Astronomers study the behaviour of the stars; philosophers of science study the behaviour of the astronomers. Philosophers of science, alongside historians and sociologists of science, are in the business of accounting for how science works and what it achieves. There is more to the philosophy of science than principled descriptions of scientific activity, since there are also all the normative questions of justification and warrant, but the descriptive task is an important part of the discipline and the primary focus of the present essay.