Go outside after sunset this month and face north. High overhead, you’ll find Ursa Major the Great Bear performing a back flip over the North Star. It’s also the time of year when the constellation’s Big Dipper (or Plow) asterism, a stunning collection of seven bright naked-eye stars of near-equal magnitude, appears to spill its contents over the thirsty Earth — “a kind of celestial metaphor,” the late Jack Horkheimer liked to say, for the nurturing power of spring.