A marine chronometer by Charles Frodsham of London, shown turned upside down to reveal the movement. Chronometer circa 1844-1860.
Greenwich clock with standard measurements
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 marine "Chronometer" of Jeremy Thacker used gimbals and a vacuum in a bell jar.
Clock in Kumasi, Ghana, set to GMT.
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.

- Marine chronometer

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.

- Lunar distance (navigation)

A fuller method was published in 1763 and used until about 1850 when it was superseded by the marine chronometer.

- Lunar distance (navigation)

The lunar distances method, initially proposed by Johannes Werner in 1514, was developed in parallel with the marine chronometer.

- 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

But this practice, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, led to GMT being used worldwide as a standard time independent of location.

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

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