The Ptolemaic model was in fact pretty accurate at accounting for the exact motions of the planets in the night sky. But this accuracy required a great degree of fine-tuning in the cosmic clockwork: the arrangement of the centres of the planets’ epicycles had to remain fixed relative to the line between the Earth and Sun, and revolution about these epicycles had to take precisely one year. What Copernicus realised is that the need for this forced fine-tuning dissolves away if we accept that we are watching the rest of the solar system from a viewpoint that is itself revolving around the sun, taking one year for each circuit. Copernicus’s model of the solar system is much neater than Ptolemy’s, and Weinberg explains how it is the classic example of an explanation gaining acceptance based on aesthetic criteria – the simplicity or elegance of the theory – without experimental evidence to support it over alternative explanations.