Re: Fwd: Flat Earth?

From: D. F. Siemens, Jr. <dfsiemensjr@juno.com>
Date: Wed Nov 23 2005 - 21:52:32 EST

On Wed, 23 Nov 2005 13:59:13 -0700 (MST) gordon brown
<gbrown@euclid.colorado.edu> writes:
> One of the earliest proofs that the Greeks used to convince
> themselves
> that the earth is spherical was that its shadow as seen in a lunar
> eclipse
> was always circular. I think Plato used this, but I don't know the
> reference. In the third century B.C. Eratosthenes obtained a fairly
> accurate estimate for the circumference of the earth when he
> compared
> differences in latitude (determined from the angle of the sun above
> the
> horizon) with actual measured distance. This enabled the Greeks to
> use
> a lunar eclipse to determine the size of and distance to the moon.
> They
> also made a valiant attempt to find the distance to the sun, but
> their
> result was very inaccurate because of the difficulty in precisely
> determining the angle of the sun-moon-earth triangle at the earth
> vertex.
>
The reference is Aristotle, /De Caelo/ (On the Heavens), Book II, Chapter
14, 297a9 et sqq. He begins with a logical point on sphericity, which he
considers more telling than observation. The shadow of the earth on the
moon is noted 297b23-31; the change of the appearance of the stars as one
moves on the earth's surface, 297b31-298a8. 298a15-20 gives an estimate
of the circumference of the earth as 400,000 stades, which I calculate to
be about 71,000 Km or 46,000 miles.
Dave
Received on Wed Nov 23 21:56:10 2005

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