The confusion occurs as a result of the way navigation, as an art and science developed.
Drawing lines of Longitude around the globe from the two poles was obvious - and easy to see that it simplified the definition of how far West or East a point is. Segment lines that make the chocolate orange possible.
The lines of latitude are not so obvious, it's one way of doing it, but it could be argued that adding another pair of poles on the equator could generate a second set of segment lines that would intersect at points to give relative coordinate.
The former method looks good on a map, but what it doesn't show is that it is not a straight line in any sense.
We're brought up on maps that have nice straight lines on them. Nobody tells us until we're older that it's all an enormous fudge. Not only are lines of latitude not straight, but the type of projection means that not much actually looks like a map from a satellite. The only viewpoint that would see a line of latitude as straight is from a satellite sitting in the same plane as that line. From there, every other line of latitude would not be straight.
All lines of longitude are great circles.
The only line of latitude that is a great circle is the equator.
A Rhumb line is a quick and dirty way to get a heading from A to B. It needs no finessing if you're going North or South, but the closer to East West you head, and the farther up or down the globe you go the more a Great Circle will save you distance. You'll also need to factor in crosswinds/tides (aircraft/ship), but that's obvious.
Last edited by boguing; 3rd Mar 2011 at 12:00.
Reason: Glaring mistake that nobody seemed to spot!