5 ms·

Lines on a Mercator map aren't straight on a globe. They are constant-compass bearing, but that's not the same as straight. Also, as defined by the French acad

Lines on a Mercator map aren't straight on a globe. They are constant-compass bearing, but that's not the same as straight. Also, as defined by the French academy of science ~1800, the meter is 1/10,000,000th the distance from the north pole to the equator, on the meridian through Paris.

kqr 10y ago
Not sure what your metre comment is about, but it turned out that definition wasn't accurate enough to confidently state the speed of light, so the speed of light was set to 299792458 m/s and a metre is consequently the distance light travels in a 299792458th of a second. (The second is in turn precisely defined by particle physics.)
- dalke 10y ago
  It's a reference to Justin_K's earlier statement "a Knot is standard for navigation world wide because the unit is standardized on earth's size". If that justifies knot then it also justifies kilometers, because it too was originally standardized on the earth's size. Neither are exactly the size as originally defined. A modern knot is 1852 meters. One minute of latitude is "about 1,855.325 m on the WGS 84 ellipsoid" says https://en.wikipedia.org/wiki/Nautical_mile https://en.wikipedia.org/wiki/Nautical_mile. The difference is 0.16%. A quarter of the Earth is: 1855.325m * 60 seconds/degree * 360 degrees/circle /4 = 10018755.0 = ~10018 km. The difference is 0.19%.