Jupiter, Saturn will look like a ‘double planet’ on December 21, they won't be this close again until 2080
The last time Jupiter and Saturn looked so close from Earth’s vantage point was nearly 800 years ago. You will have to wait another 60 years to see this again
Jupiter and Saturn have been traveling across the sky together all year, but this month, get ready for them to put on a show that has not happened in almost 800 years. On December 21, the two largest planets in our solar system will appear closer to each other in the sky than they have been since the Middle Ages.
The planets will be so close to each other that it will be extremely difficult, if not completely impossible to see them separately with naked eyes. This will make them appear to be overlapping. In other words, they will look like a “double planet.” The last time they looked this close from Earth’s vantage point was on March 4, 1226, according to Patrick Hartigan, a professor of physics and astronomy at Rice University, US.
The 21st is also the date of the December solstice, which is the winter solstice in the Northern Hemisphere and the summer solstice in the Southern Hemisphere.
What’s this phenomenon?
When two planets are seen close to each other in the sky the event is called ‘conjunction’ of planets. A conjunction of the giants Jupiter and Saturn is called “great conjunction.”
Conjunction occurs every 20 years this century as the orbits of Earth, Jupiter, and Saturn periodically align making the two outer planets appear close together in the nighttime sky. Even then, this is considered to be an exceptionally rare event. “Alignments between these two planets are rather rare, occurring once every 20 years or so, but this conjunction is exceptionally rare because of how close the planets will appear to one another,” explains Hartigan.
According to Arvind Paranjpye, Director of the Nehru Planetarium, Nehru Centre, India, this is the second closest conjunction of these two planets since the invention of the telescope in 1609. The first one was on July 16, 1623, 14 years after Galileo Galilei made his first telescope.
The two planets will not appear this close in the sky until March 2080. “The major challenge there is you’ll have to stay alive for another 60 years to see it,” adds Hartigan.
Where to look for the planets?
Over the first three weeks of December, you can watch each evening as the two planets get closer in the sky. And on December 21, the planets will appear just a tenth of a degree (0.1) apart. This implies that the planets and their moons will be visible in the same field of view through binoculars or a small telescope. Saturn will also appear as close to Jupiter as some of Jupiter’s moons.
“From December 16-25, the two will be separated by less than the diameter of a full moon. On the evening of closest approach on December 21, they will look like a double planet, separated by only 1/5th the diameter of the full moon. For most telescope viewers, each planet and several of their largest moons will be visible in the same field of view that evening,” says Hartigan.
While the best viewing conditions will be near the equator, the event can be observed from anywhere on Earth if the weather permits. Hartigan explains the planetary duo will appear low in the western sky for about an hour after sunset each evening. He suggests that to be successful in observing this conjunction, a clear southwestern horizon with no low clouds in the distance is needed.
“No matter where you are, there is maybe an hour or so to observe this conjunction before the planets sink into the haze. The further north a viewer is, the less time they’ll have to catch a glimpse of the conjunction before the planets sink below the horizon. Fortunately, the planets will be bright enough to be viewed in twilight, which may be the best time for many US viewers to observe the conjunction. By the time skies are fully dark in Houston, for example, the conjunction will be just 9 degrees above the horizon. Viewing that would be manageable if the weather cooperates and you have an unobstructed view to the southwest,” emphasizes Hartigan.
The astronomer further says, “But an hour after sunset, people looking skyward in New York or London will find the planets even closer to the horizon, about 7.5 degrees and 5.3 degrees respectively. Viewers there, and in similar latitudes, would do well to catch a glimpse of the rare astronomical sight as soon after sunset as possible.”
Hartigan believes that observers without a telescope may find it a challenge to resolve both planets, but it can be done. “This is an event that could be impressive to see, but you will have to be prepared and binoculars will likely be very helpful for seeing it well in most skies. If you can set up a small telescope on them before it gets fully dark that will be optimal, as then you will be able to see both planets together in the field of view along with Saturn’s rings, and the brightest moons of both planets,” he recommends.