CAPE CANAVERAL, Fla.—Jupiter and Saturn will merge in the night sky Monday, appearing closer to one another than they have since Galileo’s time in the 17th century. Astronomers say so-called conjunctions between the two largest planets in our solar system aren’t particularly rare. Jupiter passes its neighbor Saturn in their respective laps around the sun every 20 years. But the one coming up is especially close: Jupiter and Saturn will be just one-tenth of a degree apart from our perspective or about one-fifth the width of a full moon. They should be easily visible around the world a little after sunset, weather permitting. Toss in the winter solstice in the Northern Hemisphere, the longest night of the year—and the summer solstice in the Southern Hemisphere—and this just-in-time-for-Christmas spectacle promises to be one of the greatest of Great Conjunctions. “What is most rare is a close conjunction that occurs in our nighttime sky,” …