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Conjunction of Jupiter (upper right) and Saturn (lower left), known as a great conjunction, photographed on 21 December 2020. A great conjunction is when these two planets appear close together in the night sky, and occurs around once every 20 years. This conjunction was the closest the two planets had passed one another in the sky for nearly 400 years. (KEYSTONE/SCIENCE PHOTO LIBRARY/DAMIAN PEACH)