"Time, said Austerlitz in the observation room in Greenwich, was by far the most artificial of all our inventions, and in being bound to the planet turning on its own axis was no less arbitrary than would be, say, a calculation based on the growth of trees or the duration required for a piece of limestone to disintegrate, quite apart from the fact that the solar day which we take as our guideline does not provide any precise measurement, so that in order to reckon time we have to devise an imaginary, average sun which has an invariable speed of...