Meridian of the heavens

Although the facts are equally explicable on the alternative assumption that the stars are fixed and that the earth is revolving about the same axis in the opposite direction to the apparent revolution of the celestial sphere, the earlier and less sophisticated view embodied a tremendous gain. It involved the first step towards a world map. In counting the shadow hours and learning to use the star clock, man had begun to use geometry. He had begun to find his local bearings in cosmic and terrestrial space. An important step towards an art of measurement was made when men began to trace circles on the sand or the soft earth around the shadow pole to mark the moment when the shadow was shortest. In discovering the constant direction of the noon shadow pointing to the pole, they fixed two planes of reference. One is the horizontal plane, the north and south points of which divide the observer's terrestrial horizon into an easterly and a westerly half. The other was bounded by the great semicircle or Meridian of the heavens, with its highest point, the zenith, directly overhead. The axle of the heavenly clock of star transit and shadow connected the Pole Star to the earthly pole in the meridian plane. Sun, moon, and stars are highest in the heavens where the circles they describe on the surface of the celestial sphere cut the meridian.