View Instrument (3D) Diagram Instrument φ Observer Solar declination δ Equinox December June Session Meridian ring scale Zodiac Declination Dates ▶ Noon View Whole scene Instrument Reading Layers Ground Marks Celestial axis Celestial equator An equatorial ring and a meridian ring, welded together. The polar style casts its shadow on the equatorial ring's inner face, where the hours are 15° apart. At noon the bead on the style casts its shadow on the meridian ring, marking the date.