Watch Jupiter and Saturn Slow-dance to their Squeaky-tight "Great Conjunction"

Saturn (left) and Jupiter have been a familiar sight in the southern sky this summer and fall. Keep an eye on them. In the weeks ahead Jupiter will close the gap. Bob King

Jupiter and Saturn have been our nighttime companions all summer and now into the fall. Just lift your head and face south any clear evening and you'll find them staring back at you. Both planets shine from the teapot-shaped constellation Sagittarius. First visible at nightfall they wheel slowly westward with Earth's rotation and slink below the horizon shortly after midnight.

Jupiter and Saturn are paired up in Sagittarius. Watch for the waxing moon to swing below Jupiter on Sept. 24 and Saturn on the 25th. Stellarium

In late September the two planets are separated by 7.5° or about the same distance between bright Betelgeuse and its neighbor Bellatrix in Orion. But that's changing. 

Each planet orbits the sun at its own speed, moving eastward — to the left in the northern hemisphere — over the months and years. Jupiter travels at 29,230 mph (13 km/sec) and completes an orbit in just under 12 years. From Earth, we see the planet make a complete circle around the sky during that time as it travels through all 12 zodiac constellations, from Sagittarius to Gemini and back to Sagittarius. 

This is where the planets are today (Sept. 23). Jupiter is currently 441 million miles from the Earth while Saturn is more than twice as far at 887 million miles. in-the-sky.org

More distant Saturn orbits at 21,675 mph (9.7 km/sec) and takes 29 years to circle the sun. Because Jupiter is both closer to the Earth and travels faster it overtakes Saturn about once every 20 years, an event called a great conjunction.

Because the orbits of Jupiter and Saturn are tilted slightly with respect to Earth's orbit, 1.3° and 2.5° respectively, when they do line up the distance between them varies, making every conjunction different. If they were in exactly the same plane Jupiter would always pass directly in front of Saturn, but that's extremely rare. 

This diagram shows the decreasing separation between the two planets as faster Jupiter approaches Saturn this fall and then overtakes it on Dec. 21. Stellarium with additions by the author

If you're a conjunction watcher you know that the closer two bright objects appear in the night sky the more eye-catching the sight. This fall we'll all have ringside seats to Jupiter's slow approach to Saturn and their shrinking separation. 

On Dec. 21 they'll be closest at just 1/10th of one degree (0.1°).That's tight! Will we be able to split them apart with the naked eye or will binoculars be required? I suspect the unaided eye only because most of us can separate Mizar and Alcor, the star in the bend of the Big Dipper's handle, and they're only slightly farther apart.

Dec. 21 also happens to be the winter solstice. What a wonderfully cosmic way to brighten the longest night of the year! On that evening the two planets will shine from western Capricornus (one constellation over from Sagittarius) low in the southwestern sky at dusk. 

Whoa! Jupiter and Saturn through a telescope on the evening of Dec. 21, 2020. Stellarium

The view through a telescope should be nothing short of incredible. What a rare treat to see both the rings and moons of Saturn in the same field of view as Jupiter and its little family of satellites. I can't wait. Of course it will be cloudy.

The actual moment of closest approach occurs around noon Central Time on the solstice, but they'll be nearly as tight at dusk. Their last great conjunction occurred in June 2020 when Saturn and Jupiter were about 1° apart and challenging to spot in the solar glare before sunrise. This time around they'll be 10 times closer. You have to go all the way back to the year 1623 to find a great conjunction this squeaky tight.

Following the conjunction, the two planets will remain near each other for a week or two, but Jupiter moves on, and their separation increases. We won't see them face to face again until the summer of 2040.