I hope this incipient thread from Classics-L is of general interest.
Bill Thayer
ListOwner, RomanSites-L
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Date: Sun, 27 Apr 1997 17:58:56 -0400 (EDT)
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From: "Daniel p. Tompkins" <[EMAIL PROTECTED]>
To: classics <classics>
Subject: wondering about Eratosthenes
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Handbook lore from history of science texts: Eratosthenes in Aswan
happens to notice that the sun at high noon on a summer solstice
reflects directly off the water in a well. Bingo! He hastens to find the
angle of a shadow cast by the sun in Alexandria, 500 miles away, on the
solstice (did he wait a year in between? Did someone just happen to
be measuring shadow angles in Alexandria? Weren't there more interesting
diversions in that capital of concupiscence?).
The angle in Alexandria turned out to be 7+ degree, more or less 1/50
of a cirlce, 360. So Eratosthenes simply multiplied 50x500 and got the
circumference of the earth, 25,000--a few hundred miles over the
correct modern figure.
This neat and conclusive narrative, packed with coincidences, may well
be true. But it sounds too much like the life of Oedipus--things "just
happening" all over the place. I don't have the sources here at home
but wonder if anyone has written on this almost too good to be true story.
(Eratosthenes himself did, in his *On the Measurement of the Earth,*
perhaps part of his *Geographica,* to which Strabo alludes frequently
and disparagingly according to the OCD.)
I'd be interested in what others think.
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Date: Sun, 27 Apr 1997 20:13:19 -0400 (EDT)
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From: Phillip Snider <[EMAIL PROTECTED]>
To: classics <classics>
Subject: Re: wondering about Eratosthenes
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I've had a look at Eratosthenes a while back for a presentation I
co-taught with one of my professors at a public school last year. My
impression is that most scholars accept the calculations as being
Eratosthenes. As I recall, Strabo is pretty clear on that point, even if
he thinks that Eratosthenes is wrong. As for his method, I think you are
right that it does sound a bit neat as the textbooks suggest. I would
suggest that the shadow angles in Alexandria had probably been well known
in this period and, since Eratosthenes did do some astronomy, he would
probably either know them or could gain access to the information. So, the
missing piece is the Aswan information which Eratosthenes could have
gathered himself, or may also have been known. Mind you, the well story
may well be apocryphal, or not. I'm not sure that matters. What does
matter is that Eratosthenes is credited with making that (frankly, quite
wild) theoretical leap. Keep in mind that, if we go by Strabo,
Eratosthenes didn't exactly win a ringing endorsement of his view. It
really is we moderns who feel the itch to invoke arguments of personal
incredulity (sorry, I like the term and couldn't resist :) ).
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Date: Mon, 28 Apr 1997 17:55:38 -0500 (CDT)
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From: [EMAIL PROTECTED] (thomas winter)
To: classics
Subject: Re: Eratosthenes
No need to be in Aswan, no need to measure a baseline:
Herodotus early in bk two gives distances from coast to town
to town up the Nile to Elephantine (which is just past Syene
=modern Aswan). Just getting a distance from Syene upriver
to Elephantine and subtracting it from Herodotus' coast - to -
Elephantine distance would've given a distance. My point is,
Herodotus encourages me to bet the farm Eratosthenes just stayed in
Alexandria, __heard about__ the solstitial well in Syene, saw the
possibilities, and just looked up a baseline distance
somewhere -- perhaps even in Herodotus!
I sometimes suspect Strabo's problem with Eratosthenes is that he
didn't understand the geometry. I think it certain that Vitruvius did
not. He treats it as a marvel of nature, for instance, that gnomon
angles have to be different for different cities!
E's achievement was to realize what the solstitial sunlit well
meant: that the 3 points--sun, Syene, earthcenter--lay on a straight
line and that any vertical--like his at Alexandria--would if extended
down intersect that line. Solar rays being essentially parallel, his
solstitial noon shadow angle at Alexandria equalled the angle
Syene-earthcenter-Alexandria (intersection of parallels by a straight
line). Compared to this realization, the numbers stades, miles,
whatever, are trivial.
A young classics prof once complained to an old philosophy prof
"Philosophy's basic assumption is that truth can be reached from an
armchair." The philosophy prof replied "Not all truths.
Just the best ones." I think Eratosthenes' measurement of the
earth is one of them.
