Figure 56 is a photograph of the planet gear shown in Figure 55 meshing with the ring gear that is not shown in the diagram. The outer diameter of the ring gear is about 3 times greater than the outer diameter of the planet gear. The ring gear is a narrow ring of material with 50 teeth arranged around its inner surface. The planet gear is a standard spur gear with 15 teeth. There is a hole in the centre of the ring gear to accommodate a rotating shaft and arm connected to the planet gear. This shaft and arm have been removed. You will have to imagine that as the shaft rotates, the gear wheel will be forced to move round the inside of the ring gear and in order to be able to do this it will have to rotate. In this case, the planet gear will rotate 50/15 = 3 and a third times in one revolution of its rotating drive shaft.