This is probably the most realistic reproduction of the changes in the moon's phases which has ever been contrived for use with clockwork. It consists of a sphere, half white and half black, rotating vertically in a circular opening in the arch of a dial, driven from the main movement by a bevel gear. As the 29 1/2 days of the lunar month pass, the moon turns imperceptibly, showing an approximately true visual image of the moon at any given time. One of the earliest clockmakers, Ahasuerus Fromanteel, who worked in London during the reign of Charles II, knew of the principle of the rotating moon.