets — Jupiter’s psychedelic cloud ribbons and Saturn’s silvery rings — cuddled up within the same small field of vision like they’ve shrugged off all social distancing. But don’t worry: Although they will appear just 0.1 degree apart in the sky, in fact, they’re nearly 456 million miles apart, Todd said.
Decent binoculars will also reveal Jupiter’s four largest moons as tiny white points huddled close to the planet and changing position each night; 10X-powered binoculars may start to show Saturn as “elongated” rather than just a point of light, Seeberg said; at 20X power those elongations become distinct rings circling the planet.
Put that together in the same tight aperture as the colorful, complicated orb of Jupiter, which should be pretty big and detailed at this magnification, for truly once-in-a-lifetime viewing, he said.
All of these grand observations are weather permitting, it goes without saying in the overcast Pacific Northwest. Looking through the atmosphere is like looking through a pond, Seeberg said. It can be clear or cloudy, calm or rough. “When a planet or star is close to the horizon, you are looking at it through many more miles of atmosphere” and lots more potential turbulence, he said.
But that’s one nice aspect of a Great Conjunction (and most astronomical phenomena): conditions do change, night by night, while Jupiter and Saturn take their time moving across the sky.
“This isn’t just on Dec. 21,” Seeberg said. “I would go out and look every night starting now. You can see the planets come together and then move apart again.”
It’s an early evening show only, so don’t dawdle over dinner. Best viewing is low in the southwestern sky beginning 45 minutes after sunset, and both planets will set just before 7 p.m., Todd said. Tall trees and nearby buildings may get in your way, so a wide-open patch of ground is best.
If all else fails, check out NASA.gov/nasalive for photo updates or the livestream page of the Lowell Observatory, based in Arizona.
Star of wonder
With plenty of ambiguity about the actual date of the event, there’s no scientific final word on the Star of Bethlehem that supposedly guided the three magi to the baby Jesus, Todd said. Theories abound that the Star of Bethlehem could have been a Great Conjunction. But it could just as easily have been a comet, a triple conjunction of Jupiter and Saturn with Venus, the constellation Pisces, or even a star explosion known as a supernova.
If contemplating those possibilities makes you eager for more stargazing, the Great Conjunction isn’t the only thing to see in the night sky right now. Jupiter and Saturn set soon after the sun does, but if you look higher in the southern sky, you’ll spot reddish Mars, which doesn’t set until after 2 a.m. Very early risers can enjoy a super-bright treat: the planet Venus rising in the east, well before the sun. It will shine much more brightly than Jupiter, Seeberg said.
Orion, one of the largest and most recognizable constellations in the sky, rises in the southeast in mid- evening. Look for the three stars that form the hunter’s belt, rising in a vertical line. Orion appears to be on his back, looking up at the sky. Maybe you are too.