Aspects of this topic are discussed in the following places at Britannica.
...zenith is defined by gravity; i.e., by sighting up a plumb line. If the line were not deflected by such local irregularities in the Earth’s mass as mountains, it would point to the geographic zenith. Because the Earth rotates and is not a perfect sphere, the geocentric zenith is slightly different from the geographic zenith except at the Equator and the poles. Geocentric zenith...
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