Greenwich Mean Time

Greenwich Mean Time (GMT) is a time zone and the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight. Historically, GMT has been used as the international civil time standard, referred to as "world time". Before the adoption of Coordinated Universal Time (UTC) in 1972, GMT functioned as the basis for standard time worldwide. Although GMT and UTC share the same current time in practice, GMT is now a time zone designation rather than a time standard.

History

The concept of GMT dates back to the late 19th century when it was adopted at the International Meridian Conference in 1884. This conference also established the Prime Meridian, passing through Greenwich, as the reference line for longitude. The choice of Greenwich as the reference meridian was due to the presence of the Royal Observatory, Greenwich, where significant contributions to astronomy and navigation had been made.

GMT and UTC

While GMT was based on the Earth's rotation and celestial observations, UTC is maintained by atomic clocks, making it far more precise. UTC was introduced to provide a more stable and consistent time standard that does not vary with the Earth's irregular rotation speed. Although GMT is no longer used for official timekeeping purposes, it is still widely used in some contexts, such as the time zone for the United Kingdom during the winter months and in other countries in West Africa.

Time Zone

GMT is used as a time zone without any offset from UTC, denoted as UTC+0. Countries and territories using GMT as their standard time during the winter months switch to Daylight Saving Time (DST) during the summer months, moving to UTC+1, known as British Summer Time (BST) in the UK.

Usage

Despite the official adoption of UTC, GMT remains a term in common usage. It is often used interchangeably with UTC, although technically, this is not accurate. GMT is also used in aviation, where it is referred to as "Zulu time". In the fields of telecommunications, military, and navigation, GMT is still frequently used.

See Also

• Time zone
• Coordinated Universal Time
• Prime Meridian
• Daylight Saving Time
• Royal Observatory, Greenwich