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Eratosthenes
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One of the greatest of the ancient Greek sages and the first great geographer of the ancient world, Eratosthenes is considered the founder of physical and mathematical geography. He studied in Athens with Ariston of Chios and Archesilaus, then moved to Alexandria, where he spent most of the rest of his life. Ptolemy III Euergetes appointed him director of the famous Library of Alexandria. It was Eratosothenes who coined the term "philologist". Archimedes, his senior by 11 years, held him in very high esteem, and dedicated two of his works to him: "On the Method of Mechanical Theorems" and "The Cattle Problem". He is cited by Stobaeus. One of the moon's craters has been named "Eratosthenes" in his honour.
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Eratosthenes is most famous for making the first good measurement of the size of Earth. He did that sometime around 240 B.C. He knew that there was no shadow at the bottom of a well in the town of Syene on the summer solstice. That meant that the Sun must be straight overhead in Syene on that day. He measured the length of the shadow of a tall tower in Alexandria on the same day. He ... measured the distance between Syene and Alexandria. With this information, he was able to calculate the circumference of the Earth.
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Now, in reality, Eratosthenes, being a good administrator, probably had assistants measuring the altitude of the noon sun on a regular basis from Alexandria, so he could probably simply consult the records of solar observations from Alexandria. However he actually obtained the data, the difference between the noon sun elevations at Syene and Alexandria on the first day of summer was approximately 7 degrees. Furthermore, Eratosthenes probably did not need to measure the distance between the locations since he had the reliable geographic data accumulated by Alexander The Great.
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Eratosthenes had someone measure the distance between Alexandria and Syene. He used that distance, what he knew about the Sun's angles, and a bit of geometry to figure out the size of the Earth.
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Having ... used astronomy for the purpose of estimating the magnitude of the earth, Eratosthenes set himself to determine by the same method the boundaries of its habitable surface; to use his own words, to correct the old geographical map. He conceived this habitable area as extending from Cape St. Vincent eastwards through the Mediterranean and along the range of the Caucasus to the mouth of the GAnges for 78,000 stadia:about one-third of the earth's circumference. It would be possible, he said, for a vessel starting from Spain westward along the same parallel to reach the Ganges. He constructed a meridian from Alexandria northwards to Rhodes and Byzantium, and so onward to the mouth of the Borysthenes: southwards up the Nile to Meroe, and the land of the Sembritæ (Sennaar). His estimates of longitude were defective, owing to the want of any adequate means of measuring time. Thus the Mediterranean is represented as 26,500 stadia in length, an error of between 600 and 700 miles, which, however, in the maps of the 17th century still remains uncorrected.
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Eratosthenes lived around 276? to 194? B.C. He was born in Cyrene, Greek town in Northern Africa. His teachers included the scholar Lysanias of Cyrene and the philosopher Ariston of Chios who had studied under Zeno, the founder of the Stoic school of philosophy. Eratosthenes ... studied under the poet and scholar Callimachus who had also been born in Cyrene. Eratosthenes then spent some years studying in Athens.
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