'Methods of Ascertaining the Longitude', c 1850
© Science Museum / Science & Society Picture Library
One of a set of teaching cards published by James Reynolds & Sons, with a chart drawn and engraved by John Emslie. The diagram shows three astronomical methods for discovering longitude (east-west distance) on the Earth. These include the timing of the occultation (disappearance) of a star behind the Moon, timing a lunar eclipse and measuring accurately the angular distance of stars from the Moon. Calculating longitude was a thorny navigational problem solved when John Harrison produced an accurate shipboard chronometer which enabled longitude to be calculated by comparing local time to Greenwich Mean Time. One hour's time difference is equivalent to 15 degrees of longitude.