> > The knowledge that the earth was a sphere and the proof of it and the
> > measurement of the size was made well before Christ's time. And the
> > measurement was reasonably accurate.

> I've never heard of this before.  I am intrigued.  Could you elaborate on
> when, where, etc., this knowledge first came to be please?

In about 200 BC Eratosthenes determined the earth's circumference. He noted
that at Alexandria (essentially on the equator) the sun shone vertically
down a well at mid-day but that at Syene (now Aswan) the angle on the same
day of the year was about 7 degrees. By measuring the distance between he
was able to calculate the size of the full circle.

Eratosthenes was the director of THE Library in Alexandria - the pre-eminent
repository of knowledge in the world at that time, so he had the best
available resources available and quite possibly drew on the thinking of his
underlings (which is often enough how Science tends to work.)
Notwithstanding, the available information was there for the thinking
thinker to think about.

        http://www.phys-astro.sonoma.edu/observatory/eratosthenes/

Eratosthenes result was about 6% wrong. It has been suggested (I don't know
on what basis) that the method he used was really to measure the distance to
the horizon from a suitably high point and calculate the size of the earth
accordingly. Such a method would give an error of about 6% if atmospheric
refraction was not taken into account. While he did have one of the highest
man-made landmarks available (the famed Alexandrian light-house - one of the
7 wonders of the world) )

        http://ce.eng.usf.edu/pharos/wonders/pharos.html

It used fire by night and a mirror by day. The reflected rays from its
mirror could be seen from 35 miles out to sea in daylight.

The chances that this was the true method used for the circumference
calculation seems less likely as it is argued that the accuracy measurement
of horizon distance  is difficult. That said, a counter argument can be made
from the ability to easily detect or not detect the mirror reflections at a
distance - the cutoff point IS the horizon if the light is bright enough.

Other people anywhere could have performed similar calculations using
suitably high natural features. It may be interesting to try this without
use of modern resources.


            Russell McMahon

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