- Jupiter and Saturn will appear to align this December, creating a “double planet” in the night sky.
- It’s the first time since the Middle Ages that the planets have aligned this close.
- You can see this planetary conjunction on December 21.
For the first time since the middle ages, Jupiter and Saturn will become so close to each other in the night sky that they’ll appear as a brilliantly bright “double planet.” On December 21, the solar system’s two largest planets will be visibly on top of each other in the western sky.
If you live in the northern hemisphere, you can observe this planetary conjunction setting in the southwestern sky, shortly after sunset. If you’ve been waiting to take out your telescope, this may be the perfect opportunity. Even if you have an amateur telescope with the right eyepiece, you may be able to see the Galilean moons, the bands of Jupiter, and Saturn’s rings, all in one spectacular view.
But don’t wait too long, because the planets will dip below the horizon only a couple hours after the sun sets. You won’t see a similar conjunction this close again until March 15, 2080.
In what astronomers are calling a great conjunction, or the meeting of the two biggest worlds in our solar system, the planets will appear perfectly aligned from Earth’s perspective and be only 0.1 degrees apart (that’s only one-fifth the diameter of the moon). The last time Jupiter and Saturn appeared this close was in 1623, just 14 years after Galileo made his first telescope and discovered the moons of Jupiter.
The true solar system alignment of these two gas giants occurred a few weeks ago, but from Earth’s perspective as it orbits the sun quickly, Jupiter and Saturn will appear to inch closer and closer together over the coming weeks. If you take out your telescope early, the planets are already close together and setting later in the night, so you’ll have ample opportunities to track their progress.
Jupiter is five times the distance of Earth from the sun—a whopping 484 million miles away. Saturn is almost double that. Because of their great distances, the massive planets take ages to make a single revolution around our sun.
Your birthday on Jupiter would happen once every 12 years, and once every 30 years on Saturn. At this rate, Jupiter aligns with Saturn every 20 years in the orbital plane of the solar system, creating a conjunction viewed from Earth. But these conjunctions don’t always appear as close as they do this year.
The German astronomer Johannes Kepler suggested in 1614 that a conjunction of Jupiter and Saturn occurring in 7 B.C. may be what the three wise men in the nativity story called the Star of Bethlehem. Whether the famed “Christmas star” was a real astronomical event, like a planetary conjunction or comet, remains a mystery. Jupiter and Saturn aren’t the only planets that regularly align. In 2015, Venus and Jupiter came within one-third of a degree apart.
The best time to view the spectacle on December 21 will be around an hour after sunset, right near the constellation Capricorn. Get outside early for the matinee show.
This article originally appeared on Cosmopolitan.com. Minor edits have been made by the Cosmo.ph editors.