to get some notion of the size of the Earth. Since Earth was known to be a
round ball, the key measurement was the size of a degree of longitude, that is,
(^1) ⁄ 360 of its circumference. The first (and as it turned out the best) estimate was
given in the third century B.C.E. by the Greek geographer and mathemati-
cian Eratosthenes. He arrived at his estimate by measuring the angle of shad-
ows cast at noon both in Alexandria, where he was director of the great
library, and also about 800 kilometers (500 miles) to the south in what is
today called Aswan. With simple geometry, he estimated a degree of longi-
tude at 59^1 ⁄ 2 miles. Much of the scientific speculation of the Greeks was lost as
a result of the collapse of the western part of the classical world, but their
tradition was carried on by Muslim scholars. So it was that a medieval
Muslim geographer, al-Faraghani, recalculated Eratosthenes’ experiment
and arrived at 56^3 ⁄ 4 miles for a degree of longitude.
Given the tools that Eratosthenes and al-Faraghani had to work with,
these measurements were remarkably precise, but when passed down to
later readers, they were distorted by one crucial factor: the length of a mile.
That measure varied from place to place. When Columbus read of al-
Faraghani’s measurement, he transposed al-Faraghani’s mile into the
shorter Italian mile and came up with a degree nearly 12 miles shorter than
al-Faraghani had meant. For Columbus, therefore, the world was thus
about 25 percent smaller than the real world. He was in good company. In
the influential work Imago Mundi,Cardinal Pierre d’Ailly argued (without
objective reasons) that the “Ocean Sea” was not immense; and in 1474 the
Florentine cosmographer Paolo Toscanelli (again with no objective reason)
put a number on it: he pontificated that only about 5,000 miles separated
Europe and Cathay (China), instead of 15,000 miles.
Reaching Cathay was what Columbus desperately wanted to do. He was
inspired by Marco Polo’s account, as we can see from the fact that his own
annotated copy of The Travels(Il Milione) has survived. Marco Polo’s
Cathay was the magnet that drew Columbus’s mind to the Atlantic. So, with
the blessing and finances of Queen Isabella of Castile, he set off in three little
carracks from Palos, on the southern Atlantic coast of Spain, on August 3,
- A week later they were in the Canaries. On September 6, with sails
flapping in the calm air, his ships set off again. Then, picking up the “wester-
lies” offshore, they sailed steadily to the west for a month of nearly perfect
days until, on October 12, they caught sight of an island in the Bahamas.
The Fearsome Atlantic 27