Alan B. Pearce wrote:
> I believe that 5 decimal places is enough to calculate the
> circumference of the earth to within a handspan. Sorry don't have a
> reference, I just remember being told it at one stage.

That's easy enough to check.  The circumference of the earth is about 25,000
miles = 132,000,000 feet = 1,584,000,000 inches.  Let's say your "handspan"
is a unit Palm, which is 4 inches.  4/1,584,000,000 = 1/396,000,000 =
10**-8.6, so you need about 9 significant digits.  Sorry, 5 won't cut it
unless you meant 5 digits right of the decimal point with another 4 to the
left.

We can also look at the reverse.  The best accuracy you can get with 5
digits is 99,999.  With a implied error of +- 1/2, that is a error of
1/199,998.  1,584,000,000 * 1/199,998 = 7920 inches = 660 feet = 1/8 mile =
220 yards = about two soccer fields end to end.


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