Brian {Hamilton Kelly} wrote:

Anyway, to work (using \TeX notation for the mathematics, which is
portable across plain ASCII and O/S independent):

T^2 = {4\pi^2}\over{GM}r^3

Where T is the orbital period, G the universal graviational constant
(6.672\times10^{-11} Newton-metres-squared per kg squared), M the
mass of the Earth (5.976\times10^{24} kg: the mass of the satellite
itself is, of course, negligible in comparison and omitted) and r the
radius of the orbit relative to the centre of the planet.

I agree with your mathematics,
but not with your belief that any schoolboy could come up with it.
I doubt if 0.01% of UK schoolboys (and a lower proportion of schoolgirls)
could follow your argument, let alone repeat it.

Regarding the argument about the angle of the dish,
I misunderstood your comment,
as I have never heard the point where the directrix meets a parabola
referred to as its "centre".

--
Timothy Murphy
e-mail (80k only): tim /at/ birdsnest.maths.tcd.ie
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College, Dublin 2, Ireland