The Heavens Above: A Popular Handbook of Astronomy

by J. A. Gillet; W. J. Rolfe

The dome of the heavens appears to rest on the earth, as shown in Fig. 5. This is because distant objects on the earth appear projected against the heavens in the direction of the horizon.

Fig. 5.

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.

Fig. 6.

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.