Longitude: A journey through time, astronomy, and horology

by Dava Sobel

The measurement of longitude meridians, in comparison, is tempered by time. To learn one’s longitude at sea, one needs to know what time it is aboard ship and also the time at the home port or another place of known longitude—at that very same moment. The two clock times enable the navigator to convert the hour difference into a geographical separation.

The moon travels a distance roughly equal to its own width every hour.

This power over purse strings made the Board of Longitude perhaps the world’s first official research-and-development agency.