From: HJSmith@ix.netcom.com (Harry J. Smith)
Newsgroups: sci.math
Subject: Re: What is the diameter of earth?
Date: Fri, 07 Jul 1995 00:16:49 GMT

bpc@netcom.com (Benjamin P. Carter) wrote:

>The earth is approximately an ellipsoid of revolution.  Any plane
>that contains the north and south poles intersects the ellipsoid
>in an ellipse of the following shape:


>   Equatorial radius:      a = 6378.145 km

>        Polar radius:      b =6356.785 km

>        Eccentricity:      e = 0.08182


>Reference: Bate, Mueller, and White: _Fundamentals of Astrodynamics_
>(1971).  The "flattening" defined as  f = (a-b)/a = 1/298.6
>is a measure of the equatorial bulge.  One can determine e from f
>(or vice versa) by solving  1 - e^2  =  (1 - f)^2.  The idea that
>the earth is pear-shaped is something of an exaggeration.  (The pear-
>shapedness is a small correction compared to the flattening.)
>-- 
>    Ben Carter                  internet address: bpc@netcom.com

This data is really out of date. In the 60's we were using a value of
f = 1/297 later revised it to 1/298.3 then 1/298.25 and now 1/298.26
(but never 1/298.6).

The standard model is
Equatorial radius:      a = 6378.165 km
Flatting:                    f = 1/ 298.25

This gives

Eccentricity:      e = 0.081820179996
Polar radius:     b = 6356.7797 km

The best estimates are

Equatorial radius:      a = 6378.140 +/_ 0.003 km
Flatting:                    f = 1/ 298.26 +/- 0.01

But the standard model is normally used because many sites have been
surveyed with respect to the standard model.

Harry

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