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
The clockwork in John Harrison's H4 marine chronometer on display at the Royal Observatory, Greenwich
Marine sandglass MMM
Geodetic datum
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A geodetic datum or geodetic system is a global datum reference or reference frame for unambiguously representing the position of locations on Earth by means of either geodetic coordinates or geocentr
City of Chicago Datum Benchmark