 The dial of the Zytglogge 's astronomical clock is built in the form of an astrolabe . It is backed by a <unk> projected planisphere divided into three zones : the black night sky , the deep blue zone of dawn and the light blue day sky . The skies are crisscrossed with the golden lines of the horizon , dawn , the tropics and the temporal hours , which divide the time of daylight into twelve hours whose length varies with the time of year . 
