The document discusses the history and properties of the pendulum as a timekeeping device. It describes how Galileo Galilei discovered the regular motion of pendulums in 1602 and how pendulums became the most accurate timekeeping technology until the 1930s. The key properties that make pendulums useful for timekeeping are explained - the period of a pendulum's swing depends on its length and local gravity, and is independent of amplitude, meaning successive swings take the same amount of time. External factors like temperature, atmospheric pressure and gravity that can affect pendulums are also outlined.