Most of the Earth’s interior is far beyond access by visual observation or direct sampling, and therefore concepts of its nature must be based upon indirect geophysical measurements. The simplest geophysical measurement of the Earth is its surface dimension. At least as long ago as 550 B.C., Pythagoras believed the Earth to be a sphere, because the sun and moon were observed to be so. About 350 B.C. Aristotle, tutor to Alexander III of Macedon, agreed, noting that the Earth casts a circular shadow during eclipses of the moon. Moreover, at the many seaports of the Mediterranean coasts there must have been innumerable observations that only the sails of ships were visible at a distance, their hulls being concealed by the curve of the Earth between the seaports and the ships. This probably was the reason why the Pharos (lighthouse) at Alexandria built in 280 B.C. was so high (135 m). Presumably, on the basis of such information, Aristotle guessed the circumference of the Earth to be 400,000 stadia; at 1 stadium = 185.2 m, this 74,000-km circumference is 85 per cent too large.
Weitere Kapitel dieses Buchs durch Wischen aufrufen
- Internal Igneous Structure
K. O. Emery
- Springer New York
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