This picture shows the trajectory of a pendulum, as seen from the bottom. The pendulum bob is equipped with a white punctual light source, which emits in all directions. On the picture you can notice a drop in the amplitude, mainly caused by air friction. You can also notice that the ellipse traced during each cycle undergoes a precession, or a rotation of the major axis. As the amplitude increases relatively to the length of the pendulum, the period, or the time to complete a to and fro cycle, tends to increase also (non-harmonic oscillator). Since the amplitude of the major axis of the ellipse is greater than that of the minor axis, the periods of the major and minor axes are different, which brings the axes to oscillate out of phase and rotate.
Credits Richard Germain
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