Just after sunset on the evening of Dec. 21, Jupiter & Saturn will appear closer together in Earth’s night sky than they are here since the middle Ages, offering people the world over a celestial treat to ring in the winter solstice.
“Alignments between these 2 planets are rather rare 2 occurring just once every 20 years approximately but this conjunction is exceptionally rare because of close the planets will appear to one another,” said Rice University astronomer Patrick Hartigan. “You’d need to go all the way back to just before dawn on March 4,1226 to see a closer alignment between these 2 objects visible in the night sky.”
Jupiter & Saturn are approaching each other in Earth’s sky since the summer. From Dec. 16 to Dec. 25, the 2 are going to be separated by less than the diameter of a full moon.
“On the evening of closest approach on Dec 21, they’re going to appear as if a double planet separated by only 1/5th the diameter of the full moon,” said Hartigan, a professor of physics & astronomy. “For most telescope viewers, each planet & several of their largest moons are going to be visible in the same field of view that evening.”
Though the best viewing conditions are going to be near the equator, the event are going to be observable from 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 viewer is the less time they’ll need to catch a glimpse of the conjunction before the planets sink below the horizon,” he said. Fortunately, the planets are going to be bright enough to be viewed in twilight which may be the best time for many U.S. viewers to observe the conjunction.
“By the time, skies are fully dark in Houston, for instance, the conjunction are going to be just 9 degrees above the horizon,” Hartigan said. “Viewing that might be manageable if the weather cooperates & you’ve got 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 about 7.5 degrees & 5.3 degrees respectively. Viewers there & at similar latitudes would do best to catch a glimpse of the rare astronomical sight as soon after sunset as possible, he said.
Those who like better to wait and see Jupiter & Saturn this close and higher in the night sky will got to stick around until March 15,2080, Hartigan said. After that, the pair won’t make such an appearance until sometime after the year 2400.