A pendulum is a better clock than the solar day. The duration of the solar day varies from day to day. But how do we know this except by comparing it to another clock such as the pendulum? How do I know it is not the pendulum which varies from day to day and the solar day is constant? Is it a theoretical assignment? Our theory tells us that the pendulum is periodic while the solar day is imperfectly periodic, so we choose the pendulum as the standard between the two? Or is there a purely empirical means of distinguishing them?