What is "sun and planet gear"

n. A device for converting reciprocating motion to rotary motion, used in the first rotative beam engines.

The sun and planet gear (also called the planet and sun gear) is a method of converting reciprocating motion to rotary motion and was used in the first rotative beam engines.

It was invented by the Scottish engineer William Murdoch, an employee of Boulton and Watt, but was patented by James Watt in October 1781. It was invented to bypass the patent on the crank, already held by James Pickard. It played an important part in the development of devices for rotation in the Industrial Revolution.