This paper investigates students' conceptions of certain astronomical phenomena. The 1,920 participants were drawn from junior secondary science classes in four Australian educational jurisdictions. Participants completed a modified version of the Astronomy Diagnostic Test to elicit information about their knowledge and understanding of certain astronomical phenomena and to identify any alternative conceptions held by them. Results showed that students exhibited many alternative conceptions about concepts they should have covered in elementary or the first year of high school. Discussion centres on the extent to which school science takes account of the important construct of pedagogical content knowledge.