20 Prologue A Tale That Begins onthe Mn
In Ancient Greece, Where the Size of Earth Was
The ancient Greeks tried to explain the shape of the universe by using the logical thinking
that permeates modern mathematics and physics. One of their greatest achievements was the
discovery that Earth is a spherical body floating in space. The ancient Greeks were also the
first to calculate the size of Earth.
Eratosthenes (who lived from about 276 BC to 195 BC) was a Greek scholar who was
active in Egypt during the Hellenistic period. He calculated the size of Earth using the fol-
Eratosthenes' Calculation Method
Eratosthenes discovered an account that said that a rod standing vertically at midday on
the summer solstice in Syene (in the southern part of Egypt) did not cast a shadow. It
seemed that this phenomenon could only occur south of the Tropic of Cancer, when the
Sun appeared at the zenith (directly overhead).
The astonished scholar wondered what would happen in Alexandria, in the northern
part of Egypt, and he immediately performed the experiment under the same conditions.
The result was that the shadow of the rod remained visible. Eratosthenes concluded from
this evidence that Earth is a sphere, a theory that was being discussed among some schol-
ars at the time.
Eratosthenes also used his observations to try to measure the size of Earth. First, he
measured the length of the rod’s shadow. He calculated that in Alexandria at the same time
on the same day, the Sun’s rays arrived from a direction that was offset from the vertical by
Next, he had a man walk from Alexandria to Syene and determined from the
man’s stride that the distance was 5,000 stadia (an ancient unit of measurement), or
Tropic of Cancer
Tropic of Capricorn
The Tropics and the Equator