In astronomy the celestial sphere refers to an imaginary sphere with the Earth at the center of the sphere. Though the sky extends infinitely in all directions stars are assumed to lie on the surface of the celestial sphere. From different locations on the surface of the Earth the visibility of stars changes. The zenith refers to a point directly above the observer, which is defined by the line from the center of the Earth , and perpendicular to the horizon.
If the observer is at the Earth's equator, the celestial equator passes though the zenith. The north celestial pole moves closer to the zenith as the observer moves towards the North and the two coincide at the North pole. Similarly as the observer moves South, the south celestial pole moves closer to the zenith and the two coincide at the south pole.