Something amazing is happening in the southwestern sky right now. Two planetary lights are on course to nearly merge into one. Jupiter and Saturn, which have been drawing closer together since September, are just days before their Great Conjunction, an event that happens every 20 years.

The current Great Conjunction is the closest one observed since 1226 when the two planets were separated by less than a tenth the apparent diameter of the full moon at dawn.
(Stellarium)
The current Great Conjunction is the closest one observed since 1226 when the two planets were separated by less than a tenth the apparent diameter of the full moon at dawn. (Stellarium)

This conjunction is "greater" than most, even historic, because the last time they came this close was July 16, 1623. No records exist of anyone seeing that event most likely because the planets were hidden in the solar glare low in the west just after sunset. The last observable closer pairing of the two gas giants occurred on March 5, 1226 in the morning sky — almost 800 years ago!

Viewing circumstances for the current Great Conjunction are favorable. All you need are a pair of eyes and a location with a wide open view of the southwestern sky. The best time to see them is between 45 and 90 minutes after sundown. For many skywatchers that falls between 5:15 and 6:15 p.m. local time. Click here to find out your sunset time so you can plan accordingly.

This scene depicts Jupiter and Saturn at their closest separation on the evening of December 21st about an hour 15 minutes after sunset low in the southwestern sky. (Stellarium)
This scene depicts Jupiter and Saturn at their closest separation on the evening of December 21st about an hour 15 minutes after sunset low in the southwestern sky. (Stellarium)

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You'll notice Jupiter first because it's considerably brighter (magnitude -2) than Saturn (0.6). I last saw them on Dec. 14 when they were 0.75° (1.5 full-moon diameters) apart. Snuggled together like parent and child at storybook time, the two little lights made a beautiful impression. The duo was closer yet on Dec. 16 at 0.5°, equal to one moon diameter, and their separation shrinks to a minimum of just 1/10th of one degree on Monday night, December 21st. Despite nearly merging, the planets' combined light increases by only 9 percent, too little for the eye to detect.

This simulated view depicts Jupiter and Saturn through the telescope around 5:30 p.m. Central Time on December 21st. Despite appearances, Jupiter is 724 million kilometers closer to us than Saturn. North is at upper right.
(Stellarium)
This simulated view depicts Jupiter and Saturn through the telescope around 5:30 p.m. Central Time on December 21st. Despite appearances, Jupiter is 724 million kilometers closer to us than Saturn. North is at upper right. (Stellarium)

We should be able to split the two apart without optical aid, but you'll have to look closely. I don't think they'll be obvious at a casual glance. Bring along binoculars just to be sure. As long as you have a pair take a closer look at Jupiter. You should be able to see one or two of its four bright moons. A small telescope will show all four along with Saturn's rings and its brightest moon Titan. Observing both planets simultaneously in the same telescopic field of view is a rare sight — many amateur astronomers are eagerly anticipating the opportunity.

On Dec. 16 (Wednesday evening), the fresh-faced crescent moon returned to the evening sky just in time to join the Jupiter-Saturn pair in a stunning celestial trio. Even your mobile phone will suffice for a photo of the grouping though a tripod-mounted camera is still the best option. If you're using the latter, a standard 35mm or 50mm lens will capture the scene nicely. After using the live view feature on the camera back to carefully focus on the planets, set the ISO to 400 (or 800 for "slower" lenses) and expose from 2-5 seconds.

Jupiter and Saturn's ever-changing alignment and separation will make for several nights of enjoyable viewing. On Dec. 16th the crescent moon adds its own spice to the scene. (Stellarium with additions by the author)
Jupiter and Saturn's ever-changing alignment and separation will make for several nights of enjoyable viewing. On Dec. 16th the crescent moon adds its own spice to the scene. (Stellarium with additions by the author)

Sometimes clouds get in the way, so don't wait until the 21st. The planetary pair will be strikingly close through Christmas. Seize the next opportunity. Remember that the sky doesn't have to be perfectly clear either. Light clouds can sometimes enhance the view, creating soft, glowing halos of light around each planet that look especially pretty in the camera.

The planets orbit the Sun in approximately the same plane. Conjunctions occur when two or more of them line up across our line of sight as Jupiter and Saturn will on Dec. 21. Not to scale. (JPL Horizons with NASA images)
The planets orbit the Sun in approximately the same plane. Conjunctions occur when two or more of them line up across our line of sight as Jupiter and Saturn will on Dec. 21. Not to scale. (JPL Horizons with NASA images)

Conjunctions of Jupiter and Saturn occur every 20 years because that's how long it takes Jupiter, the faster planet, to catch and pass Saturn. The planets' separation varies at each Great Conjunction because both Jupiter's and Saturn's orbits are slightly tilted with respect to the Earth's, 1.3° and 2.5° respectively. As we look across the solar system in their direction, their orbits line up along nearly the same line of sight and we see them pair up.

Very rarely, their orbits align precisely with Earth's, and Jupiter passes directly in front of Saturn, occulting it. The last near-central occultation occurred in 6857 BC and the next will be in AD 7541. The next Great Conjunction occurs on November 4, 2040 with the planets 1.2° apart in Virgo at dawn. Not until March 15, 2080 will a closer pairing occur. That morning, Jupiter and Saturn will be just as tight as in the current conjunction. After that, they won't approach as closely again until sometime after the year 2400.

Coming so close to Christmas, the conjunction naturally invites comparison to the biblical Christmas Star that heralded the birth of Christ. Assuming the star refers to a real astronomical event, possibilities include a nova or supernova, a bright comet or a close conjunction of two bright planets. Evidence is scant or nonexistent for a bright comet or nova eruption at the time, but we do know of several notable conjunctions.

On August 12th of the year 3 BC, Jupiter and Venus were shoulder to shoulder at dawn in the eastern sky as seen from Bethlehem. They squeezed even closer in the western sky on June 17 in 2 BC, so close they would have appeared as a single brilliant "star" at dusk. To our best knowledge Christ was born between 4 and 6 BC, making it hard to know for sure what the Wise Men may have witnessed.

I took this photo on Dec. 14 when the two were just 0.75° apart. What a pretty sight! (Bob King)
I took this photo on Dec. 14 when the two were just 0.75° apart. What a pretty sight! (Bob King)

Every day I check the forecast hoping for a clear night so I can see what Jupiter and Saturn are up to. I love the slow-motion show. And how serendipitous that the Great Conjunction occurs on the winter solstice, when the sun reaches its lowest point in the sky, and nights are longest. The planets have come together just in time to buoy our spirits with light and dish up a full serving of heavenly joy.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.