# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Size of a nautical mile
From: Lu Abel
Date: 2009 Jan 17, 20:21 -0800
Makes a lot of sense.  I got so caught up in the simple oblate spheroid I forgot that it's not really an accurate model of the surface of the earth.

Peter Fogg wrote:
Simplicity.  That's my understanding; that 1,852m = minute of arc at sea surface was adopted as being appropriate for practical naviagtion in the zones of latitude where most nav gets practised - ie; excluding high latitudes around the poles.

When it comes to precision there is no end to potential discrepancies.  After the earth's oblate shape come the uneven surface of the oceans: there are local hills and valleys, the Pacific and Atlantic don't share the same levels (one reason the original, simple through-canal, plan for the Panama Canal was doomed to failure, as it would have resulted in a non-stop rush of water surging from one side to the other).  The Pacific even has different levels on the eastern side compared to the western.  Each time the level of the sea changes so does the length of a minute of arc mesured along its surface.

On Fri, Jan 16, 2009 at 8:35 AM, Lu Abel wrote:

As has been pointed out, the length of a degree of latitude varies from
pole to equator.

The nautical mile is supposed to equal a degree of latitude.   Today,
however, it is defined as precisely 1852 meters.

When one multiplies 1852 by 7200 (the number of minutes in 90 degrees),
one gets 10 000 800 meters as an answer.

My understanding is that the distance from equator to pole is 10 002 000
meters.   It's a small discrepency, on the order of 0.01%, but I'm
wondering why it was allowed to stand instead of defining the NM more
precisely.

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