  For centuries, the rich nocturnal environment of the starry sky could be
modelled only by analogue tools such as paper planispheres, atlases, globes and
numerical tables. The immersive sky simulator of the twentieth century, the
optomechanical planetarium, provided new ways for representing and teaching
about the sky, but the high construction and running costs meant that they have
not become common. However, in recent decades, "desktop planetarium programs"
running on personal computers have gained wide attention. Modern incarnations
are immensely versatile tools, mostly targeted towards the community of amateur
astronomers and for knowledge transfer in transdisciplinary research. Cultural
astronomers also value the possibilities they give of simulating the skies of
past times or other cultures. With this paper, we provide an extended
presentation of the open-source project Stellarium, which in the last few years
has been enriched with capabilities for cultural astronomy research not found
in similar, commercial alternatives.
