LONGITUDE

Longitude is a geographical coordinate, expressed as an angular value, indicating the position of a point on Earth relative to the Greenwich meridian (0° meridian). All points with the same longitude form a line connecting the North and South Poles, called a meridian.

Accurately determining longitude is of vital importance for navigation. Its calculation requires the precise measurement of time intervals with great accuracy. This quest for precision in navigation contributed to the development of chronometry (marine chronometers) in the 18th century, notably through the work of John Harrison, winner of the Longitude Act in 1773.

In watchmaking, longitude determines various astronomical information (sunrise and sunset times, date change, local time, and time zones, etc.).