Every science has roots that antedate acceptance as a discipline. Neglecting these intimations, there is a consensus that population genetics became a recognised science about 1920, and that it split into two branches about 1960. One branch is concerned with historical processes, uniting genetics with anthropology and (in other species) with paleontology and taxonomy. The other branch is concerned with inheritance in contemporary populations, uniting genetics with epidemiology, medicine, psychology, and (in other species) with agriculture. Anthony Edwards (1995) identified this dichotomy as ‘differential-equation biology’ vs ‘statistical biology’. The distinction becomes fastidious when current gene frequencies are taken as indirect evidence for selection by disease in the remote past: such inference from a unique event is more characteristic of evolutionary genetics but concerns all population geneticists. From time to time interest groups are formed around twins, behaviour, demography, or quantitative traits, but they are generally recognised as aspects of genetic epidemiology. If the young science is in jeopardy, it is not by fission but by exclusion of studies that are only indirectly related to disease, which provides a focus but does not specify the contents. I will argue that every aspect of population genetics that is not primarily concerned with evolution is part of genetic epidemiology, and that the field must suffer if it is not inclusive. As with all human activity progress is punctuated, often cyclical, and therefore suggests the real and apparent movement of celestial bodies for which the language of astronomy is appropriate.