On 08/14/2015 12:36 PM, A. John Mallinckrodt wrote:
I don't even know how to define "center of the Earth" or "radial."
If you can define "equator" and define "axis" then you
can define a center.
In fact all of those thing are known quite precisely.
More specifically, all modern geodesy is referenced to
a geoid, for which the center (and other details) are
verrrry precisely defined. The definitions are good
within fractions of a millimeter, and are consistent
with the relevant benchmarks within a meter or so. https://en.wikipedia.org/wiki/World_Geodetic_System