A cam with a profile resembling a kidney commonly used for indicating solar time. Mean time and solar time coincide four times a year and differ by as much as 16 minutes at other times. A kidney-shaped cam rotated by the clock's mechanism may be used to drive an additional hand, indicating solar time, and a sundial may then by used to regulate the clock to mean time, without reference to equation tables. Clocks having a hand indicating solar time are known as 'equation clocks'.