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 Also, some of the various values are older values that have been replaced by later, more precise and more accurate values.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 ========================
