Greenwich Mean Time

Greenwich Mean Time (GMT) is the mean solar time referenced to the prime meridian passing through Greenwich, a district in southeast London, United Kingdom. Greenwich was chosen because it is home to the Royal Observatory, a center for astronomical science and timekeeping.

GMT became the standard reference for the global system of time zones. Other zones are described as hours ahead or behind GMT, for example GMT+3 or GMT−5. Somalia and most of East Africa use GMT+3.

Origins and the Greenwich Meridian

In the eighteenth and nineteenth centuries, ships at sea needed accurate time references to calculate longitude. Ship chronometers were set to Greenwich time. At an international conference held in Washington in 1884, the Greenwich meridian was adopted as the prime meridian (0° longitude), which solidified the use of GMT.

How it is measured

The term 'mean' refers to the use of a fictitious sun moving at a constant rate, because the actual time of solar noon varies throughout the year. This variation arises from the Earth's elliptical orbit and the tilt of its axis. For this reason, the mean solar day is uniform across longitude.

Relationship to UTC

Today, the official reference for universal time is Coordinated Universal Time (UTC), based on atomic clocks of high precision, whereas GMT is based on the Earth's rotation. The difference between them is negligible and the terms are used interchangeably in everyday practice. To maintain coordination, UTC is occasionally adjusted by the addition of a leap second.

Current use

The United Kingdom observes GMT during winter and British Summer Time (GMT+1) during summer. The term GMT is still used in aviation, maritime operations, shortwave radio, and general practice, although accurate technical documentation prefers UTC.