"although the Greek prefixes tend to alter as they form compounds
(so for instance "penta" becomes "pente" in "penteconta" = 50),
this is less natural in English."
This is the wrong way round: five is pente, fifty is penteconta (but in
fact the 'e' in 'penteconta' is eta, while that in 'pente' is epsilon);
in compounds of the type we were interested in (pentagon = with five
corners) the form was always 'penta-', as in 'pentateuchos' = 'consisting
of 5 books'.
In just the same way, 'eight' = 'octo' while in compounds of this type we
find 'octa-', e.g. 'oktaknemos' = 'having 8 spokes' So it's not right to
say '"octagon" is the established English spelling for what would be
"octogon" if taken directly from the Greek.'
The Greeks will have had 'oktagon', so he who coined 'octagon' for English
knew what he was doing. What needs explaining is why we have 'octopus', not
'octapus' . (The answer is that both 'oktapous' and 'oktopous' are found
in reputable Greek writers, so even a pedant will probably admit that one
is entitled to draw on either form.)
Paul Pritchard
Swffryd