Re: Semi-minor Axis

The confusion may lie in the term "semi-major axis", which is not an axis in
its own right but a term for half the length of the "major axis". As the
"major axis" is the straight line perihelion-Sun-aphelion (or their
equivalents for other primary bodies), half this distance is the average of
the perihelion and aphelion distance. If I have got my geometry right it is
also the distance from the Sun to either end of the "minor axis", but is
more than half the length of the "minor axis" itself because the Sun is not
on that "axis".

"JG" <jg@xxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:313030303331393043B5CA7669@xxxxxxxxxxxxxxxxxxxxxxxxxx
> 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


.

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