Since the early nineteenth century, the number of "Outlines of Astronomy" has increased and given birth to a literary genre in its own right, in spite of the mathematical nature of the astronomical science. We aim to show how great astronomers such as John Herschel, Alexander von Humboldt, Francois Arago or Camille Flammarion tried to resolve the dilemma of using a defined form to describe a subject that didn't belong to the discursive sphere. In their books, they delineated new boundaries between science and literature. In the so-called "popular Astronomies", the history of science and the history of literature are linked, sometimes in a very surprising way.