The pendulum clock improved accuracy by using a swinging weight to regulate each tick.

The longer version

A weight swinging on a rod returns to each extreme in nearly the same time, so it can release an escapement tooth on every swing. That converts continuous motion into countable ticks and lets a falling weight or spring drive the hands at a regulated pace. Each swing is an almost identical interval.

Earlier mechanical clocks drifted by minutes a day; the swinging regulator cut the error dramatically and made seconds hands worth fitting. Temperature changes the rod's length and thus the rate, which led to compensation designs using metals with different expansion. Accuracy went from hopeless to within seconds per day within a few decades.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.