measurement. He compared the length of the shadow thrown by the sun
at Alexandria and at Syene, near the first cataract of the Nile,
which he assumed to be on the same meridian of longitude, and to be
at about 5000 stadia (500 miles) distance. 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. Eratosthenes
based his calculations on two fundamental lines, corresponding in a
way to our equator and meridian of Greenwich: the first stretched,
according to him, from Cape St. Vincent, through the Straits of
Messina and the island of Rhodes, to Issus (Gulf of Iskanderun); for