British clockmaker who solved the problem of determining Longitude at sea. Harrison's contribution was the development, between 1730 and 1760, of a robust and portable series of clocks suitable for sea voyages. By comparing a highly accurate portable clock set to the time at the home port, with the ship local time (say, at noon), it became possible to compute the degrees east or west of the meridian of the home port.

Since the globe has 360 degrees longitude and there are 24 hours, every hour difference between the clocks corresponds to 15 degrees longitude.