1Any of various notched, toothed, or spoked wheels forming part of a given mechanism, especially (a) a notched or toothed wheel used in clockwork to regulate or measure movement; (b) (in later use) a ratchet wheel used in drum brakes to regulate brake pad wear.

2A device used to determine the position and visibility of the stars at a given time and date, typically composed of a circular star chart overlaid by an adjustable cover with an elliptical opening.