Phew, I'm overwhelmed by the considerable and considered lengths that
respondents have made on this topic. (Of course it's tempting to be a
smart-alec and ask 'Yes, but why an ellipse?' as if to off-handedly
tilt at the signficance of mathematics in answering my question, but
that would lazy and disingenuous).

** To clarify, as was requested by some, the extended version of my
question is 'Why an ellipse and not a circle?' And thanks to those who
recognised this -- an imprecision on my part. **

It is a fascinating topic and I wish I understood (read 'could
configure my life so I had the time to learn') the maths.

Sorry that I've only had time to skim the thread, can't wait to read it
all in detail.

Greg.