The variation of the period of oscillation of a pendulum with the amplitude is known as "circular error" to clockmakers, and can easily be observed even without modern laboratory instruments. There have therefore been many approximate formulae for the pendulum period as a function of the amplitude, but the simple equation due to Denman, "T"[subscript D]([phi][subscript 0])="T"[subscript 0]"/"(1-[phi]20/16), which is more accurate than many in the literature, seems to have been forgotten.