Posted 8:20 a.m. Monday, Dec. 14, 2020

UWL Physics Professor Bob Allen says the upcoming conjunction of Jupiter and Saturn just before Christmas has some calling it “The Christmas Star.” He says all eyes will focus on the astronomical rarity: these two planets will be visibly closer in the sky than they’ve been since medieval times.

UWL astronomer says the celestial show will be unusual

So will the close conjunction of Jupiter and Saturn Monday, Dec. 21, re-enact the Christmas Star the wisemen saw in the Bible story of Jesus’ birth? Retired UWL Physics Professor Bob Allen might have the answer.

There are some reasons why the "Christmas Star" comparison comes up, says Allen.

“As I always said at our planetarium presentations about it, we don't have a video recording — or very good astronomical, historical, or biblical records of it — so nobody knows for sure what it was,” he explains.

That said, Allen knows some things it wasn’t. Over the years, astronomers have ruled out a comet, a supernova and a meteor. But the upcoming conjunction of the two planets just before Christmas has the focus on the astronomical rarity.

“It could have been a ‘miracle’ and no physical explanation is needed,” reasons Allen. “But one possibility is a planetary conjunction.”

Allen has suggested that possible Christmas Star explanation since he started giving Planetarium shows at UWL in 1969. One is a "triple conjunction" of Jupiter and Saturn in 7 and 6 BC in Pisces.

“They weren't as close together then as they will be this time, but this one also is not happening near them being at opposition to the Sun — as seen from Earth,” he explains. “So this is just a single conjunction.”

Allen says around 1980 some records were found that pointed to another possibility: a conjunction of Venus and Jupiter in Leo near the star Regulus in 3 and 2 BC. 

You can see Jupiter and Saturn any evening in December, but they’ll be especially striking around Dec. 16 and 17, when the young moon will sweep past them.

While Jupiter passes by Saturn about every 20 years, their separation can be up to about four degrees, says Allen. On Dec. 21, the day of solstice and the shortest daylight day of the year in the Northern Hemisphere, they will be about 1/10 of a degree apart, so they will be very close and in the same binocular and telescope field of view. But people with good eyesight should still see them as separate objects.

Allen says the last time the two planets were this close was about 400 years ago, in 1623. At that time, they were too close to the Sun to actually be viewed. It was around 800 years ago, 1226, when they were this close and could have been viewed similar to this year’s show. And you might want to catch it now. The conjunction won’t happen again until March 15, 2080.

Allen encourages even novice star gazers to look for Jupiter and Saturn in the southwest sky during twilight in the coming days. The two are setting around 7 p.m. — so look to the skies right after dusk. By Dec. 21, they will set around 6:30 p.m.  

“They are bright enough to see in the city,” he notes, adding that a fairly clear view to the southwest without buildings, trees or obstruction will be key. 

Allen notes that only the Moon and Venus, and sometimes Mars, are brighter than Jupiter. Saturn always looks like a fairly bright star, he says, with Saturn just left of Jupiter. “It would be best to watch them any clear night between now and the 21st to watch them draw closer,” he says.


The La Crosse Area Astronomical Society is holding observing sessions open to the public at 4:30 p.m. Friday, Dec. 18, and Monday, Dec. 21. If it’s cloudy on Dec. 18, the viewing will take place the following evening. Observations will be at Ridge History Park, N1724 Korn Clements Road, just west of Middle Ridge on the south side of Highway 33.

Discover more about the “Christmas Star”:

https://earthsky.org/human-world/was-the-christmas-star-real

Even more:

https://earthsky.org/astronomy-essentials/great-jupiter-saturn-conjunction-dec-21-2020#:~:text=Saturn%20has%20been%20closely%20following,night%20for%20most%20of%202020.&text=The%20next%20great%20conjunction%20of,conjunction%20since%20the%20year%20162