ASTRONOMICAL TERMS
THE CELESTIAL SPHERE is an imaginary sphere upon the surface of which all the stars in the sky appear to be studded to an observer stationed at its centre.
THE ZENITH is the intersection of a vertical line through the observer's station with the upper portion of the celestial
sphere. It is the point on the celestial sphere immediately above the observer's station.
THE NADIR is the intersection oI a vertical line through the observer's station with the lower portion oi the celestial sphere. It is the point on the celestial sphere vertically below the observer's stations.