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The Nautical Mile is internationally recognised as 1852m which is an approximation of 1' of latitude subtended to the earths surface. However it is an average and the geographical length on the earths surface of 1' of lat subtended will vary according to the radius of the earth at that point.

The earth is an oblate spheroid (flattened at the poles) and has a smaller radius at the poles of 6356.752Km compared to 6378.137Km at the equator (wikipedia).

Accordingly I would expect the 1' of arc of Lat subtended at the poles would give a shorter figure for a geographical NM than at the equator.

However I note that the reverse is the case. Wikipedia under the heading Nautical Mile gives a figure of 1861.57M at the pole compared to 1842.90M at the equator.

Is there something I am not understanding?

Please help.


  • Physics -

    latitude is based on 1 degree of arc, which depends on the radius of curvature, not the distance from the center of Earth. Wiki is correct.

  • Physics -

    BOB, Wiki might well be correct but I need to understand why it is correct. I wrote the question, not to challenge what I had read but to further my understanding. When you say lat is based on 1 degree of arc what are you referring to?


  • Physics -

    Thanks Bob, Looked at your link and the penny has dropped. I still find it difficult to accept that the pole having a smaller radius creates a longer NM!


  • Physics -

    Read the link I placed in the post.

    Degree of arc is measured at the center of curvature, which for Earth is not the center of Earth. Did I write degree...oops...minute of latitude is based on the angle subtended on Earth by the radius of curvature (not radius of Earth) from the center of curvature. At the "flat" poles, the radius of curvature is longer. At the poles, the distance from center of Earth to the surface is shorter. Your question, I am certain, was confusion about the shorter radius should subtend a shorter distance on Earth, however, because Latitude is based on angles measured from the center of curvature, which is further at the poles than at the equator, the latitude subtends a longer distance. Take a look at the link I posted.

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