The study investigated the mental models of primary school children related to the day/night cycle. Semi-structure interviews were conducted with 40 fourth-grade and 40 sixth-grade children. Qualitative and quantitative analysis of the data indicated that the majority of the children were classified as having geocentric models. The results also indicated that a large number of primary school children did not appropriately conceptualize the essential prerequisites for understanding the day/night cycle, and that their observational skills were limited. It was also concluded that children's age (class) and their mathematical achievement were good predictors for the quality of their mental models. (Contains 5 tables.)