by Robert Vanderbei
The conjunction of Jupiter and Saturn will take place in the late afternoon and early evening of Monday, December 21. On that day, the Sun will set at 4:35pm.At that time, Jupiter and Saturn will be low in the southwestern sky, just 20degrees above the horizon. Also, at sunset, the sky is still bright like daytime. For those observers with binoculars, it is possible to find Jupiter at sunset. But, it’s a challenge. Fortunately, darkness comes quickly and by 4:45pm, or maybe 4:50pm, Jupiter should be easy to find in binoculars and also findable without binoculars. At 4:50pm, Jupiter and Saturn will still be 18 degrees above the horizon. It’s low but not terribly low. The separation between the two planets will be just roughly 6 arcminutes. So, for those who have a telescope the conjunction will be an awesome sight. And, for those members who are into astrophotography, it will be a once in a lifetime opportunity to take pics of the event.
But, there will be some serious challenges. Jupiter will be 2.5 magnitudes brighter than Saturn. That’s a factor of 10 times brighter. So, a photograph that nicely shows Jupiter will have Saturn looking very dark. To make a good picture, one will need to take images with different exposures and then do some sort of “high dynamic range (aka HDR)” combo of the images. In addition to Jupiter and Saturn themselves, we’ll also get to see some of their moons. But, the moons will be even fainter and that makes the HDR imaging an even bigger challenge. Also, the fact that this event will only be about 18 degrees above the horizon will mean that the atmospheric “seeing” is likely to be bad. Shown here are two screenshots from the planetarium program Cartes du Ciel (aka Sky Charts) showing how things will look at 4:50pm. One picture just shows Jupiter, Saturn, and their moons. The other picture shows the various stars that are also in this field of view. Jupiter has four bright moons. From left to right they are Callisto, Io, Ganymede, and Europa. Saturn has lots of moons. From left to right, they are Lapetus, Hyperion, Rhea, Mimas, Enceladus, Dione, Tethys, and Titan. Of Saturn’s moons, Hyperion is the dimmest. It’s magnitude 14.9. That magnitude can be seen in, say, 10 second long astrophotographs, but is not visible visually through most amateur telescopes. And, the not completely dark sky will also be a problem. Saturn’s brightest moon is Titan at magnitude 9.0. If the skies are dark enough, that moon could be seem visually through a telescope. The four moons of Jupiter are all about magnitude6 and things of that magnitude do appear in astrophotographs taken at dusk.