A note on rods and clocks in Newton's Principia

There is an asymmetry in Newton's Principia between the status of rods and that of clocks. This can be seen by considering issues highlighted by Harvey Brown in his book Physical Relativity, in which he emphasizes the importance of thinking carefully about the relationships between the metrics of space and time, the spatial and temporal behaviors of rods and clocks, and dynamics. Brown's book focuses primarily on Einstein's theories of relativity, but the general lesson applies more widely. In this paper, I examine the status of rods and clocks in Newton's Principia. I argue that rods are geometrical whereas clocks are dynamical (section 2), in a sense to be explained, and comment on some aspects of this asymmetry that I find interesting (section 3). I am not suggesting that there is, in fact, an asymmetry in how rods and clocks should be treated, nor that Newton thought that there was. On the contrary, Newton sought a dynamical treatment of both rods and clocks. Nevertheless, for the purposes of the Principia, rods do not receive a dynamical treatment, do not need to receive dynamical treatment, and (given the resources of the Principia) would struggle to get such a treatment. The same is not true for clocks.