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. 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," said Vanderbilt University’s David Weintraub, an astronomy professor. "I think it’s fair to say that such an event typically may occur just once in any one person’s lifetime, and I think ‘once in my lifetime’ is a pretty good test of whether something merits being labeled as rare or special.”