This paper analyses a process of co-construction of knowledge and its multiple forms of communication in a country of the European periphery in the early twentieth century. It focuses on Lieutenant Manuel Soares de Melo e Simas, a politically engaged Portuguese astronomer, who moved from amateur to professional during the political transition from the monarchy to the republic. Melo e Simas paralleled his professional career in continuous activity of communicating science to the public in the context of republicanism in a double way, by responding to the agenda of republicanism and by playing an active role in shaping it. He aimed at educating lay audiences in the various ways of astronomy, and he reached out to as many people as possible by exploring a multitude of communication channels, from lectures to articles in newspapers and journals. Voiced often within newly created republican institutions, the praxis and the ideas of Melo e Simas helped to mold the new republican scientific ethos. By going beyond mere emphasis on scientism and positivism, usually taken to be the defining characteristics of the new republican ethos, this paper argues that science and the specificities of its multiple forms of communication were central to the way Melo e Simas shaped the republican ideology. Furthermore, popularization of science was used to legitimize the status of professional scientists at the same time that it helped reinforce their institutional setting, still to be negotiated in the forthcoming decades through a complex process which deserves further historical analysis.