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*From*: Hugh Haskell <hhaskell@MINDSPRING.COM>*Date*: Wed, 6 Jun 2001 17:53:23 -0400

At 17:14 -0400 6/6/01, John S. Denker wrote:

[snip]

Measuring the size of the earth is an extreme case, because the answer is

so well known. Indeed, you don't even need a highway map! To get a good

estimate of the circumference of the earth, you can just measure the length

of a meter stick (which is pretty easy :-) then multiply by 40,000. Even

in less extreme cases, known quantities manage to insinuate themselves into

measurements in all sorts of ways.

Philip and Phylis Morrison did the Eratosthenes experiment some years

back for the PBS series "The Ring of Truth." They made their

independent distance measurement using the odometer of their truck as

it drove down one of those ramrod straight north-south highways in

the midwest for several hundred miles. They measured the elevation of

a star (Antares, as I recall) as it crossed the local meridian each

night.

The fact that the distance unit they used was miles, whose modern

definition is related to the meter, which was originally related to

the circumfirence of the earth is really irrelevant. They could have

measured the distance traveled in truck lengths, or (as E. did) in

stadia (which were related to the average length of a human stride),

or any other arbitrary unit (furlongs or chains, or any other unit

whose origin is unrelated to the circ. of the earth). Then measuring

the earth's circ. in these units and comparing that to a nominal

definition of the meter as 1/40,000,000 of the earth's circ., would

give the relationship between the units, but no prior assumptions

about the size of the earth enter into this sort of procedure.

Unfortunately, when you have two static groups and cannot directly

measure the distance between them, the problem is a bit more

complicated, as John D. and John B. have eloquently pointed out.

Hugh

--

Hugh Haskell

<mailto://haskell@ncssm.edu>

<mailto://hhaskell@mindspring.com>

(919) 467-7610

Let's face it. People use a Mac because they want to, Windows because they

have to..

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