An Stainless Steel Chain epicyclic gear train (also referred to as planetary gear) contains two gears mounted to ensure that the centre of 1 gear revolves around the centre of the various other. A carrier links the centres of both gears and rotates to carry one gear, called the earth gear or world pinion, around the other, called sunlight gear or sun wheel. The earth and sunlight gears mesh so that their pitch circles roll without slide. A spot on the pitch circle of the planet equipment traces an epicycloid curve. In this simplified case, the sun gear is fixed and the planetary equipment(s) roll around the sun gear.
An epicyclic gear train can be assembled so the planet equipment rolls within the pitch circle of a fixed, outer gear band, or ring gear, sometimes called an annular equipment. In this instance, the curve traced by a point on the pitch circle of the earth is a hypocycloid.
The combination of epicycle gear trains with a planet engaging both a sun gear and a ring gear is called a planetary gear train. In cases like this, the ring equipment is usually fixed and sunlight gear is driven.
Epicyclic gears get their name from their earliest application, that was the modelling of the movements of the planets in the heavens. Believing the planets, as everything in the heavens, to end up being perfect, they could just travel in perfect circles, but their motions as seen from Earth could not be reconciled with circular movement. At around 500 BC, the Greeks invented the thought of epicycles, of circles traveling on the circular orbits. With this theory Claudius Ptolemy in the Almagest in 148 AD could predict planetary orbital paths. The Antikythera System, circa 80 BC, had gearing which was in a position to approximate the moon’s elliptical path through the heavens, and also to improve for the nine-year precession of that path. (The Greeks could have seen it not as elliptical, but instead as epicyclic motion.)