From the difference in the length of the shadows he deduced that this distance represented one-fiftieth of the circumference of the earth, which would accordingly be about 250,000 stadia, or 25,000 geographical miles.

As the actual circumference is 24,899 English miles, this was a very near approximation, considering the rough means Eratosthenes had at his disposal.
Having thus estimated the size of the earth, Eratosthenes then went on to determine the size of that portion which the ancients considered to be habitable.

North and south of the lands known to him, Eratosthenes and all the ancients considered to be either too cold or too hot to be habitable; this portion he reckoned to extend to 38,000 stadia, or 3800 miles.

In reckoning the extent of the habitable portion from east to west, Eratosthenes came to the conclusion that from the Straits of Gibraltar to the east of India was about 80,000 stadia, or, roughly speaking, one-third of the earth's surface.

The remaining two-thirds were supposed to be covered by the ocean, and Eratosthenes prophetically remarked that "if it were not that the vast extent of the Atlantic Sea rendered it impossible, one might almost sail from the coast of Spain to that of India along the same parallel." Sixteen hundred years later, as we shall see, Columbus tried to carry out this idea.