A day—on any planet—is considered to be a full rotation. We’re used to this rotation taking around twenty-four hours—and so it seems bizarre to think of spending a day on Mercury, whose rotation lasts for nearly sixty Earth days. We’d be waiting a long, long time for morning to come. (Funnily enough, we can experience a rather similar phenomenon by traveling to Antarctica, where the sun stays below the horizon for the whole winter.) Due to Mercury’s orbit around the sun, a year on that planet is the equivalent of around eighty-eight Earth days—which means that on Mercury, there are less than two days in a year. But that’s not all: due to the planet’s strange orbit, the sun actually seems to travel backwards and forwards in the sky.