In the third century b.c.e., Eratosthenes of Cyrene made a famous measurement of the circumference of the Earth. This was not the first such measurement, but it is the earliest for which significant details are preserved. Cleomedes gives a short account of Eratosthenes’ method, his numerical assumptions, and the final result of 250,000 stades. However, many ancient sources attribute to Eratosthenes a result of 252,000 stades. Historians have attempted to explain the second result by supposing that Eratosthenes later made better (...) measurements and revised his estimate or that the original result was simply rounded to 252,000 to have a number conveniently divisible by 60 or by 360. These explanations are speculative and untestable. However, Eratosthenes’ estimates of the distances of the Sun and Moon from the Earth are preserved in the doxographical literature. This essay shows that Eratosthenes’ result of 252,000 stades for the Earth’s circumference follows from a solar distance that is attributed to him. Thus it appears that Eratosthenes computed not only a lower limit for the size of the Earth, based on the assumption that the Sun is at infinity, but also an upper limit, based on the assumption that the Sun is at a finite distance. The essay discusses the consequences for our understanding of his program. (shrink)

Recent investigations have thrown new light on such topics as the early Greek belief in heliocentricity, the relation between Greek and Babylonian astronomy, the reliability of Ptolemy's Syntaxis, Hipparchus's theory of motion for the sun, Hipparchus's value for the obliquity of the ecliptic, and Eratosthenes' estimate of the size of the earth. Some claims resulting from these investigations are controversial, especially the reevaluation of Ptolemy (though it is notable that no one any longer uses data from the Syntaxis for investigating (...) such things as the spin of the earth). This essay presents the evidence for these claims; it makes no pretense of presenting the evidence against them. (shrink)