A relatively simple demonstration and quantitative determination of the rotation of the earth using a simple pendulum several meters long, the so-called Foucault pendulum. The frequency of rotation can be measured in less than one minute. After carefully damping any spurious vibrations of the pendulum held at its maximum excursion, it is released using a mechanism that avoids shaking it. On the projection screen one can see how the maximum excursion of the pendulum wire shifts from one period to the next. A quantitative evaluation leads to the well known rotational frequency of the earth (1/day). Note that for this evaluation the geographic latitude has to be considered at which the experiment is being performed (f = 51.5 degrees). The actual rotational frequency of the observed rosette trace is here smaller than on the pole. On the equator, it would vanish.