What is a clock?

Abstract Without differences between what we experience in different instants, we could never have a notion of time and its passage. The hands of a clock must move if it is to tell time and measure duration. As regards the direction of time, evidence from the Newtonian theory of gravitating point particles suggests that time’s arrow is not, on the scale of the Universe, determined by the statistical growth of entropy and a very special initial condition but has a direct dynamical origin manifested as the growth of structure. This paper also draws attention to the fact, first revealed by geologists, that a single static structure contains information about both its own age and even the age of the whole Universe.