Instrument φ Observer Variation on the theme Solar declination δ Equinox December June Session ▶ Noon View Whole scene Instrument Shadow Layers Ground Celestial axis Celestial equator The ring lies in the plane of the celestial equator. Away from the equinoxes the Sun is above or below that plane, so the shadow of the sunlit arc misses the opposite arc and a sliver of light survives on its inner face. At the equinox the Sun lies exactly in the plane: the shadow covers the inner face and the light vanishes.