[ATM] eratosthenes

[Guy Brandenburg]

Part of what Bob says I agree with, but I think part is in error.

Eratosthenes lived in Alexandria. His other point was a well on Elephantine Island at some cataract on the Nile near or in a town called Syene (modern-day Aswan, where the big dam is). Why that well? Well, I'll tell you. (Har de har) The well happened to be located very, very close to the  Tropic of Cancer (which hadn't been invented yet) and Eratosthenes had heard that every year on the summer solstice, the sunlight would shine directly to the bottom of the well. So, he only really needed one other measurement: the angle from the zenith of the sun as viewed on the same day of the year at his location in Alexcandria. Plus, he needed to find out how far it was from Syene to Alexandria.

As far as I know, Athens didn't come into play at all.

This is off-topic, of course. Eratosthenes didn't make telescvopes.

Guy Brandenburg, Washington, DC 

My blog, mostly on Education in DC:
http://gfbrandenburg.wordpress.com/

My home page on astronomy, mathematics, education:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
or else   
http://tinyurl.com/r6fh2
=====================================


--- On Tue, 9/7/10, Bob May <rmay at nethere.com> wrote:

> From: Bob May <rmay at nethere.com>
> Subject: Re: [ATM] eratosthenes
> To: atm at atmlist.net
> Date: Tuesday, September 7, 2010, 8:30 PM
> The exact time of noon isn't needed
> for this experiment.  All
> that is needed is the local noon and that is obtained by
> watching
> the sun's shadow on the ground.    The larger the
> distance
> between the two nlocations north to south, the better the
> results
> will be.  The only tools of any accuracy needed ar the
> angle
> measuring tools of finding a vertical and being able to
> measure
> it accurately.  I will note that you can usually
> assume tall
> buildings are pretty vertical which makes for some nice
> high
> gnomons for the sun's shadow.
> The Erastonethes measurement was from Athens to
> Alexendria,
> across the Med.  The actual measurements were done on
> successive
> years at the summer solstice.
> Bob May
> 
> rmay at nethere.com
> http: slash /nav.to slash bobmay
> http: slash /bobmay dot astronomy.net
> 
> 
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