Greenwich clock with standard measurements
A marine chronometer by Charles Frodsham of London, shown turned upside down to reveal the movement. Chronometer circa 1844-1860.
Clock in Kumasi, Ghana, set to GMT.
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.

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.

- Marine chronometer

As the United Kingdom developed into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was considered to have longitude zero degrees, by a convention adopted in the International Meridian Conference of 1884.

- Greenwich Mean Time
Greenwich clock with standard measurements

3 related topics

Alpha

Flamsteed House in 1824

Royal Observatory, Greenwich

Observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north.

Observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north.

Flamsteed House in 1824
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.

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).

1818 Oversight of the Royal Observatory was transferred from the Board of Ordnance to the Board of Admiralty; at that time the observatory was charged with maintaining the Royal Navy's marine chronometers.

These automatic signal clocks were synchronized by telegraphy in 1905 before the widespread use of radio

Time signal

Visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.

Visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.

These automatic signal clocks were synchronized by telegraphy in 1905 before the widespread use of radio
The time ball on the roof of Greenwich Observatory, London
Advertisement for a telegraph time signal service (1900)
A modern LF Radio clock
A low cost LF radio clock receiver, antenna left, receiver right.

Accurate knowledge of time of day is essential for navigation, and ships carried the most accurate marine chronometers available, although they did not keep perfect time.

Greenwich Mean Time was distributed by telegraph from the Greenwich Observatory.

Finding Greenwich time while at sea using a lunar distance. The lunar distance is the angle between the Moon and a star (or the Sun). The altitudes of the two bodies are used to make corrections and determine the time.

Lunar distance (navigation)

Angular distance between the Moon and another celestial body.

Angular distance between the Moon and another celestial body.

Finding Greenwich time while at sea using a lunar distance. The lunar distance is the angle between the Moon and a star (or the Sun). The altitudes of the two bodies are used to make corrections and determine the time.

The lunar distances method uses this angle, also called a lunar, and a nautical almanac to calculate Greenwich time if so desired, or by extension any other time.

A fuller method was published in 1763 and used until about 1850 when it was superseded by the marine chronometer.