Fortunately, as it resulted, this overestimate was not allowed to stand uncontested. Posidonius of Rhodes (B.C. 135-51), by an independent calculation based upon the difference in altitude of Canopus at Rhodes and at Alexandria, reached a result which is reported by Cleomedes as 240,000, and by Strabo as 180,000 stadia.[271] The final judgment of Posidonius apparently approved the smaller number; it hit, at all events, the fancy of the time, and was adopted by Marinus of Tyre and by Ptolemy,[272] whose authority imposed it upon the Middle Ages. Accepting it as an independent estimate, it follows that Posidonius allowed but 500 stadia to a degree, instead of 700, thus representing the earth as about 28 per cent. smaller than did Eratosthenes.[273] To the earliest writers the known lands constituted the earth; they were girdled, indeed, by the river Oceanus, but that was a narrow stream whose further bank lay in fable-land.[274] The promulgation of the theory of the sphericity of the earth and the approximate determination of its size drew attention afresh to the problem of the distribution of land and water upon its surface, and materially modified the earlier conception. The increase of geographical knowledge along lines of trade, conquest, and colonization had greatly extended the bounds of the known world since Homer’s day, but it was still evident that by far the larger portion of the earth, taking the smallest estimate of its size, was still undiscovered,—a fair field for speculation and fantasy.[275]