Greenwich clock with standard measurements
Flamsteed House in 1824
Clock in Kumasi, Ghana, set to GMT.
Royal Observatory, Greenwich c. 1902 as depicted on a postcard
Greenwich Observatory (Latinized as "Observatorium Anglicanum Hoc Grenovici prope Londinum"), as illustrated in Johann Gabriel Doppelmayr's map of the southern celestial hemisphere, ca. 1730
Royal Observatory, Greenwich
Dome of the Greenwich 28 inch refractor telescope and tree
The Airy Transit Circle, used for over a century (1851–1953) as the reference point when charting the heavens and determining times, thus earning for it the epithet "the centre of time and space"
The building housing the origin of the Greenwich Prime Meridian
Laser projected from the observatory marking the Prime Meridian line
Laser at night
Shepherd Gate Clock at Royal Greenwich Observatory
One of the hyper-accurate timekeepers at the observatory
The time ball is the red ball on a post – when it drops a certain time is signalled. This allowed clocks to be set from afar with great accuracy, particularly the chronometers of ships on the River Thames below, prior to sailing. The observatory would first determine the time by stellar observations.
Dome of the Great Equatorial Building overlooking Greenwich Park
21st-century view of the Altazimuth Pavilion
Standard lengths on the wall of the Royal Observatory, Greenwich, London – 1 yard (3 feet), 2 feet, 1 foot, 6 inches (1/2-foot), and 3 inches. The separation of the inside faces of the marks is exact at an ambient temperature of 60 °F and a rod of the correct measure, resting on the pins, will fit snugly between them.
Aerial view of the Royal Greenwich Observatory, Herstmonceux site in East Sussex; the dome that formerly housed the Isaac Newton Telescope is the single dome to the right. The telescope was moved to La Palma in the Canary Isles in 1979.
Former Royal Greenwich Observatory, Herstmonceux, East Sussex
Greenwich House at Cambridge
The Queen's House (centre left) at Greenwich, with the Royal Observatory on the skyline behind, in 2017.
The Magnetic Pavilion, 1900
Tourists flock to the Observatory museum, 2009
The centuries-old Flamsteed House overlooking Greenwich Park in London. The statue at left is of Major General James Wolfe, who died capturing Quebec in 1759, and was buried in St Alfege Church, Greenwich.

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight.

- Greenwich Mean Time

It played a major role in the history of astronomy and navigation, and because the Prime Meridian passes through it, it gave its name to Greenwich Mean Time, the precursor to today's Coordinated Universal Time (UTC).

- Royal Observatory, Greenwich
Greenwich clock with standard measurements

2 related topics with Alpha

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Standard time zones of the world (February 2021). The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time.

Universal Time

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ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians

ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians

Standard time zones of the world (February 2021). The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time.
An 1853 "Universal Dial Plate" showing the relative times of "all nations" before the adoption of universal time

Starting in 1847, Britain established Greenwich Mean Time, the mean solar time on the Prime Meridian at Greenwich, England, to solve this problem: all clocks in Britain were set to this time regardless of local solar noon.

Using telescopes, GMT was calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK.

A marine chronometer by Charles Frodsham of London, shown turned upside down to reveal the movement. Chronometer circa 1844-1860.

Marine chronometer

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Precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation.

Precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation.

A marine chronometer by Charles Frodsham of London, shown turned upside down to reveal the movement. Chronometer circa 1844-1860.
The marine "Chronometer" of Jeremy Thacker used gimbals and a vacuum in a bell jar.
Henry Sully (1680-1729) presented a first marine chronometer in 1716
John Harrison's H1 marine chronometer of 1735
Drawings of Harrison's H4 chronometer of 1761, published in The principles of Mr Harrison's time-keeper, 1767.
Ferdinand Berthoud's marine chronometer no.3, 1763
Pierre Le Roy marine chronometer, 1766, photographed at the Musée des Arts et Métiers in Paris
Harrison's Chronometer H5 of 1772, now on display at the Science Museum, London
Ferdinand Berthoud chronometer no. 24 (1782), on display at the Musée des Arts et Métiers, Paris
Einheitschronometer pattern MX6 marine chronometer mass-produced in the Soviet Union after World War II
Mechanical boxed Marine Chronometer used on Queen Victoria's royal yacht, made about 1865
A chronometer mechanism diagrammed (text is in German). Note fusee to transform varying spring tension to a constant force
Einheitschronometer pattern marine chronometer (A. Lange & Söhne, 1948) displaying its second hand advancing in ½ second increments over a 60 seconds marked sub dial for optimal timing of celestial objects angle measurements at the GFZ
Omega 4.19 MHz (4,194,304 = 222 high frequency quartz resonator) Ships Marine Chronometer giving an autonomous accuracy of less than ± 5 seconds per year, French Navy issued,1980. The second hand can advance in ½ second increments for optimal timing of celestial objects angle measurements.
The inner working of a Marine Chronometer

It is used to determine longitude by comparing Greenwich Mean Time (GMT), or in the modern world its successor Coordinated Universal Time (UTC), and the time at the current location found from observations of celestial bodies.

It was common for ships at the time to observe a time ball, such as the one at the Royal Observatory, Greenwich, to check their chronometers before departing on a long voyage.