DAVIS Mark 3, Manual

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A nautical mile is equal to one minute of arc of a great circle. Since latitude is
measured north or south from the equator, it is measured along a meridian
(a great circle). One minute of latitude equals one nautical mile anywhere on the
earth. Since longitude is measured east or west from the prime meridian (zero
degrees) at Greenwich, England, it is measured along a parallel of latitude
(a small circle). One minute of longitude equals one nautical mile only at the
equator. Approaching the poles, one minute of longitude equals less and less of
a nautical mile (Fig. 10).
LATITUDE, LONGITUDE, and the NAUTICAL MILE
A great circle is a circle on the surface of the earth, the plane of which passes
through the center of the earth. A small circle is a circle whose plane does NOT
pass through the center of the earth. The equator and the meridians are great cir-
cles, while parallels of latitude are small circles which become progressively
smaller as the distance form the equator increases. At the poles (90° N or S), they
are but single points (Fig. 9).
The plane of a meridian (a great
circle) divides the earth into two
equal halves.
The plane of a parallel of latitude
(a small circle) divides the earth
into two unequal parts.
Figure 9
Figure 10
Note that the nautical mile is about
15% longer than the statute mile:
Nautical Mile Statute Mile
6076 feet 5280 feet
1852 meters 1609 meters
The earth measures 21,600 nautical
miles in circumference (24,856 statute
miles).