Approx. 13.5 mi difference between the polar and equatorial radii.
But it's not a perfect ellipsoid either so calculation for distance
around a particular latitude may not be so easy. So you may as well use
the equation for a sphere and carry a little extra gas.

This is somewhat interesting reading:
http://exchange.manifold.net/manifol..._Ellipsoid.htm

Tony P.

"Peter Duniho" wrote in message
...
"Don Tuite" wrote in message
...
[...]
I am attempting to determine how the circle distances change as you
move from the equator to either of the poles.

r times two pi times the cosine of the latitude.

As a rough estimate, that's not bad. However, the Earth is not perfectly
spherical. It bulges out at the equator, and so you have to adjust r
based on latitude for that equation to come out right.

Pete