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Modern agreement with the study done by Eratosthenes in ancient Greece, requires the a priori premise that the earth is spherical and the sun’s rays are parallel.
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To be accurate, what his proposition provides for modern times, objectively interpreted, is as follows:
GIVEN: EARTH IS SPHERICAL & SUN’S RAYS PARALLEL; ERGO: CIRcuмFERENCE OF EARTH = 25,000 MILES.
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The argument flat-earthers make against it is, by replacing the “SPHERICAL” with “FLAT” and replacing “PARALLEL” with “DIVERGENT,” that 25,000 mile circuмference is no longer a valid conclusion!
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For by using the same mathematics but inserting formulas with “flat” earth and “diverging” sun rays (IOW they deny that the sun’s rays are parallel!), the same results of different length shadows are obtained, when the sun is presumed to be about 3,000 (or 3,400) miles away, instead of 93 million.
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Effectively, the essential proposition may be reduced to: "The sun’s rays are parallel."
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