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Le 24.04.2011 18:03, [email protected] a écrit :
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In my class I was asked when the tropics of Cancer and Capricorn
were identified as two of the five great circles around the earth.
Wikipedia has failed me on the question. Surely these go back before the
Greeks who named them. I would like to impress my students with an
answer and would appreciate whatever you can give me.
The next one will probably be the equinoctial line. (Sigh!)
Tom Goodrich
Sorry for this late post but I was in Italy during the Easter vacation.
The notion of "tropic" is something easy to apprehend without deep
knowledge in astronomy. The zone of the earth situated between the 2
tropics is the the only area where the sun passes, at least, twice a
year at the exact zenith (=exactly perpendicular to the ground/- in
fact, to a plane tangent to our sphere/). In Europe, we cannot observe
the real zenith.
Eratosthenes experimented the exact zenith from the bottom of a dry well
at Aswan (=Syene) and by a comparison with the zenith angle at
Alexandria, he was able to calculate the earth circumference.
My opinion is that Eratosthenes well should have been rather at Amada,
today under the lake Nasser, 180 km south of Aswan, but, anyway, that
doesn't make a big difference for an eye measure.
Atb
_____________________________
Christos Nüssli
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Europe Maps - http://www.euratlas.org
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