Greenwich Mean Time

Greenwich Mean Time (GMT) is a time standard historically based on the mean solar time at the Royal Observatory in Greenwich, London. It remains a civil time zone in some contexts and a reference in navigation and broadcasting.

Mean solar time: Time measured by the Sun’s average apparent motion, smoothing out seasonal variations in the length of solar days. GMT originally tracked this averaged solar time at Greenwich.
Astronomical transit instrument: A telescope mounted to observe when celestial objects cross a fixed meridian. Greenwich observers used transit instruments to determine local time precisely.
Earth rotation: The turning of Earth around its axis, producing the apparent daily motion of the sky. GMT’s solar-time basis follows Earth’s rotation, which is not perfectly uniform.
Coordinated Universal Time: The global civil time standard maintained by atomic clocks and adjusted with leap seconds to stay near Earth-rotation time. UTC is the modern standard commonly meant when GMT is used as a global time reference.
Solar time: Time defined by the apparent position of the Sun in the sky at a particular location. GMT began as a location-specific form of solar time.
John Flamsteed: The first Astronomer Royal, who directed the Royal Observatory from its founding in 1675 until his death in 1719. Flamsteed’s observatory work began Greenwich’s long tradition of precise meridian observations.
Universal Time: A family of time standards derived from Earth’s rotation, including UT1, which tracks observed rotation angle. Universal Time replaced GMT as the precise name for rotation-based global time.
British Summer Time: The United Kingdom’s summer civil time, one hour ahead of GMT. It contrasts with GMT because clocks in the UK advance seasonally during summer.
Longitude: The angular position of a place east or west of a reference meridian, measured in degrees. Longitude connects local solar time to Greenwich’s reference meridian.
Nautical almanac: A yearly publication of astronomical data used to determine position and time at sea. Greenwich astronomical tables helped navigators compare local time with a standard reference.

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