<P> The history of longitude is a record of the effort, by astronomers, cartographers and navigators over several centuries, to discover a means of determining longitude . </P> <P> The measurement of longitude is important to both cartography and navigation, in particular to provide safe ocean navigation . Knowledge of both latitude and longitude was required . Finding an accurate and reliable method of determining longitude took centuries of study, and involved some of the greatest scientific minds in human history . </P> <P> Eratosthenes in the 3rd century BC first proposed a system of latitude and longitude for a map of the world . By the 2nd century BC Hipparchus was the first to use such a system to uniquely specify places on the earth . He also proposed a system of determining longitude by comparing the local time of a place with an absolute time . This is the first recognition that longitude can be determined by accurate knowledge of time . In the 11th century Al - Biruni believed the earth rotated on its axis and this forms our modern notion of how time and longitude are related . </P> <P> Determining latitude was relatively easy in that it could be found from the altitude of the sun at noon (i.e. at its highest point) with the aid of a table giving the sun's declination for the day, or from many stars at night . For longitude, early ocean navigators had to rely on dead reckoning . This was inaccurate on long voyages out of sight of land and these voyages sometimes ended in tragedy as a result . </P>

How did the early explorers determine local time