Unit 5.Lab Part1. Determine the Pendulum Period. A swinging pendulum keeps a very regular beat. It is so regular, in fact, that for many years the pendulum was the heart of clocks used in astronomical measurements at the Greenwich Observatory. There are at least three things you could change about a pendulum that might affect the period (the time for one complete cycle): the amplitude of the pendulum swing the length of the pendulum, measured from the center of the pendulum bob to the point of support. the mass of the pendulum bob