This study investigated the effect of non-traditional guided inquiry instruction on middle school students' conceptual understandings of lunar concepts. Multiple data sources were used to describe participants' conceptions of lunar phases and their cause, including drawings, interviews, and a lunar shapes card sort. The data were analyzed via a constant comparative method to produce profiles of each participant's conceptual understandings and nonparametric tests also were used. Results revealed very positive performance for observable moon phases and patterns of change, as well as the cause of moon phases. Results indicated that significantly more participants shifted from drawing nonscientific shapes on the pretest to drawing scientific shapes on the post-test. Results for the drawings of moon phase sequences were similar in that significantly more participants shifted from drawing alternative waxing and waning sequences on the pretest to drawing scientific sequences on the post-test. Also, significantly more participants shifted from alternative understanding of the cause of the moon phases on the pretest to scientific understanding on the post-test. Implications of these findings and recommendations for further research are provided.