in places of business and railroad stations and used to schedule work and set other clocks. The need for extremely accurate timekeeping in celestial navigation to determine longitude on ships during long sea voyages drove the development of the most accurate pendulum clocks, called astronomical regulators. These precision instruments, installed in naval observatories and kept accurate within a second by observation of star transits overhead, were used to set marine chronometers on naval and commercial vessels. Beginning in the 19th century, astronomical regulators in naval observatories served as primary standards for national time distribution services that distributed time signals over telegraph wires. From 1909, US National Bureau of Standards (now NIST) based the US time standard on Riefler pendulum clocks, accurate to about 10 milliseconds per day. In 1929 it switched to the Shortt-Synchronome free pendulum clock before phasing in quartz standards in the 1930s. With an error of less than one second per year, the Shortt was the most accurate commercially produced pendulum clock. Pendulum clocks remained the world standard for accurate timekeeping for 270 years, until the invention of the quartz clock in 1927, and were used as time standards through World War 2. The French Time Service included pendulum clocks in their ensemble of standard clocks until 1954. The home pendulum clock began to be replaced as domestic timekeeper during the 1930s and 1940s by the synchronous electric clock, which kept more accurate time because it was synchronized to the oscillation of the electric power grid. The most accurate experimental pendulum clock ever made may be the Littlemore Clock built by Edward T. Hall in the 1990s (donated in 2003 to the National Watch and Clock Museum, Columbia, Pennsylvania, USA). Mechanism The mechanism which runs a mechanical clock is called the movement. The movements of all mechanical pendulum clocks have these five parts: A power source; either a weight on a cord or chain that turns a pulley or sprocket, or a mainspring A gear train (wheel train) that steps up the speed of the power so that the pendulum can use it. The gear ratios of the gear train also divide the rotation rate down to give wheels that rotate once every hour and once every 12 hours, to turn the hands of the clock. An escapement that gives the pendulum precisely timed impulses to keep it swinging, and which releases the gear train wheels to