An observer on the earth at any place sees all the bright heavenly objects in the sky to be fixed on the innersurface of a hypothetical sphere of infinit radius and the center at the observer's eye. This sphere is called the celestial sphere.If a straight line is drawn throught the position of the observer in the direction of gravity and extended to either directions,it means the celestial sphere at 2 points,one vertically above called the zenith(z) and other vertically below called the Nadir(z')
