geocentric zenith
Technical term in astronomy and geodesy; the point on the celestial sphere directly above the observer, defined relative to the Earth's center rather than the local vertical.
The geocentric zenith is in the direction of a line drawn from the Earth's center at the observer's location to the celestial sphere.
天頂 is the general term for zenith, usually meaning the point directly overhead based on local gravity (astronomical zenith). 地心天頂 specifies the zenith defined by the Earth's center, used in precise astronomical calculations.
Compound of 地心 (geocentric) and 天頂 (zenith). The exact historical derivation is uncertain; the term is a modern technical coinage.