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Eratosthenes, Measuring the Earth > 
Eratosthenes of Cyrene was a Greek scholar who lived and worked in Cyrene and Alexandria. Eratosthenes was director of the famous library in Alexandria, and is known for numerous important contributions to mathamatics, geography, and astronomy. In particular, he is remembered for a technique he introduced which enabled him to compute the first reliable determination of the true size of the earth.
This technique is based on the observation by Eratosthenes that the sun is directly overhead at noon in Syene in southern Egypt on the first day of summer. (This is the time of the summer solstice.) While visiting Syene Eratosthenes apparently stopped at a well and noted that the noon sun reflected directly back from the water in the bottom of the well.
Since Eratosthenes knew that the earth was a sphere, he correctly reasoned that if he could determine the altitude of the noon sun at some other location on the first day of summer, AND if he knew the distance between these two locations, he could compute the circumference of the earth as a simple ratio.
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More on this Website >
• http://www.phys-astro.sonoma.e ... vatory/eratosthenes/
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Ptolemy, Astronomer / Geographer
Claudius Ptolemy lived in Alexandria (in Egypt) from approx. 87 -150 AD. He was an astronomer, mathematician and geographer. He codified the Greek geocentric view of the... |
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John Harrison, Solved Longitude - 1773
John Harrison was an English clockmaker, who designed and built the world's first successful chronometer (maritime clock), one whose accuracy was great enough to allow th... |
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Delambre, The Metric System
In 1795 Jean Baptiste Joseph Delambre was admitted to the Bureau des Longitudes, becoming President in 1800. In 1801 he was appointed secretary to the Académie des Scienc... |
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