FIRTH@TL-20B.ARPA (03/14/86)
I don't think Aristarkhos' theory was "mere conjecture". There is quite
a lot of evidence for it.
(1) The planets Mercury and Venus remain close to the Sun. This is
good evidence that they are associated with that body, rather
than with the Earth. In fact, I believe even Ptolemy was ready
to concede that the "inner" planets revolved about the Sun
(2) Similarly, the perceived brightness of Venus varies greatly
as it moves. We know this to be due to a combination of
distance and phase, but even if you don't know that, it is
hard to accept that Venus rotates about the Earth in a circular
orbit.
(3) But more significant is the variation in the brightness of
MARS. This is the best evidence that the "outer" planets,
also, don't move in geocentric orbits.
(4) Finally, the Sun is much bigger than the Earth, and it is more
reasonable that the smaller body should move about the larger.
The ancients knew this: they had measured the distance of the
Moon by triangulation. They then measured the relative distances
of Sun and Moon by finding the Sun-Earth-Moon angle when half the
lunar disc was lit. They got a very poor answer, of course - that
the Sun was about 30 times as far away as the Moon - but even at
that under estimate the Sun is more than 4 times as big (linear) as
the Earth.
The one killer argument against the heliocentric theory is the absence
of a perceived stellar parallax, as has been mentioned. The only counter
argument is that "the stars must be too far away", and that is both a
bad argument and pretty unbelievable. That it happens to be true is
rather unfair. On the basis of the evidence as available, the best
theory is the "Tychonic" - that the Sun moves round the Earth, but all
the (other) planets move round the Sun. First proposed, I believe, by
Hipparkhos of Nicea.
[For a good overview of the whole matter, see Koestler: The Sleepwalkers.
This also discusses the question of the "death" of Classical thought]
Robert Firth
-------