Meridian arc
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In geodesy and navigation, a meridian arc is the curve between two points near the Earth's surface having the same longitude. The term may refer either to a segment of the meridian, or to its length.
The north and south sections of the meridinal survey met at Rodez Cathedral, seen here dominating the Rodez skyline at left.
A copy of the "provisional" metre located in the wall of a building, 15 rue de Vaugirard, Paris. These metres were provisional (French: provisoire) because the expedition and the calculations to determine the definitive length of metre were not completed until 1799.
Montjuïc Castle in Barcelona, Spain – the southern end of the meridian arc
Longitude
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Longitude is a geographic coordinate that specifies the east-west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees
Meridian arc
…In geodesy and navigation, a meridian arc is the curve between two points near the Earth's surface having the same longitude. The term may refer either to a segment of the meridian, or to its length. Both the practical determination of meridian arcs (employing measuring…
The clockwork in John Harrison's H4 marine chronometer on display at the Royal Observatory, Greenwich
Marine sandglass MMM