On March 20, as on every vernal equinox, schoolchildren learn and the media proclaim that day and night are equal everywhere. It’s an appealing thought. Our planet, flying through space in a Charlie Chaplin wobble, momentarily, at 4:29 P.M. eastern time, tilts neither toward nor away from the sun. In theory this balancing act bestows a universal equilibrium. And in fact it is a time of approximate equality in most of the world, a far howl from the long nights and short days we’ve endured the past several months. But the truth is both more complex and more interesting than the common wisdom.