Determination of Longitude and Chronometer Rating

This chapter provides a simplified view of developments that led to visual time signals. Astronomers of many different nationalities provided the essential foundation. The search for a method to determine longitude at sea lasted for centuries. Recognition of methods that would allow determination of longitude preceded development of accurate instruments and provision of nautical almanacs. It was recognised that longitude might be determined using a timekeeper, later known as a chronometer, that could display accurate time at a chosen meridian on the earth’s surface. Astronomical and chronometer solutions became available at almost the same time in the second half of the eighteenth century. The challenge then was to check and maintain the accuracy of time measurement during long sea voyages. Time signal observations on successive days could be used to rate chronometers, complemented by understanding of factors that caused rate variation at sea. That was the justification for visual time signals around the world.