The _sensible_ horizon is a plane tangent to the earth at the point of observation. The _rational_ horizon is a plane parallel with the sensible horizon, and passing through the centre of the earth. As it cuts the celestial sphere through the centre, it forms a great circle. _SN_ (Fig. 6) represents the sensible horizon, and _S'N'_ the rational horizon. Although these two horizons are really four thousand miles apart, they appear to meet at the distance of the celestial sphere; a line four thousand miles long at the distance of the celestial sphere becoming a mere point, far too small to be detected with the most powerful telescope. [Illustration: Fig. 6.] [Illustration: Fig. 7.] 4. _Rotation of the Celestial Sphere._--It is well known that the sun and the majority of the stars rise in the east, and set in the west. In our latitude there are certain stars in the north which never disappear below the horizon. These stars are called the _circumpolar_ stars. A close watch, however, reveals the fact that these all appear to revolve around one of their number called the _pole star_, in the direction indicated by the arrows in Fig. 7. In a word, the whole heavens appear to rotate once a day, from east to west, about an axis, which is the prolongation of the axis of the earth. The ends of this axis are called the _poles_ of the heavens; and the great circle of the heavens, midway between these poles, is called the _celestial equator_, or the _equinoctial_. This rotation of the heavens is apparent only, being due to the rotation of the earth from west to east. 5. _Diurnal Circles._--In this rotation of the heavens, the stars appear to describe circles which are perpendicular to the celestial axis, and parallel with the celestial equator. These circles are called _diurnal circles_. The position of the poles in the heavens and the direction of the diurnal circles with reference to the horizon, change with the position of the observer on the earth. This is owing to the fact that the horizon changes with the position of the observer. [Illustration: 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. [Illustration: 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_.