On the evening of the Winter Solstice (Dec. 21), there will be a rare event as Jupiter and Saturn come together in a Great Conjunction. These two giant planets have been bright in our evening skies since the summer and slowly moving closer together. And they are still easy to spot. Head out on Dec. 1 and look low in the southwest in the twilight skies after the sun sets around 4:30. Jupiter is the brightest object in that part of the sky. Saturn is dimmer and more yellowish above and to the left. They are only 2 degrees apart. That’s the width of four full moons or your index finger held at arm’s length. Back in July, when so many of us were outside being awed by Comet NEOWISE, they were over 6 degrees apart. They should be visible until they set around 7:45. Use your finger as a guide each clear evening to measure them moving ever closer together. On Dec. 16 and 17, the waning crescent moon passes by them, and they are less than 1 degree apart. By Dec. 21, only a 10th of a degree separates them. That’s about a fifth of the width of the full moon. Test your eyesight. Do you see one planet or two? Be sure to savor the view in binoculars if you have them. If you are able to look through a telescope before they set around 6:45, you will see Jupiter with its four brightest moons and Saturn with its rings in a single field of view. Continue to watch the following evenings as Jupiter moves left and above Saturn as they draw apart. As 2020 ends on Dec. 31, they will be a degree (about one pinkie finger) apart.
Great Conjunctions happen only once every 20 years. The last one was May 31, 2000, but was very difficult to see because Jupiter and Saturn were so close to the sun and rose just a half hour before sunrise. They have not appeared so close together since July 16, 1623, the year Anne Hathaway died and Shakespeare’s First Folio was published. But skygazers would have struggled to see them since they set soon after the sun. The last Great Conjunction that was so close and easily visible was almost eight centuries ago on March 5, 1226, the year Saint Francis of Assisi died. We can look forward to Great Conjunctions on Nov. 5, 2040, April 10, 2060, and March 15, 2080.
The closer a planet is to the sun, the less time it takes to orbit. Earth takes a year, Jupiter takes 12 years, and Saturn takes 30 years. In the time it takes Earth to round the sun 60 times, Saturn has only gone around twice and Jupiter five times passing between Earth and Saturn three times. When Jupiter “laps” Saturn every 20 years, a Great Conjunction is visible from Earth. It’s a syzygy with Earth, Jupiter and Saturn all in a line.
Enjoy the Winter Solstice Great Conjunction of 2020. Like transits and eclipses and auroras, there are only so many in a lifetime.
John Heasley is an astronomy educator and stargazer who enjoys connecting people with the cosmos. For more information about stargazing in southwestern Wisconsin, like Driftless Stargazing LLC on Facebook and find out whenever there’s something awesome happening in the skies above.