Dear Bill, Thanks for your thorough, and insightful, comments.
To answer your final question, 'Why do you need anything more precise?', I was thinking about a universal grid for cartography, and I was thinking along somewhat similar lines to Marcus Berger, in that I was thinking of a 40�000 kilometre reference sphere � I particularly like Marcus' idea of a reference sphere that gives an exact (theoretical) circumference of 40�000�kilometres or 40�megametres. Cheers, Pat Naughtin LCAMS Geelong, Australia -- on 3/4/04 2:57 AM, Bill Hooper at [EMAIL PROTECTED] wrote: > Pat Naughtin wrote: >> Infoplease.com says that the equatorial diameter is 12 760 km and the >> polar >> diameter is 12 720 km, so taking a simple average of these two I get a >> diameter of 12 740 km, which gives a circumference of 40 087 >> kilometres. > > He asked, what is the REAL circumference (and diameter) of the Earth? > The answer is in his final example (quoted above): there IS NO SUCH > THING as a single circumference (or diameter) because the Earth is not > a perfect sphere. It is not even possible to use a "simple average" > found by averaging the largest and smallest values because, generally > speaking, the average of a large number of different values of anything > is seldom equal to the simple average of the largest and smallest > value. > > The reason for the wide variety of values that Pat cites is that they > are values obtained for variously measured diameters (including polar > and equatorial) and perhaps by using various averages. Also, some of > the various values are older values that have been replaced by later, > more precise and more accurate values. > > In any case, there is no reason why any of these values should give a > circumference > of exactly 40 000 km (40 Mm). That was the value chosen for defining > the metre in the earliest stages of development of the metric system, > leading to SI. It was abandoned as technology advanced making different > ways of defining the metre preferable. > > Since every time the metre (or other base unit) was redefined, the new > definition was ALWAYS made so that the metre would not change in size > as far as the precision of then-current technology permitted > measurements to be made. Thus, the present day metre is still very > close to the size necessary for the Earth's circumference to be > approximately 40 000 000 of them, making he Earth's circumference close > enough > to 40 000 km to be a good approximation for general use. ALL of the > various diameters Nat quotes are within about a quarter of one percent > of the nice round > approximation of 40 000 km. Why do you need anything more precise? > > > Regards, > Bill Hooper > Fernandina Beach, Florida, USA > ======================== > SIMPLIFICATION begins with SI > ======================== >
