 Around the planisphere moves the rete , a web @-@ like metal cutout representing the zodiac , which also features a Julian calendar dial . Above the rete , a display indicates the day of the week . Because leap days are not supported by the clockwork , the calendar hand has to be reset manually each leap year on 29 February . A moon dial circles the inner ring of the zodiac , displaying the moon phase . The principal hand of the clock indicates the time of day on the outer ring of 24 golden Roman numerals , which run twice from I to XII . It features two suns , the smaller one indicating the date on the rete 's calendar dial . The larger one circles the zodiac at one revolution per year and also rotates across the planisphere once per day . Its crossing of the horizon and dawn lines twice per day allows the timing of sunrise , dawn , dusk and sunset . 
