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# Eratosthenes

[ er-*uh*-**tos**-th*uh*-neez ]

## noun

- 276?–195? b.c., Greek mathematician and astronomer at Alexandria.

Eratosthenes

/ ˌɛrəˈtɒsθɪˌniːz /

## noun

- Eratosthenes?276 bc?194 bcMGreekSCIENCE: mathematicianSCIENCE: astronomer ?276–?194 bc , Greek mathematician and astronomer, who calculated the circumference of the earth by observing the angle of the sun's rays at different places

Eratosthenes

/ ĕr′ə-tŏs**′**thə-nēz′ /

- Greek mathematician and astronomer who is best known for making an accurate estimate of the circumference of the Earth by measuring the angle of the Sun's rays at two different locations at the same time. He also invented a method for listing the prime numbers that are less than any given number.

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## Biography

Had he been born and raised farther north or south, the ancient astronomer Eratosthenes might never have come to think about the circumference of the Earth. It so happened that in his hometown of Syene (now Aswan), Egypt—which lies just north of the Tropic of Cancer—the Sun's rays were almost exactly perpendicular to the ground at noon on the summer solstice. One year in Alexandria, about 500 miles away, he noticed that the Sun's rays hit the ground at a deviation of about 7 degrees from the vertical on the same date and time. He believed, correctly, that the Sun was very distant and that its rays were essentially parallel when they hit the Earth. Therefore, he reasoned that the difference between the angles of incidence in Syene and Alexandria could only be due to curvature of the Earth's surface. Fairly basic geometry allowed him to use these figures to calculate the circumference of the Earth. Although one usually reads that his calculation was very close to that of modern scientists, we do not know this for certain; the units of length that he used (called stadia) were not fixed throughout the ancient world, and there is no record of precisely which length he had in mind. His measurement could have been anywhere from 0.5 to 17 percent off from modern measurements. Whatever the case, Eratosthenes's calculations were remarkably good, not only for being the very first known measurement of the Earth's circumference, but also for being made when not everybody even thought the Earth was round.

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## Example Sentences

There is no ground for considering the Geography of Strabo an improved edition of that of Eratosthenes.

From Project Gutenberg

In conclusion, he again corrects Eratosthenes as regards the magnitude and divisions of the inhabited world.

From Project Gutenberg

The next scheme of musical ratios is that of Eratosthenes, who makes no use of the natural Major Third.

From Project Gutenberg

The places in a straight line with these, as far as India, are described in the same manner by Artemidorus and Eratosthenes.

From Project Gutenberg

Eratosthenes relates the same facts, for he speaks of a winter and a summer sowing, and of the rain at the same seasons.

From Project Gutenberg

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