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    Re: Uses of almanacs for astrology
    From: Herbert Prinz
    Date: 2004 Oct 20, 19:12 -0400

    Frank Reed wrote:
    
    > George wrote:
    > "This is due to the ellipsoidal shape of the Earth."
    >
    > Lisa added:
    > "Actually the earth is an oblate spheroid, sort of squashed at the top
    >
    > and stretched at the equator."
    >
    > They're essentially synonymous. Any cross-section of the Earth
    > containing the Earth's axis of rotation is an ellipse. Hence the
    > Earth's shape can be called "ellipsoidal". An ellipsoid of revolution
    > where revolution is about the minor axis is, of course, an "oblate
    > spheroid". Different ways of refering to the same shape.
    >
    
    Now it's my turn for a little nit picking!
    
    Ellipsoid and spheroid are not synonymous because a spheroid is a
    special case of an ellipsoid. While George is not wrong, Lisa is more
    specific.
    
    But the real and absolute truth is that the shape of the Earth is nearly
    a geoid. The geoid is not a perfect spheroid, but it is more or less the
    shape of a body that has the mass distribution of the Earth and rotates
    with the angular speed of the Earth.
    
    The difference between the spheroid and the geoid is probably of more
    relevance to aircraft pilots (especially during landing), and mountain
    climbers than to the boater, but I am not 100% sure of this. If I
    remember correctly the maximum geoidal separation of WGS84 is in the
    order of  30 to 50 meters.
    
    Herbert Prinz
    
    
    

       
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