Planetary gearing is a gear system consisting of one or more outer pinions, revolving about a central sun gear. Simple planetary gears have one sun, one ring, one carrier, and one planet set. A carrier connects the canters 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.