Semi-minor Axis

Hi,

I've been lurking and gaining a great deal of knowledge from this
newsgroup for quite some time now but I have a very basic question that
I have not seen asked.

I used to think that Aphelion referred to the semi-major axis of the
ellipse described by the Earth (or any planet) on its journey around the
Sun and Perihelion was the semi-minor axis. Having read most of the
planetary data from 'The Nine Planets' web site I now see that the sum
of Aphelion and Perihelion is in fact the major axis of that ellipse.
What I cannot understand is the statement in the Glossary that Aphelion
is also the 'average' or mean distance of the planet from the Sun.

Surely the maximum distance cannot also be the mean?

What I really want to know is how to calculate the semi-minor axis.
Given the 'Mean' and the eccentricity I can readily calculate the Major
as a(1+e) and the Minor as a(1-e) but if the mean is also the Major then
this doesn't make sense.

JG