Since the vibration of a single pendulum is very periodic, measuring its period is a very interesting topic. When students are asked to measure the period of a single vibration, they start and stop the stopwatch when the pendulum reaches the top point as a reference point. In this paper, we try to show that the error can be reduced more by using the equilibrium point, that is, the bottom position as the reference rather than the top position. We think it would be beneficial for students to measure the period in both cases, compare the errors, and think about the reasons for error differences. Students believe that the moment the pendulum moved slowly and nearly stopped at the top was more likely to measure the time to be more accurate. The reason for this is that they think the pendulum will move so fast when it passes the bottom point that they will not be able to start or stop the stopwatch accurately at the instant passing the lowest point. We are also able to obtain the error of the position measurement by using the recorded video of a simple pendulum.