The Italian scientist Galileo Galilei (1564–1642) was among the first to observe that the steady, predictable rate of a pendulum’s swing made it ideal for use in timekeeping. The first ever fully operational clock powered by a pendulum was built in 1656 by Dutch horologist Christiaan Huygens (1629–95). Initially accurate to within a minute per day, later refinements brought this down to around 10 seconds. By the end of the century, pendulum clocks were keeping time to within 0.5 seconds per day – accurate enough to warrant the introduction of the first-ever second hand.
See also First pendulum clock design