Astronomers use the word conjunction to describe meetings of planets and other objects on our sky’s dome. They use the term great conjunction to describe meetings of the two biggest worlds in our solar system, mighty Jupiter and the glorious ringed planet Saturn. The next great conjunction of Jupiter and Saturn will be December 21, 2020. That date is, coincidentally, the date of the December solstice. It’ll be the first Jupiter-Saturn conjunction since the year 2000, and the closest Jupiter-Saturn conjunction since 1623, only 14 years after Galileo made his first telescope. However, that conjunction was only 13 degrees east of the sun (closely following the sun at sunset), and it is considered unlikely that it was noticed by many. The closest observable Jupiter-Saturn conjunction before that was as long ago as during medieval times, in 1226! At their closest in December, Jupiter and Saturn will be only 0.1 degree apart. That’s just 1/5 of a full moon diameter.
The extra-close Jupiter-Saturn conjunction in 2020 won’t be matched again until the Jupiter-Saturn conjunction of March 15, 2080.
But don’t wait until December to start watching these worlds. They’re visible tonight and every night – near each other for the rest of 2020 – an appealing and mind-expanding sight!
At the 2000 great conjunction, 20 years ago, Jupiter and Saturn were near the sun in our sky and difficult to observe. We’re due for a more observable great conjunction, and we’ll get one. In November, Jupiter and Saturn are noticeable for their nearness to each other, and they are in the sky when darkness falls.
By December, Jupiter and Saturn will still be easily visible. They’ll be, if anything, even more beautiful than now, in the western twilight shortly after sunset.
You’ll recognize Jupiter and Saturn easily, from now through the end of the year. Jupiter is brighter than any star. Saturn is not as bright as Jupiter, but it’s as bright as the brightest stars and shines with a distinctly golden color. Also, Jupiter is near Saturn! Saturn is just to the east of Jupiter on the sky’s dome. Unlike the twinkling stars, Jupiter and Saturn both shine steadily.
This month – during the period when the moon swept past Jupiter and Saturn (about November 16 to November 21) – the two gas giant planets are some 3 degrees apart.
Over the coming month – from November 21 to the day of the conjunction itself, December 21 – Jupiter will travel about 6 degrees and Saturn 3 degrees on the sky’s dome. That movement will mean that Jupiter bridges the 3-degree gap between itself and Saturn.
Bingo! Great conjunction!
Saturn, the sixth planet outward from the sun, is the farthest and slowest-moving planet that we can easily see with the eye alone. Dazzling Jupiter, the fifth planet outward from the sun, is the second-slowest bright planet, after Saturn.
For that reason, Jupiter/Saturn conjunctions are the rarest of bright-planet conjunctions, by virtue of their slow motions in front of the constellations of the zodiac. Saturn takes nearly 30 years to go around the sun full circle whereas Jupiter takes nearly 12 years.
Thus, every 20 years, Jupiter catches up to Saturn as viewed from Earth.
From the years 2000 to 2100 inclusive, as viewed from our planet Earth, these Jupiter/Saturn conjunctions (in ecliptic longitude) happen on these dates:
May 28, 2000
December 21, 2020
October 31, 2040
April 7, 2060
March 15, 2080
September 18, 2100
Now, here’s why these great Jupiter/Saturn conjunctions happen every 20 years. Each year, Saturn completes about 12 degrees of its orbit around the sun, whereas Jupiter completes about 30 degrees. Therefore, in one year, Jupiter closes the gap between itself and Saturn by about 18 degrees (30 – 12 = 18 degrees).
In a period of 20 years, then, Jupiter gains 360 degrees on Saturn (18 x 20 = 360 degrees), therefore lapping the ringed planet once every 20 years.
So start watching Jupiter and Saturn now! And mark your calendar for the great conjunction of Jupiter and Saturn on December 21, 2020.
Bottom line: The upcoming great conjunction of Jupiter and Saturn will be on December 21, 2020, the day of the December solstice. These two worlds will be visibly closer in our sky than they’ve been since medieval times, in 1226. At their closest, Jupiter and Saturn will be only 0.1 degree apart. That’s just 1/5 of a full moon diameter. Charts and more info here.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.