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# Re: Would you like to share some data?

Regarding John Denker's method of estimating the earth's circumference:

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.

First off it seems that John has inadvertently made a decimal error of
3 orders of magnitude (probably because of accidentally ignoring the
distinction between a meter and a kilometer).

Secondly, this method would (IMO) constitute an example of "cheating".
If the experiment was done when the meter was defined according to its
original 18th century definition, then this procedure would not really
constitute a *measurement* of the size of the earth. It would really be
merely quoting the definition of the meter. An honest measurement of
some quantity does not use the quantity measured as the definition of the
unit of measurement. The unit of measurement need to be specified
*independently* of the quantity to be honestly measured. For instance
the fact that the vacuum speed of light is exactly 1 ly/yr or exactly
299792458 m/s is not a measurement of that speed. It is really just the
definition of the unit of length.

Even if the experiment was done nowadays using the modern definition of
the meter, then the cheat would be in relying on the fact that
experimentally, the new definition of the meter is numerically close to
the original definition of the meter, and using that numerical fact is
tantamount to merely using a previous measurement of the size of the
earth (made when the new definition was calibrated to agree with the
old one) and then quoting it as some sort of new independent measurement
of the Earth's size.

Another example of another "measurement" of the circumference of the
earth which would constitute "cheating" is to give the circumference in
Nautical miles. Since a nautical mile was originally the surface distance
corresponding to a surface arc angle of 1 minute of arc we know that the
circumference of the earth is 60 x 360 = 21600 naut. mi. by the original
definition of a nautical mile. However, (like other units that have been
"metrified") the current international standard definition of the
nautical mile is exactly 1852 m rather than being based on the size of
the earth.

David Bowman
David_Bowman@georgetowncollege.edu