Is there an expression out there for determining the distance around
the globe, at let's say the tropic of cancer and the tropic of
capricorn? I know that the radius of the earth is 6371 - is there
perhaps an expression which relates the circle distance at the tropic
of cancer/capricorn to the radius or even perhaps the GCD?

I am attempting to determine how the circle distances change as you
move from the equator to either of the poles.


thanks