Measuring the Earth (Geodesy)
On a single midsummer noon, two shadows in two cities let one man weigh the whole earth's girth in numbers.
Around 240 BCE, the Alexandrian polymath Eratosthenes realized he could measure the entire earth without leaving Egypt. He knew that at noon on the summer solstice the sun stood directly overhead at Syene (modern Aswan), casting no shadow, while at Alexandria to the north a vertical pole still cast one. By measuring that shadow's angle—about 1/50th of a circle—and the distance between the two cities, he scaled up to a circumference of roughly 252,000 stadia, strikingly close to the true value. It was one of antiquity's most elegant feats: pure geometry turning sunlight and shadow into the size of the world.
How it traveled
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Key passages(12)
9. Those who know names for very many winds will perhaps be surprised at our setting forth that there are only eight. Remembering, however, that Eratosthenes of Cyrene, employing mathematical theories
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Artemidorus says that, as one goes from Physcus, in the Peraea of the Rhodians, to Ephesus, the distance to Lagina is eight hundred and fifty stadia; and thence to Alabanda, two hundred and fifty more
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The complete voyage around its shores, as one would encircle an island, is a distance of 23,000 stadia, as is asserted by Eratosthenes, Hecataeus, Ptolemy, and other very accurate investigators of suc
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As for its size, Sosicrates, whose account of the island, according to Apollodorus, is exact, defines it as follows: In length, more than two thousand three hundred stadia, and in breadth, . . . , so
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The voyage from Cyrenaea to Criumetopon takes two days and nights, and the distance from Cimarus to Taenarum is seven hundred stadia, Cythera lying between them; and the voyage from Samonium to Aegypt
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It is said that the last part of the Taurus, which is called Imaïus and borders on the Indian Sea, neither extends eastwards farther than India nor into it; but that, as one passes to the northern sid
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Eratosthenes gives the distances as follows: From Mt. Caspius to the Cyrus River, about one thousand eight hundred stadia; thence to the Caspian Gates, five thousand six hundred; then to Alexandreia i
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Sicily is triangular in shape; and for this reason it was at first called “Trinacria,” though later the name was changed to the more euphonious “Thrinacis.” Its shape is defined by three capes: Pelori
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Now the above distances are put down in accordance with the data of Artemidorus; but according to the Chorographer, the distances from Brentesium as far as Garganum amount to one hundred and sixty-fiv
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Of this seaboard, then, the first parts are those about Epidamnus and Apollonia. From Apollonia to Macedonia one travels the Egnatian Road, towards the east; it has been measured by Roman miles and ma
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Now the Peloponnesus is like a leaf of a plane tree in shape, its length and breadth being almost equal, that is, about fourteen hundred stadia. Its length is reckoned from the west to the east, that
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Polybius states that the distance from Maleae towards the north as far as the Ister is about ten thousand stadia, but Artemidorus corrects the statement in an appropriate manner by saying that from Ma
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