To ancient peoples, the sky appeared to be the inside of a immense dome covering the Earth and extending as far as they could see. Little wonder that they conceived of the Earth as a small sphere (some saw the Earth as being more like a flat plate) at the center of a huge sphere, with the stars located just inside or on the large sphere. This two-sphere concept is still used by astronomers for organizing the sky. The imaginary outer sphere is known as the celestial sphere.
Watching the sky, early peoples could see that stars rise above the eastern horizon--the circle that divides the visible celestial hemisphere from the invisible one--cross the sky during the night, and later set below the western horizon. This daily (or diurnal) behavior was attributed by most peoples, including the Greeks, to the rotation of the celestial sphere from east to west. Today we know that the apparent rising and setting of stars is actually caused by the Earth's rotation in the opposite sense, that is, from west to east.
Ancient peoples also observed that during their diurnal motion the stars move around two points on the celestial sphere, the north and south celestial poles (NCP and SCP). To them these were the ends of the axis about which the celestial sphere rotated. To us they are the points of intersection of the Earth's axis of rotation with the celestial sphere. (The significance of Polaris, the North Star, is that it lies within 1o of the north celestial pole and is a relatively bright marker of the NCP's position).
For any observer, the point directly overhead is called the zenith. And the imaginary arc on the celestial sphere running from the north point of the horizon through the north celestial pole and zenith to the south point of the horizon is called the celestial meridian. This line is the dividing line between rising and setting, since the highest point above the horizon that any star reaches in its daily motion occurs when the star crosses the celestial meridian. For this reason, the celestial meridian is a basic reference in timekeeping.
Which stars are or are not visible depends on the observer's geographic latitude. An observer in northern latitudes notices that not all stars rise above and set below the horizon daily. Stars near the NCP are always above the horizon, while stars near the SCP are always below the horizon. As ancient peoples traveled to different latitudes, they noticed that the stars that were visible to them changed. For example, as travelers moved northward, they could see stars near the northern horizon that previously had not been visible, and stars previously visible near the southern horizon were now below it. Such an effect along with some others was used as evidence that the Earth is spherical and not flat.