Fig. 8.
When the observer is north of the equator, the north pole of the heavens is elevated above the horizon, and the south pole is depressed below it, and the diurnal circles are oblique to the horizon, leaning to the south. This case is represented in Fig. 8, in which PP' represents the celestial axis, EQ the celestial equator, SN the horizon, and ab, cN, de, fg, Sh, kl, diurnal circles. O is the point of observation, Z the zenith, and Z' the nadir.
Fig. 9.
When the observer is south of the equator, as at O in Fig. 9, the south pole is elevated, the north pole depressed, and the diurnal circles are oblique to the horizon, leaning to the north. When the diurnal circles are oblique to the horizon, as in Figs. 8 and 9, the celestial sphere is called an oblique sphere.
When the observer is at the equator, as in Fig. 10, the poles of the heavens are on the horizon, and the diurnal circles are perpendicular to the horizon.
When the observer is at one of the poles, as in Fig. 11, the poles of the heavens are in the zenith and the nadir, and the diurnal circles are parallel with the horizon.
Fig. 10.