Geography Reference
In-Depth Information
Geographers from ancient Greece through the 19th century were largely devoted to exploring
the world, gathering information about newfound lands, and indicating their locations as ac-
curately as possible on maps. Sometimes the great explorers and thinkers got it right, and
sometimes they did not (see the sidebar called “Measuring the Earth”). But in any event, geo-
graphy and exploration became intertwined; so, “doing geography” became closely associated
with making maps, studying maps, and memorizing the locations of things (see Chapters 3
through 5 for information on locating things and creating and reading maps).
Measuring the Earth
In the third century B.C., the Greek scholar Eratosthenes made a remarkably accurate
measurement of Earth's circumference. At Syene (near Aswan, Egypt), the sun illumin-
ated the bottom of a well only one day every year. Eratosthenes inferred correctly this
could only happen if the sun were directly overhead the well — that is, 90º above the
horizon. By comparing that sun angle with another one measured in Alexandria, Egypt,
on the same day the sun was directly overhead at Syene, Eratosthenes deduced that
the distance between the two locations was one-fiftieth (1/50th) of Earth's circumference.
Thus, if he could measure the distance from Syene to Alexandria and multiply that num-
ber times 50, the answer would be the distance around the entire Earth.
There are diverse accounts of the method of measurement. Some say Eratosthenes
had his assistants count camel strides (yes, camel strides) that they measured in
stade
,
the Greek unit of measurement. In any event, he came up with a distance of 500
miles between Syene and Alexandria. That meant Earth was about [500 x 50 =] 25,000
miles around. (“About” because the relationship between stade and miles is not exactly
known.) The actual average circumference is 24,680 miles so Eratosthenes was very
close.
About a century-and-a-half later, another Greek named Posidonius calculated Earth's cir-
cumference and came up with 18,000 miles. Posidonius' measurement became the gen-
erally accepted distance thanks to Strabo, the great Roman chronicler, who simply did
not believe that the Earth could be as big as Eratosthenes said it was. About 18 A.D.
Strabo wrote his
Geography
, which became the most influential treatise on the subject
for more than a millennium.
Geography
credited the calculations of Posidonius and rejec-
