Astronomy of the Spheres

Abstract

Despite the apparent flatness of the sky canopy, explored in Chapter 1, the sky has often been depicted as a great sphere. The diurnal motions of the stars are along circular arcs, as the photographed paths of stars near the North Celestial Pole show (see Fig. 2.1). Each stellar diurnal circle, regardless of angular diameter, takes the same length-a sidereal day-to complete. The stars also maintain the same positions relative to each other. In the past when people assumed the Earth to be motionless, the easiest way to account for such a phenomenon was to imagine that the stars were embedded on a great rotating sphere.