On Sun, 24 Aug 2003 14:38:07 +0100, "W.J. \(Bill\) Dean \(U.K.\)."
wrote:

On the Equator.

Wrong.

There have been some geographical miles based on the equatorial
circumference, but I've never seen them called nautical miles. There
was one geographical mile equal to 4 minutes of arc on the equator, or
about 7.421 km; I have a copy of a map using these units.

Nautical miles have normally been defined to be some midrange,
midlatitude value for a minute of arc as you travel north-south along
a meridian.

At the equator, 1 minute of longitude is 1.001795 nmi.

But 1 minute of latitude (geodetic latitude, the kind normally used)
at the Equator is only about 0.9950 nmi.

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.


"JJ Sinclair" wrote in message
...

Navigational question for the day;
One minute of latitude = 1 nautical mile.
At what point on the earth does 1 minute of longitude = 1 nautical mile?

JJ Sinclair.

It is at some place close to the equator, where the circumference of
the earth at that latitude is 40.0032 Mm, rather than the 40.007495 Mm
at the Equator (WGS-84 ellipsoid). A latitude close to the arccosine
of 40.0032/40.007495, or about 3½ degrees from the Equator either
north or south (a more exact value depends on which type of latitude
you use, as well as which ellipsoid you use to approximate this).

To help see this better, your "minute of longitude" at a latitude of
60 degrees would be about 0.50 nmi, and at the poles a
"minute of longitude" is 0 nmi. It is actually the minute of latitude
as you travel along a meridian (constant longitude) that most people
consider in evaluating the fit of a nautical mile to the Earth.

Now, at what point is one centigrade of latitude equal to one
kilometer?

Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/