geocentric zenith

The term "geocentric zenith" refers to a point on the celestial sphere directly above an observer, defined by a line drawn from the Earth's center through the observer's position. This concept is distinct from the astronomical zenith, which is determined by gravity and a plumb line.1

The geocentric model, prevalent until the 16th century, placed Earth at the center of the solar system. In contrast, the zenith is the highest point directly overhead, while its opposite, the nadir, is directly underfoot. The geocentric zenith differs slightly from the geographic zenith due to Earth's rotation and imperfect spherical shape, except at the equator and poles. Celestial navigation uses the observed positions of celestial bodies, including those at the zenith, to determine location.2134