The Science Book

22

T

he Greek astronomer

and mathematician

Eratosthenes is best

remembered as the first person to

measure the size of Earth, but he

is also regarded as the founder of

geography—not only coining the

word, but also establishing many

of the basic principles used to

measure locations on our planet.

Born at Cyrene (in modern-day

Libya), Eratosthenes traveled

widely in the Greek world, studying

in Athens and Alexandria, and

eventually becoming the librarian

of Alexandria’s Great Library.

It was in Alexandria that

Eratosthenes heard a report that

at the town of Swenet, south of

Alexandria, the Sun passed directly

overhead on the summer solstice

(the longest day of the year, when

the Sun rises highest in the sky).

Assuming the Sun was so distant

that its rays were almost parallel to

each other when they hit Earth, he

used a vertical rod, or “gnomon,”

to project the Sun’s shadow at

the same moment in Alexandria.

Here, he determined, the Sun was

MEASURING THE

CIRCUMFERENCE

OF EARTH

ERATOSTHENES (276–194 BCE)

IN CONTEXT

BRANCH

Geography

BEFORE

6th century BCE Greek

mathematician Pythagoras

suggests Earth may be

spherical, not flat.

3rd century BCE Aristarchus

of Samos is the first to place

the Sun at the center of the

known universe and uses

a trigonometric method to

estimate the relative sizes of

the Sun and the Moon and

their distances from Earth.

Late 3rd century BCE

Eratosthenes introduces the

concepts of parallels and

meridians to his maps

(equivalent to modern

longitude and latitude).

AFTER

18th century The true

circumference and shape

of Earth is found through

enormous efforts by French

and Spanish scientists.

Sunlight reached Swenet at right

angles, but cast a shadow at Alexandria.

The angle of the shadow cast by the

gnomon allowed Eratosthenes to

calculate Earth’s circumference.

Alexandria

Swenet

Earth

7.2° south of the zenith—which is

1/50th of the circumference of a

circle. Therefore, he reasoned, the

separation of the two cities along

a north–south meridian must be

1/50th of Earth’s circumference.

This allowed him to figure out the

size of our planet at 230,000 stadia,

or 24,662 miles (39,690 km)—an

error of less than 2 percent. ■

7. 2 ̊

Sunrays

7. 2 ̊

Gnomon

See also: Nicolaus Copernicus 34–39 ■ Johannes Kepler 40–41