The Definition of GMT
GMT stands for Greenwich Mean Time. It is the time zone based on the average time it takes for the Earth to rotate once on its axis, as measured from the Prime Meridian (0° longitude) at the Royal Observatory in Greenwich, London.
Key Facts About GMT:
- Historical Standard: From 1884 to 1972, GMT was the international civil time standard, established at the International Meridian Conference in Washington, D.C.
- Solar vs. Atomic Time: GMT is a solar time standard, based on the sun's position. This differs from UTC, which is based on ultra-precise atomic clocks. GMT can vary by up to 16 minutes from UTC throughout the year due to the Earth's elliptical orbit.
- Modern Usage: Today, GMT is officially a time zone (UTC+0) used by the UK in winter and several African countries year-round. The UK switches to British Summer Time (BST, UTC+1) in summer.
- The Prime Meridian: The line of 0° longitude passes through the Airy Transit Circle at the Royal Observatory in Greenwich. This was chosen in 1884 because Britain was the dominant maritime and commercial power at the time.
GMT vs UTC vs BST
GMT and UTC are often used interchangeably in casual conversation, but they are technically different. GMT is based on astronomical observation (the Earth's rotation), while UTC is based on atomic clocks. In practice, they never differ by more than 0.9 seconds. British Summer Time (BST) is GMT+1, used in the UK from late March to late October. When someone in London says "It's 3 PM GMT" in July, they actually mean 3 PM BST (which is 2 PM GMT). This confusion is one reason UTC is preferred in technical contexts.
Countries that use GMT year-round
Countries that observe GMT all year (without DST) include: Iceland, Senegal, Ghana, Guinea, Mali, Mauritania, Sierra Leone, Togo, Burkina Faso, Gambia, Guinea-Bissau, Liberia, São Tomé and Príncipe, and parts of Antarctica. The UK and Ireland use GMT in winter but switch to BST/IST in summer.