Just after sunset on the evening of December 21, 2020, Jupiter and Saturn they will appear closer to each other in the night sky on Earth than they have been since the Middle Ages, offering people a celestial feast to ring during the winter solstice.
“Alignments between these two planets are quite rare, happening once every 20 years or so, but this combination is extremely rare because nearby planets will look next to each other,”
Jupiter and Saturn have been approaching each other in the Earth’s sky since the summer. From December 16 to 25, the two will be separated by less than the diameter of a full moon.
“On the evening of the closest approach on December 21, they will look like a double planet divided by only 1/5 of the diameter of the full moon,” said Hartigan, a professor of physics and astronomy. “For most telescope viewers, each planet and several of its largest moons will be visible in the same field of view tonight.”
Although the best viewing conditions will be near the equator, the event will be observable anywhere on Earth, weather permitting. Hartigan said the planetary duo would appear low in the western sky for about an hour after sunset each night.
“The further north the viewer is, the less time they will have to see the connection before the planets sink below the horizon,” he said. Fortunately, the planets will be bright enough to be seen in the dark, which may be the best time for many American viewers to observe the connection.
“As long as the sky is completely dark in Houston, for example, the connection will be just 9 degrees above the horizon,” Hartigan said. “The view would be manageable if the weather worked together and you had an unobstructed view of the southwest.”
But an hour after sunset, people looking at the sky 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 such latitudes would do well to catch a glimpse of the rare astronomical sight as soon as possible after sunset, he said.
Those who prefer to wait and see Jupiter and Saturn so close together and higher in the night sky will have to stay until March 15, 2080, Hartigan said. After that, the couple will not appear until 2400.