Zenith, << ZEE nihth, >> in astronomy, is any point directly above a person on Earth. Zeniths lie on the celestial sphere, which can be pictured as an imaginary sphere that encloses the universe. A point directly below a person on Earth is called a nadir (see Nadir ).
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 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. This information can be used to describe the position of such an object.
