Greenwich Mean Time

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, which was established as the prime meridian (0° longitude) at the 1884 International Meridian Conference. GMT was the world's time reference standard from 1884 until it was superseded by UTC in 1972. In practice, GMT and UTC are often used interchangeably for civil timekeeping purposes, though they are technically different — GMT is defined by astronomical observation (Earth's rotation) while UTC is defined by atomic clocks. The difference is typically less than a second. GMT is still used colloquially and in British law. The term 'GMT' appears in international treaties, broadcasting schedules, and everyday conversation more often than 'UTC' despite the latter being the official standard. The Royal Observatory's Shepherd Gate Clock, installed in 1852, was one of the first clocks to show GMT to the public. Greenwich was chosen as the prime meridian partly because the UK's Royal Navy was the world's most powerful, and British nautical charts (which used Greenwich as zero longitude) were the most widely used. The observatory's famous red time ball, dropped daily at 1 PM since 1833, was originally a signal for ships on the Thames to set their chronometers.