Eratosthenes measures the circumference of the Earth
- Date
- c. 240 BCE
- Category
- Science, technology & invention
- Region
- Egypt (Alexandria)
- Era
- Classical Antiquity
- Significance
Around 240 BCE, Eratosthenes of Cyrene, chief librarian at Alexandria, calculated the circumference of the Earth using only geometry and shadows.
Knowing that at noon on the summer solstice the Sun shone directly down a well at Syene (modern Aswan), casting no shadow, he measured the angle of a shadow at Alexandria at the same moment and found it to be about 7.2 degrees — one-fiftieth of a full circle. Multiplying the reported distance between the two cities by fifty, he derived a circumference of about 250,000 stadia, a figure remarkably close to the modern value. The achievement demonstrated both that the Earth is a sphere and that its size could be determined by reason and observation. Eratosthenes also produced a map of the known world with a system of parallels and meridians, founding the discipline of mathematical geography and epitomising the empirical confidence of Hellenistic science centred on Alexandria.
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Empires active in c. 240 BCE
- Maurya Empire322 BCE – 185 BCE
- Roman Republic509 BCE – 27 BCE
- Odrysian Kingdom480 BCE – 46 CE
- Seleucid Empire312 BCE – 63 BCE
- Ptolemaic Kingdom305 BCE – 30 BCE
- Kingdom of Kush1070 BCE – 350 CE
Related science, technology & invention events
- Euclid compiles the Elements at Alexandriac. 300 BCE
- Aristotle founds the Lyceum and systematises the sciencesc. 335 BCE
- Archimedes formulates the principle of buoyancyc. 250 BCE
- Wootz crucible steel developed in South India and Sri Lankac. 300 BCE
- Construction of the Aqua Appia, Rome's first aqueduct312 BCE
- The Antikythera mechanism, an ancient analog computerc. 100 BCE
Further reading
The wider story
A Little History of Science
Warm, jargon-free tour of discovery from Babylonian stargazers to the genome.
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