In 1714 the British Board of Longitude announced a prize of £20,000 (in those days a fortune) for the invention of means of determining longitude to an accuracy of 30 miles.
It was already known that because the earth is round (more or less) and rotates at a reliable speed, it could theoretically be imagined as a circle and divided into 24 segments of longitude, each segment being 15 degrees of rotation (the total being 360 degrees). If mariners could maintain a reliable understanding of the time at the point of departure, by comparing this time with the local time at noon on any day when the sun was visible they could work out the time elapsed or advanced, and so calculate the number of degrees of longitude east or west from t…
Clark, Robert. "Invention of Harrison's sea-going chronometer". The Literary Encyclopedia. First published 06 January 2009
[http://www.litencyc.com/php/stopics.php?rec=true&UID=565, accessed 19 January 2017.]