The earth, as we all know, rotates about its axis once every twenty-four hours, giving us the cyclic alternation of night and day. An ingenious demonstration of this was designed in 1851 by the French scientist and mathematician Leon Foucault (1819-1868). The underlying idea is a very neat one, although its realization is actually quite problematic. It uses a pendulum. We are all familiar with the basic idea of a pendulum. One end of a string is attached to a fixed support (here referred to as the 'pivot'), the other to a weight (referred to as the 'bob'). The whole is suspended so that it hangs freely from the support. The string remains taut, but otherwise the bob is free to move. If it merely oscillates back and forth, its motion is confined to a plane and the pendulum is referred to as a 'simple pendulum'. Its behavior is analyzed in many school physics textbooks where the period of that oscillation is shown to be proportional to the square root of the length of the string.