Reckoning / Sky & sea / Latitude and longitude
Sky & sea · coordinates
Latitude and longitude
Latitude is how far north or south you are, measured as an angle from the equator; longitude is how far east or west, measured from the meridian through Greenwich. Two angles fix any point on Earth. Turn the globe, and tap anywhere to read its position.
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Position
Red: equator. Yellow: tropics and polar circles, at the obliquity of the ecliptic for this year. Blue: prime meridian and the 180th meridian.
How long is a minute?
The Earth is slightly flattened (WGS 84: equatorial radius 6 378 137 m, flattening 1/298.257 223 563), so a minute of latitude is a little longer near the poles, where the surface is flatter. Averaged over the globe it is close to 1852 m, and that is where the nautical mile comes from. A minute of longitude shrinks with the cosine of the latitude and vanishes at the poles.
| Latitude | 1′ of latitude | 1′ of longitude | Geocentric latitude |
|---|
Computed live from the WGS 84 ellipsoid (WGS 84). The build checks the code against the length of the meridian quadrant, 10 001 965.729 m from the equator to the pole. Geocentric latitude is the angle seen from the Earth’s centre; maps and GPS use the geodetic latitude; the two differ by up to 11.5′, at 45°.
No flat map can show a sphere without stretching it: Gauss proved in 1827 that distances on a curved surface cannot all be kept on a plane. Every chart therefore chooses what to keep true. Mercator keeps angles, so a constant course is a straight line; the gnomonic projection keeps great circles straight. See both on the sailing page.
The British Admiralty mile was 6080 ft (1853.184 m), close to a minute of latitude in the English Channel. The international nautical mile was fixed at exactly 1852 m in 1929; see length units. Latitudes today refer to WGS 84; old charts used local datums that can differ by hundreds of metres.