I don't recall the proper name for this amazing contraption, but it is a mechanical clock type device that was designed in roughly the 1500's to display the positioning of the planets and to set the calendar for each year in advance. The calendar, Easter for example, was set according to the position of the moon, which was all calculated by this great machine. You won't be able to see it in this picture, but on the "clock" face itself, there are many small arms with little black balls on the end that represent the planets position around the sun. Saturn is the farthest planet on the clock because no others were known at the time.