By the time we get to the winter solstice on Monday, December 21, Jupiter and Saturn will appear only 0.1 degrees apart, which is just 1/5 of a full moon diameter. In fact, they will appear so close together that Saturn will actually look to us as close to Jupiter as some of its own moons. The two planets may appear as a “double planet” to some depending on local viewing conditions or as one very bright star to others in the low southwest sky shortly after sunset. Some are already referring to this great conjunction of Jupiter and Saturn as the “Christmas Star” because of the timing just a few days before Christmas Day. Note- some Biblical scholars believe the “Star of Bethlehem” was a triple conjunction made up of the following:
Jupiter (known as the king planet)
Venus (the brightest planet in our solar system)
The star Regulus (known as the kingly star) in the constellation Leo.
Jupiter and Saturn should be visible within the same field-of-view on Monday, December 21st whether using binoculars or a small “backyard” telescope along with four of Jupiter’s largest moons spread out in a straight line. Beyond December 21, Jupiter and Saturn will move away from each other rather quickly in terms of appearance to us here on Earth and this trend will continue for the next ten years before they start to converge again during the 2030s. This will set up the next “Great Conjunction” of the year 2040; however, that one will not be as brilliant as this one. In fact, the two planets won’t appear this close again until the year 2080 and in that late 21st century close encounter, Jupiter will completely cover Saturn which is an extremely rare event and won’t happen again until the year 7541.
Make sure to mark your calendars specifically for Monday, December 21st to view this once-in-a-lifetime celestial event as this opportunity will be just a one-day affair. Just one day before and one day after the “Great Conjunction”, the planets will appear noticeably farther apart from each other and nowhere near as striking as on December 21st – hopefully skies will cooperate on the first day of astronomical winter. The best viewing on Monday, December 21st will be from about 30 minutes to an hour after sunset in the low southwestern sky until the time the two planets set (sunset time in Philly on 12/21 is 4:39 PM; the two planets will set by about 8:23 PM). And as for the crescent moon, this too is a one-time opportunity as it will be visible tonight only – depending on sky conditions – in the same region of the sky as Jupiter and Saturn.
For info on how best to photograph Monday’s “Great Conjunction” visit this NASA web site.
Meteorologist Paul Dorian
Video discussion (recorded on December 10th):