Greenwich

The only practical way of ascertaining longitude is by knowing the difference between two meridians. Until the invention of a reliable chronometer, the position of a ship sailing between two meridians could only be determined by establishing the time when the same celestial phenomenon occurred at two places.

In 1675, Charles II established an astronomical observatory in Greenwich Park for the purpose of making observations that would improve the accuracy of navigation. This entailed recording the position of various stars and the position of the moon in relation to these stars at various times.

Observations were made using a base meridian at the observatory. The earliest was a line running along the west wall of the building in which John Flamsteed, the first Astronomer Royal, set up a mural arc. The Greenwich meridian was designated 0° longitude, and the world divided at the equator into 360°. Each degree was then divided into minutes, each representing 1/60th of a degree.

With the Greenwich meridian as a reference point, navigators could use the position of the moon to establish the time at Greenwich using a series of tables. Once they knew the difference between the time of their observation at sea, and the time it then was at Greenwich, they could calculate their longitude in terms of degrees and minutes west or east of Greenwich.