The solution ultimately lay in the clocklike turning of the Earth. Every 24 hours, the planet rotates 360 degrees. This is totally predictable. A precise relationship therefore exists between time elapsed and how far the Earth has turned; one’s distance from any fixed east-west reference point can be determined accordingly. If one knows the local time on board ship and the time at the reference point, the difference can be translated into a measure of distance. Although this principle was understood by the early 18th century, no shipboard device could accurately tell the time at a distant fixed location. Therefore, the principle could not yet be . Its utilization awaited a reliable seaworthy clock