Things get complicated because of the Earth is not a sphere but an oblate spheroid. That means that it bulges slightly at the equator and the poles are slightly flattened by the centrifugal force of the earth's rotation about its axis. So a degree of latitude actually varies in length with latitude.
At 29º a degree of latitude is 59.846 nm. At 75º it is 60.269 nm and at 95º it is 60.305 nm. So the closer to the equator you are the shorter the definition nautical mile should be. Since we fixed that dimension, the closer to the equator you are, the shorter distance one degree of latitude is!
For those who are wondering what on Earth (
) we are talking about
Unlike other imperial measurements, the nautical mile derives its definition from a convenient measurement on the Earth itself. One nautical mile is supposed to equal one minute of arc in latitude anywhere or in longitude at the equator. As usual with such definitions, and similarly to the meter, inaccuracies in our ability to measure the dimensions of the Earth at the time such standards were established converted the sensibly defined unit from one verifiable by measuring the Earth itself to just another arbitrary unit! Although a nautical mile is somewhat close to a minute of arc (60 minutes equals one degree which should equal 60 miles and hence this conversation), it is not, and so is no better than the imperial foot in being a "scientifically rational" unit.
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