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ERATOSTHENES was one of the most distinguished men produced by the School of Alexandria. He is regarded by Delambre as the founder of scientific astronomy, on the ground of his alleged institution, in one of the observatories of Alexandria, or armillary spheres, with circles representing the meridian, the equator, and the winter and summer solstice.
The principal achievement of Eratosthenes was his attempt to determine by astronomical observation the dimensions of the planet. His method was perfectly simple and accurate. Taking two points on the earth's surface, lying on the same meridian, and of known distance from each other, he proposed to observe at noon, in each place, the distance of the sun from the zenith. Syene in Upper Egypt and Alexandria were the two points selected. At Syene it was known that at the summer solstice the son shone into a deep well of that place; in other words, the distance from the zenith was zero. At Alexandria on the same noon, the distance from the zenith was a fiftieth part of the circumference (i.e. 7° 12´). Therefore the distance between Syene and Alexandria, which was regarded by the Alexandrian surveyors as 5000 stadia, was one-fiftieth of the earth's circumference, presuming the earth to be spherical. Syene and Alexandria are not, however, precisely on the same meridian: nor do we know with the least precision the value of the stadium employed. The importance of the calculation lies entirely in the method adopted.
Eratosthenes made a more accurate observation of the distance between the tropics, i.e. of the obliquity of the ecliptic, than any of his predecessors. It is commonly said that he fixed it at 47° 42' 39". But this is hardly accurate, and is very misleading. The ancients were wholly without instruments enabling them to observe seconds, or even minutes. The circles used at Alexandria were graduated to sixth parts of a degree only. What Eratosthenes found was that the distance between the tropics was to the whole circumference as 11 to 83. Bringing this fraction to parts of 360, all that can be said is that Eratosthenes observed the distance to be either 47° 40' or 47° 50': more probably the former. The Greek instruments were far inferior in precision to those used afterwards by Arabians and even Turks. Ulugh Begh, the grandson of Tamerlane, by a gnomon 180 feet in height, determined the obliquity more accurately.
Having thus 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.
Of the discoveries of Eratosthenes, as of those of Aristarchus, Comte remarks that the dependence of climate, of seasons, of day and night, of the division of time, upon conditions of latitude and longitude, gave a shock to the absolute system, by showing that all these, like the direction of gravitational bodies, were relative notions without any arbitrary tendency.
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| This biography is
reprinted from The New Calendar of Great Men. Ed. Frederic
Harrison. London: Macmillan and Co., 1920. |
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