This case study assessed the use of a computer simulation and whole-class display system to promote scientific conceptions of shapes, sequence, and causes of moon phases for secondary earth science students. The instructional intervention was done by two preservice teachers and included 16 days of daily moon observations followed by in-class inquiry-based instruction on moon phases. Two treatments were compared: instruction where pupils collected moon data from natural observations (NO; n=18) and instruction where pupils collected moon data from Starry Night observations (SN; n=14). Data resources included open-ended questionnaires and interviews. The responses were analyzed using the constant comparative method. Pre- to post-instruction gains from the open-ended questionnaires and the interviews were substantial for both treatments and for all targeted moon concepts. The SN group demonstrated higher gains for all targeted moon concepts. Results indicate that the use of a computer simulation can be effective in achieving conceptual change.