The Greenwich Meridian and Hour Angle
The scale that made longitude a matter of time.
Origin
The Royal Observatory was founded at Greenwich in 1675 expressly to solve longitude. The International Meridian Conference adopted Greenwich as the prime meridian in 1884.
The catch
Longitude is not really a distance at all. It is a difference in time: carry an accurate clock set to Greenwich, compare it with local noon, and every four minutes of difference is one degree of longitude. The problem was never geometry. It was building a clock that worked at sea.
John Harrison’s marine chronometers, developed over four decades from the 1730s, finally made the method practical, and the Board of Longitude’s prize of £20,000 became one of the most consequential research grants in history. Greenwich was chosen as the prime meridian in 1884 largely because most of the world’s shipping already used British charts, which is a rare instance of a standard being adopted through sheer commercial weight rather than negotiation.