I've was studying from "Creative Illustration for Artists and Illustrators" by Ernest W. Watson, when I came upon something interesting.
To quote the author:
" Always we must remember that the long diameter of the ellipse is not a structural line of the object: it is merely an imaginary line - useful only for drawing purposes. It is never the same as the geometric diameter of the circle which the ellipse represents "
" Many beginners, especially those who are mechanically minded, are quite surprised to be told that the diameter of a circle and the long diameter of its elliptical appearance are never the same lines; and to discover that the long diameter of the ellipse is always in front of (ie nearer the spectator) the circle's structural diameter. "
I've always been under the assumption that the true diameter of the circle naturally ended up as the long-side-line of the ellipse. Anyways, I'm sure that this only information for the beginners (like myself), but I figured it couldn't hurt to post it.
I looked back through a lot of my perspective studies of ellipses, and I've noticed a lot of mistakes due to my lack of understanding in this regard.
PS: The image attached shows an example of what I am talking about.