The research presented here, a quasi-experimental study of knowledge acquisition and concept learning in the upper primary years of schooling, systematically identifies, categorises and describes children's ideas and mental models in the 'traditional' content areas of astronomy (the Earth, the Sun, the Moon, Earth-Sun-Moon System phenomena and the Solar System and wider Universe) and measures and maps the changes in those ideas and mental models as a result of exposure to a ten week period of formal astronomy teaching in one English primary school setting. In total, 62 children in two vertically grouped Year 5/6 classes were involved (9 to 11 years of age). Within the context of the quasi-experimental design employed, one class was designated the experimental group (n=31), the other the control group (n=31). Initially, only the children in the experimental group were taught astronomy and by their own class teacher, a highly experienced practitioner with a positive attitude and orientation towards science. Within the limitations of the work undertaken, research findings indicate that among children in the experimental group, and in most of the content areas investigated at least, progression was evident in many different forms and levels of scientific conceptualisations were high suggesting that an acceptable scientific knowledge and understanding of sometimes complex and abstract astronomical information is attainable in the upper primary years of schooling and can be retained over time. Research findings also draw attention to the individuality, commonality, non-linearity and dynamic nature of astronomy learning as a whole and a new, if tentative, perspective on astronomy learning involving chaos theory is presented. The place of astronomy education provision in primary schools is reconsidered and concerns are expressed over the need for extreme care at a time when classroom practices and expectations are increasingly driven by curriculum guidelines and science curricula which remain to be fully informed.