GMT (Greenwich Mean Time)
gmt
The mean solar time at the Royal Observatory in Greenwich, London, historically used as the international time reference.
Solar Time
solar-time
Definition
Solar time is time reckoned from the apparent position of the Sun in the sky. Apparent solar time sets noon at the exact moment the Sun transits the local meridian. Mean solar time smooths out the irregularities caused by Earth's elliptical orbit and axial tilt by using a fictitious 'mean sun' that moves at a uniform rate. GMT was historically defined as mean solar time at the Prime Meridian.
The Equation of Time
The difference between apparent solar time and mean solar time is called the equation of time. It arises from two factors: the eccentricity of Earth's orbit (Kepler's second law) and the 23.4-degree tilt of Earth's axis relative to the orbital plane. The equation of time varies between roughly -14 minutes and +16 minutes over the course of a year. This is why a sundial and a mechanical clock do not always agree.
Role Before Standard Time
Until the mid-19th century, cities around the world kept local solar time. Because one degree of longitude corresponds to a four-minute time difference, even neighboring towns had slightly different clocks. The expansion of railway networks made a unified time system necessary, eventually leading to the modern time zone framework in which broad regions share a single standard time. Solar time remains in use today in astronomy and sundial applications.
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Related Terms
UT1 (Universal Time 1)
ut1
UT1 is a form of universal time derived from the Earth's actual rotation angle. It reflects irregular variations in rotational speed and its divergence from UTC determines when leap seconds are inserted.
Meridian
meridian
A meridian is an imaginary semicircle running from the North Pole to the South Pole on the Earth's surface, serving as a reference line for longitude and forming the foundation of the global timekeeping system.
Standard Time
standard-time
Standard time is the officially adopted civil time of a region, defined as an offset from UTC and established by law or convention to serve as the basis for all social and commercial activity in that area.
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