Of course, today we know that the celestial sphere is not real, but it remains a very convenient model of the sky. In fact, the star charts that we use to locate and identify objects in the sky may be thought of as maps of sections of the celestial sphere.

Constructing the celestial sphere

Consider an imaginary straight line that passes through Earth's poles and extends to infinity on both sides, as shown in the picture above. The north and south celestial poles are located where this line passes through the celestial sphere. In other words, the celestial north and south poles lie directly above Earth's north and south poles, respectively, and it follows that the celestial equator lies directly above Earth's equator.  Other imaginary "lines" on the earth, such as lines of longitude and latitude, are also projected onto the celestial sphere, giving us a convenient basis for a celestial coordinate system. In particular, latitude is measured in angular degrees, with zero assigned to the equator, and 90 degrees assigned to the poles. This means that the distance in degrees from one pole the other is 180 degrees. In astronomy we use the term declination instead of latitude, and it is customary to use negative values for angles south of the celestial equator.  For example, if an observer is located at latitude 30 degrees south on the earth, the point directly overhead, i.e., the zenith, is at declination -30 degrees on the celestial sphere.  We'll talk about right ascension, the equivalent of longitude, in due course.

An observer situated anywhere on the earth's surface can see only one half of the celestial sphere at any time.  In the picture, the observer happens to be in South Afirca. The limit of the visible half of the celestial sphere, as seen by the observer, is the horizon, and the point directly overhead is called the zenith. The celestial poles, together with the observer's zenith, defines the observer's meridian. which is an imaginary arc on the celestial sphere, which starts at one celestial pole, passes through the zenith, and ends at the other celestial pole.  A celestial object that lies on the observer's meridian is said to be at culmination or in transit. Stars and other objects move across the meridian from east to west during the course of a night.

Apparent movement of the stars

For an observer standing on the earth's surface, the celestial sphere appears to "rotate" about its axis from east to west once in 24 hours, "causing" the sun, stars and other objects to rise in the east and set in the west.  But of course this apparent rotation is due to the earth's rotation about its polar axis, and not due to any large-scale movement of the sky. How this rotation is perceived by the observer depends on his or her location.  For someone standing on the earth's equator, the celestial poles lie on the horizon to the north and south, and the entire sky will appear to move slowly from east to west, with stars near the celestial poles following arcs of smaller radius than those of stars near the celestial equator; stars close to the celestial equator will appear to rise due east and follow an overhead path to the west.  On the other hand, for a person standing at one of the poles, the horizon coincides with the celestial equator, and it would appear that half the sky, i.e., the part above the horizon, is turning about the pole - clockwise at the north pole, and anti-clockwise at the south pole.

At locations other than the poles or the equator, the rotation perceived depends on the observer's latitude, and one of the celestial poles will be above the horizon.  The height of the pole in degrees corresponds to the observers latitude.  For example, if an observer in the southern hemishere is located at latitude 30 degrees south, the celestial south pole will be located 30 degrees above the horizon.  This means that some stars are circumpolar, which is simply a term used to describe the fact that they appear to revolve about the celestial pole, and hence never set. In other words, for the observer just mentioned, stars within 30 degrees of the celestial south  pole will be circumpolar. At the same time, the north celestial pole would be 30 degrees below the observer's horizon, so that there are stars that are not visible to this observer from his or her location.  The same applies to an observer in the northern hemisphere.

The ecliptic

So far in our discussion of the celestial sphere I mentioned nothing about the earth's movement about the sun.  When we look at the night sky over a period of, say, a month, we notice that the stars rise about four minutes earlier every night, so that "new" stars become visible as time progresses, and familiar ones disappear.  Therefore, not only do stars rise in the east and set in the west on the imaginary celestial sphere every night, their positions at specific times also seem to move from east to west. Why should this be so?  The answer lies in the fact that, in addition to its rotation about its axis, the earth also revolves around the sun during the course of a year, and therefore our viewpoint with respect to the stars changes continuously. Now, if we could plot the position of the sun on the celestial sphere against the background stars at a specific time every day, we would find that the sun follows an apparent track on the celestial sphere over the course of a year.  This track is called the ecliptic, and it is tilted at an angle of about 23.5 degrees with respect to the celestial equator, due to the inclination of the earth's polar axis.