Facing the South, a grid of 7 arcs (forming six parts – one for each pair of the zodiac with the upper arc defining the summer solstice, the central the equinoxes and the lowest the winter solstice) and 13 hour lines representing the hours defined the time with the help of the limit line formed by the light and shade of the Sun. The central hour line indicated the sunrise and the sunset, the left lines indicated the ante meridian hours, the right lines indicated the post meridian hours and the edge curves indicated midday.