Main Idea and Its Support

The Longitude Problem

A ship's latitude — how far north or south it is — can be found from the height of the sun at noon. Sailors could do that for centuries. Longitude, how far east or west, was the problem. It has no natural marker. The solution was known in principle: compare the time on board with the time at a fixed place. The Earth turns fifteen degrees an hour, so every hour of difference is fifteen degrees of longitude. That required a clock that stayed accurate on a rolling, salty, temperature-swinging ship. No such clock existed. John Harrison, a carpenter, spent forty-three years building four. The fourth, finished in 1759, was the size of a large pocket watch and lost five seconds in eighty-one days at sea.

Why Clocks Agree

Until the 1840s, every British town kept its own time, set by the sun. Noon in Bristol was about ten minutes after noon in London. That was no problem while the fastest thing on the road was a horse. The railways changed it. A timetable cannot work if every station measures noon differently, and trains sharing a track need to agree about when. So the railway companies imposed London time on their whole network. People called it railway time. Some towns kept two minute hands on the town clock for years, one local and one railway. In 1880 Parliament made a single time legal across the country. The sun had been overruled by the timetable.