In December, Jupiter and Saturn will appear as a double planet for the first time since the Middle Ages

Immediately after sunset, on the evening of December 21, Jupiter and Saturn will appear closer in the Earth’s night sky than they have been since the Middle Ages, offering people around the world a heavenly pleasure to play on the winter solstice.

“Alignments between these two planets are quite rare, occurring once every 20 years or so, but this conjunction is exceptionally rare because of how close the planets will appear,” said Rice University astronomer Patrick Hartigan. “You should go back until dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky.”

Jupiter and Saturn have been approaching each other in the terrestrial sky since the summer. From December 16 to December 25, the two will be separated by less than the diameter of a full moon.

“On the evening of the closest approach, December 21, they will look like a double planet, separated only by 1/5 the diameter of the full moon,” said Hartigan, professor of physics and astronomy. “For most telescope viewers, each planet and many of their larger moons will be visible in the same field of view that evening.”

Even if the best viewing conditions will be near the equator, the event will be observable anywhere on Earth, weather permitting. Hartigan said the planetary duo will appear low in the western sky for about an hour after sunset each evening.

“The further north a spectator is, the less time he will have to glimpse the conjunction before the planets sink below the horizon,” he said. Fortunately, the planets will be bright enough to be seen at dusk, which may be the best time for many US viewers to observe the conjunction.

“When the sky is completely dark in Houston, for example, the conjunction will be just 9 degrees above the horizon,” Hartigan said. “Vision that would be manageable if the weather cooperates and you have an unobstructed view to the southwest.”

But an hour after sunset, people looking skyward in New York or London will find the planets even closer to the horizon, at around 7.5 degrees and 5.3 degrees, respectively. Viewers there, and at similar latitudes, would do well to catch a glimpse of the rare astronomical spectacle as soon as possible after dark, he said.

Those who prefer to wait and see Jupiter and Saturn so close and higher in the night sky will have to stay until March 15, 2080, Hartigan said. After that, the couple won’t make such an appearance until sometime after the year 2400.