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 centre 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.

The 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 travelled 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 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 work out the size of our planet at 230,000 stadia, or 39,690km (24,662 miles) – an error of less than 2 per cent.

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.

See also: Nicolaus Copernicus • Johannes Kepler