And even Mercury had its settlement. Although it had to be admitted that for most purposes, Mercury was too close to the sun. Its day lasted fifty-nine Terran days, its year eighty-eight Terran days, so that three of its days equaled two years, a pattern that was not a coincidence but a node on the way to being tidally locked, like Luna around the Earth. The combination of these two spins gave Mercury a very slow roll through its solar day, during which the brightside hemisphere became much too hot, while the nightside hemisphere became extremely cold. The lone city currently on the planet was therefore a kind of enormous train, running around the planet on tracks set on the northern forty-fifth latitude. These tracks were made of a metalloceramic alloy that was the first of the Mercurial physicists’ many alchemical tricks, a matrix that withstood the eight-hundred-K heat of midbrightside. The city itself, called Terminator, then ran over these tracks at a speed of about three kilometers per hour, which kept it within the planet’s terminator, the zone of predawn shadow that was in most terrain about twenty kilometers wide. A slight expansion of the tracks exposed to the morning sun farther to the east drove the city ever westward, as it rested on tightly fitting sleeves shaped to slide the city away from the expansion. This motion was so inexorable that resistance to it in another part of the sleeves generated great amounts of electrical power, as did the solar collectors trailing the city, and set on the very top of the high Dawn Wall, catching the first blasting rays of sunlight. Even in a civilization where energy was cheap, Mercury was amazingly blessed. And so it joined the worlds farther out, and became one of the brightest of all. And a hundred new floating worlds opened every year— cities in flight, little city-states, each with its own charter, settler mix, landscape, style.