An epicyclic gear teach (also called planetary gear) contains two gears mounted so that the centre of one gear revolves around the centre of the various other. A carrier connects the centres of the two gears and rotates to transport one equipment, called the planet gear or planet pinion, around the various other, called sunlight gear or sunlight wheel. 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 equipment is fixed and the planetary gear(s) roll around the sun gear.
An epicyclic gear train can be assembled so the planet gear rolls within the pitch circle of a fixed, outer gear band, or ring equipment, sometimes called an annular gear. In this instance, the curve traced by a point on the pitch circle of the planet is a hypocycloid.
The combination of epicycle gear trains with a planet engaging both a sun gear and a ring gear is named a planetary gear train. In this case, the ring gear is usually fixed and the sun gear is driven.
Epicyclic gears obtain name from their earliest program, that was the modelling of the motions of the planets in the heavens. Believing the planets, as Auto Chain everything in the heavens, to end up being perfect, they could only travel in ideal circles, but their motions as viewed from Earth cannot be reconciled with circular motion. At around 500 BC, the Greeks invented the idea 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, acquired gearing which was in a position to approximate the moon’s elliptical route through the heavens, and even to correct for the nine-12 months precession of that route. (The Greeks could have seen it much less elliptical, but rather as epicyclic motion.)