The study examines the use of two 3D virtual environments for teaching basic Astronomy concepts, to pre-service teachers. The motivation was the fact that pre-service teachers know very little about Astronomy and that the virtual environments can be used as tools for successfully teaching concepts related to this scientific field. Two online courses were also developed, in order to compare the learning outcomes. A hundred and twenty randomly selected students from the Department of Primary School Education, University of the Aegean participated in the study, divided into six groups. Two groups used the virtual environments, two groups used the online courses, while the last two were the control groups. Data was collected using evaluation sheets, questionnaires, and log files. Data analyses indicate that even though all groups that used an application showed significant progress regarding knowledge acquisition, the groups that used the virtual environments had better results. Implications are also discussed.