Astronomy education researchers now know that college students do not enter the introductory astronomy classroom as blank slates, but rather with a pre-existing understanding of many introductory astronomy concepts, including lunar phases. Sometimes this understanding is scientifically correct, but often students' understanding is incomplete, inadequate or simply incorrect and cannot explain observed phenomenon. Unfortunately, students' pre-existing understandings are often deeply rooted, and many students leave the classroom without a scientifically correct understanding of lunar phases. The purpose of this research study was to design instruction that enhances college students' understanding of lunar phases.

This multi-phase study utilized qualitative and quantitative research methods to fulfill this purpose by identifying students' prior understanding of lunar phases, developing the Lunar Phases Concept Inventory (LPCI) to measure conceptual change, designing and evaluating an in-class group activity designed to teach the concept of lunar phases.

Using a qualitative phenomenology, fourteen college students' conceptual understanding of lunar phases was uncovered and organized into a conceptual framework with eight dimensions of student understanding, each with alternative facets. Based upon this conceptual framework, the LPCI was developed. This instrument consists of fourteen multiple-choice items designed to assess student understanding of lunar phases.

Based on a modified Karplus Learning Cycle, an in-class group activity was developed to teach the concept of lunar phases. During the fall of 1999, this activity was implemented at a midwestern university as part of a restructured astronomy course during two fifty-minute class periods. Administered prior to and after instruction, the LPCI shows the instruction was effective. A statistical analysis of the results shows that the instruction produced an effect size of 2.99 and a normalized gain of 0.63.