The diameter of a circle is the distance from a point on the circle to a point 
radians away, and is the maximum
distance from one point on a circle to another. The diameter of a sphere
is the maximum distance between two antipodal points
on the surface of the sphere.
If 
is the radius
of a circle or sphere, then
. The ratio of the circumference 
of a circle
or great circle of a sphere
to the diameter 
is pi,
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