Just after sunset on the evening of December 21, 2020, Jupiter and Saturn appear closer together in the earth̵
“Adjustments between these two planets are quite rare and occur once every 20 years or so, but this correlation is unusually rare because of how close the planets will appear to each other,” said astronomer Patrick Hartigan of Rice University. “You will have to go all the way back to just before 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 Earth’s sky since the summer. From 16.-25. December, the two will be separated by less than the diameter of a full moon.
“On the evening of the nearest approach on December 21, they will look like a double planet, separated by only 1/5 of the full moon’s diameter,” said Hartigan, a professor of physics and astronomy. “For most viewers on telescopes, every planet and several of their largest moons will be visible in the same field of view that night.”
Although the best visibility conditions will be near the equator, the event will be observable anywhere on earth, weather permitting. Hartigan said the planet duo appears low in the western sky for about an hour after sunset each night.
“The further north a viewer is, the less time they get a glimpse of the context before the planets sink below the horizon,” he said. Fortunately, the planets will be bright enough to be seen in twilight, which may be the best time for many American viewers to observe the connection.
“By the time the sky is completely dark in Houston, for example, the connection will be only 9 degrees above the horizon,” Hartigan said. “Seeing it would be manageable if the weather cooperates and you have an unobstructed view to 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 and 5.3 degrees, respectively. Viewers there and at similar latitudes would do well to catch a glimpse of the rare astronomical sight as soon after sunset as possible, he said.
Those who prefer to wait and see Jupiter and Saturn close together and higher in the night sky should hold on until March 15, 2080, Hartigan said. Then the couple first comes out like this, before a while after the year 2400.