Minute and second of arc arcsecond on a 10 m class telescope. Minutes (′) and seconds (″) of arc are also used in cartography and navigation. At sea level one minute of arc along the equator or a meridian (indeed, any great circle) equals exactly one geographical mile along the Earth's equator or approximately one nautical mile (1852 meters, or ≈1.15078 statute miles). A second of arc, one sixtieth of this amount, is roughly 30 meters or 100 feet. The exact distance varies along meridian arcs because the figure of the Earth is slightly oblate (bulges a third of a percent at the equator). Positions
Nautical mile of latitude is not constant, but about at the poles and at the Equator, with a mean value of . Other countries measure the minute of arc at 45 degrees latitude, giving the nautical mile a length of . In 1929, the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as precisely 1,852 metres. Imperial units and United States customary units used a definition of the nautical mile based on the Clarke (1866) Spheroid. The United States nautical mile was defined as based in the Mendenhall Order foot of 1893. It was abandoned in