This year’s winter solstice will bring a rare sight to our night skies — just in time for the holidays.
For Earth viewers, Jupiter and Saturn will appear closer to one another on Dec. 21 than they have been since the Middle Ages. If you can gaze into the southwestern horizon at the right time, the two gas giants will look like neighboring points of light.
Almost a “double planet,” said Rice University astronomer Patrick Hartigan.
“Alignments between these two planets are rather rare, occurring once every 20 years or so, but this conjunction is exceptionally rare because of how close the planets will appear to one another,” Hartigan explained.
“You’d have to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky.”
Astronomers call what we’ll witness on Dec. 21 a “conjunction.” From asteroids or moons to planets and stars, a conjunction is when two objects in space appear to be close to one another, as observed from Earth. In the reality of space, they’re still hundreds of millions of miles apart.
Every 20 years, our solar system’s largest planets align during their orbits around the sun. Jupiter and Saturn’s last conjunction was in 2000. But this year is particularly special because the two will appear to separated by just 1/5th the diameter of a full moon — or 0.1 degrees — an occurrence the world hasn’t seen since the Middle Ages.
The last time Jupiter and Saturn came this close was 1623, but that conjunction was too near the sun to be seen by Earthlings. Therefore, 1226 is actually the most recent time that such a close conjunction of Jupiter and Saturn was visible to humans.
“That’s just sly of 800 years ago” said Amy Oliver, spokeswoman for the Harvard-Smithsonian Center for Astrophysics.
“Call [2020’s conjunction] a unique, holiday gift to the world,” she said. “Maybe it’s the soothing band-aid for 2020.”
Moon: Water discovered on sunlit part of the moon for the first time, NASA says
Venus: Life on Venus? Astronomers see hint of life in clouds of Venus
How to watch the 2020 conjunction on Dec. 21
Saturn and Jupiter are already visible in December’s sky, as the two planets have been moving closer to each other for much of 2020. They will look like two points of light in the sky — Saturn, which is farther from Earth, will be the fainter one.
Both Hartigan and Oliver encourage stargazers around the world to start looking now.
“Over the next couple of weeks you can watch them move which is super cool, because you’re actually seeing planets in orbit” Hartigan told USA TODAY, adding that identifying them now will also make the conjunction on winter solstice easier to identify.
And to witness the conjunction on Dec. 21? Weather permitting, it will be observable anywhere on Earth, although more difficult in high northern latitudes.
“People are going to have to work a bit,” Hartigan said. He added that the two planets seen in the sky, “are not going to suddenly look like a brilliant ‘Christmas star.'”
You won’t need a telescope, but you’ll have to find a good viewing spot and be on time. Avoid tall buildings or mountains, and look towards the low, southwestern horizon right after sunset.
If you’re too early, you might miss a fainter Saturn. If you’re too late, the two may have already slipped into the horizon for the night.
The best viewing conditions will be near the equator, with less time to catch a glimpse further north.
Oliver said the best time is between dusk and about 15-20 minutes after dark.
She added that the maximum conjunction will occur at 1:20pm ET on Dec. 21, and won’t be visible to the naked eye during daylight. Still, Americans across the U.S. will be able to see the conjunction in the early night of Dec. 21, weather permitting.
“That doesn’t make it any less special,” Oliver said. “[It will be] still so close together that that by far beats other past events.”
If a snowy day or tight schedule makes you miss winter solstice’s sky event, never fear: although the maximum conjunction is on Dec. 21, Jupiter and Saturn will still appear to be closer than the diameter of a full moon from Dec. 16-25.
Such a close conjunction won’t happen again until early March 15, 2080. Although this might be the only time most adults witness the rare occurrence, Hartigan adds that it’s a special opportunity for family.
Parents can show the conjunction to their young kids, he said, and tell them, “60 years from now, when I’m long gone…you go out in the morning sky and look at this conjunction, and you think of me.”
Hartigan feels that looking up to the sky enriches people’s lives.
“Astronomy gives you a different kind of perspective on the universe, which is different from our day-to-day experiences,” he said. “Things are going on above us all the time, they mark eras… It’s an important connection between generations.”
This article originally appeared on USA TODAY: 2020 conjunction: Rare ‘Christmas star’ with Jupiter, Saturn to align