How does a mechanical clock keep time?

A falling weight or an unwinding mainspring wants to spin the gear train as fast as friction allows. The escapement stops that. An anchor or lever sits against a toothed escape wheel, blocking it, and rocks back and forth. Each rock lets exactly one tooth slip past, hence escapement, and the same motion gives the oscillator a small push to keep it going. The oscillator is a pendulum in a clock or a balance wheel on a hairspring in a watch, and its swing period depends on its own physical properties rather than on how hard the spring is pushing. So the gear train advances in equal ticks. The rest of the mechanism is just division: gears reduce those ticks to seconds, minutes and hours.

The deeper principle is that accurate timekeeping needs a stable oscillator plus a way to count its cycles without disturbing it. Galileo noticed a pendulum's period barely changes with swing size; Christiaan Huygens turned that into a working pendulum clock in 1656 and cut typical daily error from minutes to seconds. Every later improvement chases the same two goals: make the oscillator less sensitive to its surroundings, and take less energy from it. Temperature changes pendulum length, so makers built gridiron and mercury pendulums that cancel expansion. Air pressure and friction drag on the swing, so precision regulators ran in partial vacuum. Quartz and atomic clocks replaced the pendulum with a vibrating crystal or an atomic transition, but the architecture is unchanged.

Mechanical clocks no longer set the world's time; national laboratories do, using caesium standards that define the second and drift by less than a second over tens of millions of years. Everything from GPS to power grids to financial timestamps depends on them. Mechanical movements survive as craft objects, and the vocabulary persists: a watch's beat rate, typically twenty-eight thousand eight hundred vibrations per hour, is the escapement's counting rate. Tower clocks and marine chronometers left a permanent mark on navigation and on the standardisation of time zones. Even a good modern wristwatch is judged by a chronometer standard measured in seconds gained or lost per day, a tolerance a cheap quartz movement beats effortlessly.

Common misconceptions confuse the power source with the timekeeper. Myth: the mainspring or weight determines the rate. Reality: it only supplies energy; the oscillator sets the rate, and a well-designed escapement keeps the rate steady even as the spring weakens. Myth: winding a watch more tightly makes it run fast. Reality: it does not, though a badly regulated movement can vary with mainspring tension, which is the problem the fusee was built to solve. Myth: a pendulum's swing size sets its period. Reality: for small swings the period depends mainly on length and gravity, which is why pendulum clocks must be levelled and are adjusted by moving the bob. Myth: mechanical watches are more accurate than quartz. Reality: they are not, by a wide margin.