Norman Werling wrote:
>
> Considering what those guys went through, it was and is very accurate
> indeed. I recall that one minute of the earth's surface in any circle of
> circumference is exactly 1852 meters. Thus 1852 metres times 360 degrees
> times 60 minutes equals 40 003.2 kilo-metres.
>
> So that would mean the error from the equator to either pole is 0.8
> kilo-metres (3.2 divided by 4). Can we avoid talking the pear shape of the
> earth?
....
What you are quoting at the start there, Norm, is the old thumbrule
that 1 minute of latitude (not longitude!) equals 1 nautical mile. Prior
to this becoming the commonly accepted standard, there were many
different sizes of nautical mile which varied by nationality and era.
But for most of the 18th and 19th centuries -- and partway into the 20th
century -- that was pretty much the accepted relationship. About halfway
through the 20th century, the British and later the Americans defined
the nautical mile as being exactly 1852 m. That upset the thumbrule.
The original survey conducted by Jean-Baptiste Joseph Delambre and
Pierre Fran�ois Andr� M�chain, starting in 1792, took into account the
oblate shape of the Earth (a bit like a squashed beach ball), but their
"flatness factor" (which is in the order of 1/300) was off a wee bit.
Still, considering the instruments available at the time, it was
impressively close. The so-called "pear model" was something that came
out as a result of the 1957/58 International Geophysical Year
collaboration, as I recall. It basically described the Earth as being
roughly "pear-shaped" or along the lines of a tetrahedron. Today's
geodesic models are much more sophisticated.
Sorry for the discourse. I feel myself winding up to get this May's
Celestial Navigation class ready. Stand easy, mates.
Jim
--
Metric Methods(SM) "Don't be late to metricate!"
James R. Frysinger, CAMS http://www.metricmethods.com/
10 Captiva Row e-mail: [EMAIL PROTECTED]
Charleston, SC 29407 phone/FAX: 843.225.6789
