He Measured the WHOLE EARTH With a Stick and a Shadow #Shorts
Pack: 53 - He Measured the WHOLE EARTH With a Stick and a Shadow
In the third century BCE, Eratosthenes, librarian of Alexandria, worked out the size of the whole Earth from one noon shadow. Here is how the measurement worked, and why "how accurate was it?" is still argued over. ▶ Full documentary on our channel: https://youtu.be/k3mrc7OCCNk
- Eratosthenes became the third librarian at Alexandria around 240 BCE; his own book on measuring the Earth is lost, and his method survives in later writers such as Cleomedes (MacTutor, University of St Andrews).
- At Syene (modern Aswan), a well marked the summer tropic: at midsummer noon the sun stood overhead and pointers cast no shadow (Strabo, Geography 17.1.48).
- At Alexandria, at the same noon, the shadow showed the sun about 7.2° from vertical, one-fiftieth of a full circle (MacTutor).
- Treating the sun's rays as parallel, 5,000 stadia between the cities times 50 gave 250,000 stadia for the whole Earth (J. L. Martin, University of Kansas; MacTutor). How close that is depends on the length of his stadion, which scholars still debate.
Genius, or lucky with his numbers? Tell us in the comments. Full story in our documentary, The Full History of Greece.
Quick answers
What is “He Measured the WHOLE EARTH With a Stick and a Shadow” about?
In the third century BCE, Eratosthenes, librarian of Alexandria, worked out the size of the whole Earth from one noon shadow. Here is how the measurement worked, and why "how accurate was it?" is still argued over. ▶ Full documentary on our channel: https://youtu.be/k3mrc7OCCNk
What are the key facts?
Eratosthenes became the third librarian at Alexandria around 240 BCE; his own book on measuring the Earth is lost, and his method survives in later writers such as Cleomedes (MacTutor, University of St Andrews). At Syene (modern Aswan), a well marked the summer tropic: at midsummer noon the sun stood overhead and pointers cast no shadow (Strabo, Geography 17.1.48).
What sources is this Short based on?
It cites MacTutor History of Mathematics (University of St Andrews), Eratosthenes; J. L. Martin (University of Kansas), Measuring the circumference of the earth; Strabo, Geography 17.1.48 (tr. H. L. Jones), LacusCurtius. Links are in the Sources list below.
Sources
- MacTutor History of Mathematics (University of St Andrews), Eratosthenes
- J. L. Martin (University of Kansas), Measuring the circumference of the earth
- Strabo, Geography 17.1.48 (tr. H. L. Jones), LacusCurtius
Images
- The Blue Marble, Apollo 17, 1972 — NASA (Apollo 17 crew) — Public domain —
- Eratosthenes, later engraving (imaginary likeness) — unknown — Public domain —
- The Nile at Aswan (ancient Syene) — Warren LeMay — CC0 —
- Alexandria, the harbour, c. 1890–1900 — Photoglob Co., Library of Congress — Public domain —
- Eratosthenes' world map, 19th-century reconstruction — National Library of Israel, Eran Laor Collection — Public domain —
- Blue Marble, Eastern Hemisphere — NASA (Reto Stöckli) — Public domain —
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