People in the Northern Hemisphere are in for a once-in-a-millennium treat on December 21 — the winter solstice and the longest night of the year — when Jupiter and Saturn will meet in a “great conjunction,” the closest they will be seen in the sky together for nearly 800 years.Astronomers use the term “conjunction” to describe any time two celestial bodies appear to meet in the sky, as viewed from Earth. But when Saturn and Jupiter, the two largest planets in the solar system, appear to meet in the sky, they call it a “great” conjunction.Astronomers say the two planets regularly appear to pass each other in the solar system from Earth’s perspective, with their positions being aligned in the sky about once every 20 years.What makes this year’s great conjunction so significant is that the two planets have not passed this close to one another in 400 years, and it has been nearly 800 years since the alignment occurred at night, as it will this year, allowing nearly everyone around the world to witness it.Astronomers recommend finding an unobstructed view of the western sky about an hour after sunset. Jupiter will appear first and then Saturn; Jupiter will look like a bright star, and Saturn will be slightly fainter and will appear slightly above and to the left of Jupiter. Astronomers say the conjunction will be bright enough to be seen in cities.The conjunction will be the climax, but astronomers say observers do not have to wait until then. The two planets will be clearly visible in the west through December 25, though they will reverse positions in the sky after December 21, with the brighter Jupiter to the left. 
 
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