Galileo's escapement is a design for a clock escapement, invented around 1637 by Italian scientist Galileo Galilei (1564 - 1642). It was the earliest design of a pendulum clock. Since he was blind, Galileo described the device to his son, who drew a sketch of it. The son began construction of a prototype, but both he and Galileo died before it was completed.
Galileo was the first to investigate the timekeeping properties of pendulums, beginning around 1602. His interest was sparked by his discovery that, at least for small swings, the pendulum is isochronous; its period of swing is the same for different size swings. He realized that this property made the pendulum useful for timekeeping. He also discovered that the pendulum's period is dependent on its length, and independent of the mass of the pendulum bob. He used freeswinging pendulums as timers in scientific experiments and for keeping time for music.
In 1637, when he was 73, Galileo had the idea of a mechanism that would keep the pendulum swinging by giving it pushes, an escapement, thus allowing it to be applied to clocks. Since he was totally blind, he described the mechanism to his son Vincenzio, who drew a picture from his description. Galileo's student and biographer, Vincenzo Viviani, describes the invention
One day in 1641, while I was living with him at his villa in Arcetri, I remember that the idea occurred to him that the pendulum could be adapted to clocks with weights or springs, serving in place of the usual tempo, he hoping that the very even and natural motions of the pendulum would correct all the defects in the art of clocks. But because his being deprived of sight prevented his making drawings and models to the desired effect, and his son Vincenzio coming one day from Florence to Arcetri, Galileo told him his idea and several discussions followed. Finally they decided on the scheme shown in the accompanying drawing, to be put in practice to learn the fact of those difficulties in machines which are usually not foreseen in simple theorizing.
His son Vincenzio began building a clock, but both he and Galileo died before it was completed. The first pendulum clock was built in 1657 by Christiaan Huygens using a different design. The pendulum clock remained the world's most accurate timekeeper for 300 years, until the 1930s.
Since his time, various working models of Galileo's clock have been built (see picture at top).