Strabo, Geography
H.C. Hamilton, Esq., W. Falconer, M.A., Ed.

The next objection of Hipparchus is likewise false. He attempts to prove that Eratosthenes, in his statement that the route from Thapsacus to the Caspian Gates is 10,000 stadia, gives this as the distance taken in a straight line; such not being the case, as in that instance the distance would be much shorter. His mode of reasoning is after this fashion. He says, ‘According to Eratosthenes, the mouth of the Nile at Canopus,¹ and the Cyaneæ,² are under the same meridian, which is distant from that of Thapsacus 6300 stadia. Now from the Cyaneæ to Mount Caspius, which is situated close to the defile³ leading from Colchis to the Cas- pian Sea, there are 6600 stadia,⁴ so that, with the exception of about 300 stadia, the distance from the meridian of the Cyaneæ to that of Thapsacus, or to that of Mount Caspius, is the same: and both Thapsacus and Mount Caspius are, so to speak, under the same meridian.⁵ It follows from this that the Caspian Gates are about equi-distant between Thapsacus and Mount Caspius, but that the distance between them and Thapsacus is much less than the 10,000 stadia mentioned by Eratosthenes. Consequently, as the distance in a right line is much less than 10,000 stadia, this route, which he considered to be in a straight course from the Caspian Gates to Thapsacus, must have been a circumbendibus.’

To this we reply, that Eratosthenes, as is usual in Geography, speaks of right lines, meridians, and parallels to the equator, with considerable latitude, whereas Hipparchus criticizes him with geometrical nicety, as if every line had been measured with rule and compass. Hipparchus at the same time himself frequently deciding as to right lines and parallels, not by actual measurement, but mere conjecture. Such is the first error of this writer. A second is, that he never lays down the distances as Eratosthenes has given them, nor yet reasons on the data furnished by that writer, but from mere assumptions of his own coinage. Thus, where Eratosthenes states that the distance from the mouth of the [Thracian Bosphorus] to the Phasis is 8000 stadia, from thence to Dioscurias 600 stadia,⁶ and from Dioscurias to Caspius five days' journey, (which Hipparchus estimates at 1000 stadia,) the sum of these, as stated by Eratosthenes, would amount to 9600 stadia. This Hipparchus abridges in the following manner. From the Cyaneæ to the Phasis are 5600 stadia, and from the Phasis to the Caspius 1000 more.⁷ There fore it is no statement of Eratosthenes that the Caspius and Thapsacus are under the same meridian, but of Hipparchus himself. However, supposing Eratosthenes says so, does it follow that the distance from the Caspius to the Caspian Gates, and that from Thapsacus to the same point, are equal.⁸