Zeniths lie on the celestial sphere, which can be pictured as an imaginary sphere that encloses the universe.
Astronomers speak of two kinds of zeniths, astronomical zeniths and geocentric zeniths.
An astronomical zenith is determined by gravity. It is any point where an extended plumb line would intersect the celestial sphere. A geocentric zenith is determined by geometry. It is any point where a line drawn from the earth's center through a person on its surface would intersect the celestial sphere. The angular distance of a star or other celestial body from a zenith is called the zenith distance.