How easy to take for granted the grid lines on every map and globe. Yet the entire Age of Exploration took place without accurate measurements. Once sailors lost sight of land, they were truly adrift. How could sailors more accurately plot their location, and so save many ships from sinking? They needed a precise way to tell time. Self-educated clockmaker John Harrison took forty years to develop one, and his invention, in time, changed the world. An engrossing tale, well-told.