If you managed to see the crescent Moon pass Jupiter and Saturn this week, you’ll have noticed something else about the Solar System’s two largest planets.
They’re now really, really close to each other, and on December 21, 2020—the date of the December solstice—they’re going to almost appear to collide to become one super-bright point of light.
Jupiter and Saturn will look like a “double planet” for first time since Middle Ages.
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In reality, of course, they won’t be close at all. Think about the distance from the Earth to the Sun. That’s what astronomers call an astronomical unit (au), and it’s how they measure distances in the vastness of the Solar System. Jupiter is 5 au from us. Saturn is 10 au.
These two planets aligned in the Solar System a few weeks ago, but on December 21 they’ll appear aligned to us on Earth. Our line of sight is different because we’re orbiting quickly around the Sun.
“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 be to one another,” said Patrick Hartigan, astronomer at Rice University.
“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.”
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Jupiter and Saturn’s orbital resonance is such that they align in a “great conjunction” every 19.6 years, but they’re rarely as close—from our point of view on Earth—as on December 21, 2020 when the two planets will be separated by less than the apparent diameter of a full Moon.
A conjunction is when two objects line up in the sky.
It’s thought by some—including legendary German astronomer Johannes Kepler—that the “star of Bethlehem” in the story of the Magi or “three wise men” could have been a rare triple conjunction of Jupiter, Saturn and Venus.
The rare celestial event will be observable anywhere on Earth where skies are clear. The planets will appear low in the western sky for about an hour after sunset as viewed from the northern hemisphere, and though they’ll be closest on December 21, 2020, you can look each evening that week.
Although the sight will be sinking towards the horizon, it will be bright enough to be viewed in twilight.
All you need is an unobstructed view to the southwest, and to look to the southwest from about 45 minutes after sunset where you are.
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For telescope viewers, on December 21, 2020, each planet and several of their largest moons will be visible in the same field of view that evening.
A “great conjunction” this close won’t happen again until March 15, 2080.
Wishing you clear skies and wide eyes.
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