Clock

A clock is an instrument that indicates or measures time, usually by tracking regular cycles and displaying their position within a timekeeping system.

Oscillator: A system that produces repeated cycles at a characteristic frequency. Regular oscillation supplies the repeating signal that many clocks count.
Sundial: A device that indicates time from the position of a shadow cast by the Sun. Sundials are an ancient timekeeping predecessor, but depend on daylight and weather.
Alarm clock: A clock designed to sound or otherwise signal at a preset time. It turns time measurement into a timed prompt for waking or other routines.
Hourglass: A timekeeping device that measures an interval by the flow of sand between chambers. Unlike a clock, it measures elapsed duration rather than continuously displaying civil time.
Pendulum clock: A clock that uses a swinging pendulum to regulate its timekeeping mechanism. Its pendulum provides a near-regular cycle that made accurate clocks practical.
Water clock: A device that measures time through the controlled flow of water. Water clocks tracked intervals independently of sunlight before mechanical clocks spread.
Marine chronometer: A highly accurate portable timekeeper used aboard ships to determine longitude. Keeping reference time at sea let navigators compare local noon with a known meridian.
Stopwatch: A timepiece designed to measure elapsed intervals, usually with start, stop, and reset controls. A stopwatch times events from a chosen start instead of indicating the current time.
Quartz clock: A clock whose timing is regulated by the vibrations of a quartz crystal. Quartz vibration sets the pace for common modern electronic clocks.
Mechanical clock: A clock powered by weights or springs and regulated by a mechanical device. Mechanical clocks replaced many earlier systems with continuous, countable motion.

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