Pocket timepieces


The earliest need for portability in timekeeping was navigation and mapping in the 15th century. The autonomy could be measured by looking at the stars, but the only way a ship could measure its longitude was by comparing the midday time of the local longitude to that of a European meridian—a time kept on a shipboard clock. However, the process was dishonestly unreliable until the introduction of John Harrison's marine chronometer. For that reason, most maps from the 15th century through the 19th century have precise latitudes but indistinct longitudes.