Epicyclic gearing
![This planetary gear train consists of a sun gear (yellow), planet gears (blue) supported by the carrier (green) and an annular gear (pink). The red marks show the relative displacement of the sun gear and carrier, when the carrier is rotated 45° clockwise and the annular gear is held fixed.](/Images/godic/202502/06/Epicyclic_gear_ratios0839.png")
![Bookwheel, from Agostino Ramelli's Le diverse et artifiose machine, 1588](/Images/godic/202502/06/Le_diverse_et_artificiose_machine_del_Capitano_Agostino_Ramelli_Figure_CLXXXVIII0840.jpg")
![In this example, the carrier (green) is held stationary while the sun gear (yellow) is used as input. The planet gears (blue) turn in a ratio determined by the number of teeth in each gear. Here, the ratio is −24/16, or −3/2; each planet gear turns at 3/2 the rate of the sun gear, in the opposite direction.](/Images/godic/202502/06/Epicyclic_carrier_locked0840.png")
![Compound planets of a Sturmey-Archer AM bicycle hub (gear ring removed)](/Images/godic/202502/06/1948amhub0840.jpg")
An epicyclic gear train consists of two gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates to carry one gear, called the planet gear, around the other, called the sun gear. The planet and sun gears mesh so that their pitch circles roll without slip. A point on the pitch circle of the planet gear traces an epicycloid curve. In this simplified case, the sun gear is fixed and the planetary gear(s) roll around the sun gear.