IN THE GREAT COURT of Trinity there was a sundial Isaac Newton didn't like: a flat disk divided by labeled spokes with a gnomon angling up from the center, naïvely copied from Roman designs, having a certain Classical elegance, and always wrong. Newton was constructing a sundial on a south-facing wall, using, as gnomon, a slender rod with a ball on the end. Every sunny day the ball's shadow would trace a curve across the wall – a slightly different curve every day, because the tilt of the earth's axis slowly changed through the seasons. That sheaf of curves made a fine set of astronomical data but not a usable timepiece. To tell time, Isaac (or his faithful assistant, Daniel Waterhouse) had to make a little cross-tick at the place the gnomon's shadow stood when Trinity's bell (always just a bit out of synchronization with King's) rang each of the day's hours. In theory, after 365 repetitions of this daily routine, each of the curves would be marked with ticks for 8:00 a.m. , 9:00 a.m. , and so on. By connecting those ticks – drawing a curve that passed through all of the eight o'clock ticks, another through all of the nine o'clock ticks, and so on – Isaac produced a second family of curves, roughly parallel to one another and roughly perpendicular to the day curves.