I think you can assume that the earth is treated as a sphere.
The nautical mile is 1/360 * 1/60 of the circumference.
The Nautical Mile?.
Standard description of the Nautical Mile copied from a text book. "A Nautical Mile is the distance on the Earths surface subtended by one minute of latitude at the Earths centre".
Without complicating this further by referring to the distances..........
The Earth is an oblate spheroid having a slightly greater (+/- 13 miles) Earth centre to Equator radius than Earth centre to pole. Therefore you would expect an angle of 1 minute at the Earth centre subtended to the Equator would yield a nautical mile of greater distance than 1 minute subtended to the pole. In the case of the equator the radians are diverging for a greater distance.
However this is not the case as all references to the actual distance of a nautical mile (apart from the internationally recognised figure of 1852m which is a different issue) states a nautical mile at/near the pole is greater than at/near the equator.
I have been chewing this over for some time and am thinking the nautical mile has less to do with angles of 1 minute at the earth centre and more to do with the distance along an arc (earths surface) between 2 points where the curve alters by 1 minute. That is to say tangents drawn at the 2 points would create an angle of 1 minute where they meet/cross.
This relates to the earth/oblate spheroid as the flattening at the pole creates a flatter curve of the earth surface and therefore a greater distance between the 2 points required to create a 1 minute angle at the tangents. Conversely the more acute curve at/near the equator means a shorter distance between the 2 points.
I am unable to come up with a more plausible explanation?
Additionally the flatter curve at the pole suggests it is part of a circle of greater diameter than earth and the more acute curve at the equator suggests it is part of a circle of lesser diameter than earth.
Furthermore if the longer nautical mile (1 minute of arc) at the pole is subtended to the earth centre rather than its natural greater radius the angle created at the earth centre would be greater than 1 minute. Conversely if the shorter nautical mile at the equator is subtended to the earth centre the angle created will be less than 1 minute.
Please confirm the above or correct my understanding.
Many thanks
Mike
1 answer
2 answers