Experiments with the simple pendulum are easy, but its motion is nevertheless confounded with simple harmonic motion. However, refined theoretical models of the pendulum can, today, be easily taught using software like CALCODE. Similarly, the cycloidal pendulum is isochronous only in simplified theory. But what are theoretically equal intervals of time? Newton accepted Barrow’s even tenor hypothesis, but conceded that ‘equal motions’ did not exist – the refutability of Newtonian physics is independent of time measurement. However, time measurement was the (...) key difficulty in reconciling Newtonian physics with electrodynamics. On Poincaré’s criterion of convenience, equal intervals of time ought be so defined as to make the enunciation of physics simple. Hence he postulated constancy of the speed of light. (The Michelson–Morley experiment was not critical.) The theory of relativity followed. But does there exist a proper clock? (shrink)