Greenwich Mean Time (GMT)

The mean solar time at the Prime Meridian in Greenwich, London, used as a global time standard.

Greenwich Mean Time (GMT) is a timekeeping system that started at the Royal Observatory in Greenwich, London, and acts as a reference point for global time zones. It is based on the mean solar time at the Prime Meridian (0° longitude), which goes through Greenwich. Historically, GMT was set up to offer a standardised time reference for navigation and communication, especially for maritime purposes. Before it was adopted, sailors used local solar time, which varied based on their location, causing inconsistencies in navigation and scheduling.

GMT is calculated by observing the Sun’s position relative to the Prime Meridian. The term “mean” in GMT refers to the averaging of solar time to account for variations caused by the Earth’s elliptical orbit and axial tilt. This ensures a consistent and predictable time standard. GMT was widely adopted in the 19th century, particularly after the International Meridian Conference in 1884, which designated the Prime Meridian as the global reference for longitude and timekeeping.

Although GMT has largely been replaced by Coordinated Universal Time (UTC) in most official contexts, it remains a term of cultural and historical importance. GMT is still used to coordinate activities across different time zones, ensuring consistency in navigation, weather reporting, and communication. For sailors, GMT provides a reliable framework for calculating longitude and synchronising operations, making it an enduring cornerstone of nautical timekeeping.