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 conceived in late 1656 by Dutch scientist Christiaan Huygens (1629–95) and refined into a working concept by clockmaker Salomon Coster the following year. 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.Christiaan Huygens believed (correctly) that a good pendulum clock would allow sailors to accurately determine their longitude at sea, but his designs were too fragile and easily knocked out of time to be any use. It was not until the development of John Harrison's marine chronometers in the 1740s that clock accurate enough to use for navigation became a reality.