A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Lars Bergman
Date: 2020 Oct 31, 08:40 -0700
Do not trust the definition in The Oxford Companion, it is not correct. The length of one minute of latitude varies from around 1843 m at the equator to around 1862 m at the poles. And this is, as you have correctly observed, due to different radii of curvature at different latitudes. At 45° latitude the length is around 1852 m. In earlier times these small differences didn't have any practical impact on navigation, but in 1929 (if my memory is correct) some kind of conference decided that "The International Nautical Mile" should be 1852 m, exactly. But it took many decades before it become used all over the globe. The UK, for example, used 6080 feet (≈ 1853.2 m) at least until the nineteen sixties.