In view of students' alternative conceptions about basic concepts in astronomy, we conducted a series of constructivist activities with future elementary and junior high school teachers aimed at changing their conceptions about the cause of seasonal changes, and of several characteristics of the Sun-Earth-Moon relative movements like Moon phases, Sun and Moon eclipses, and others. The activities concerning the characteristics of the Sun-Earth-Moon relative movements and their results are reported. The experimental class as well as the control groups improved their grasp of basic astronomy concepts at a statistically significant level. Regarding subjects relevant to this study (Sun-Earth-Moon relative movements), only the experimental class and one of the control groups showed a statistically significant improvement, and in both cases the experimental class made the most impressive progress of all. (Contains 4 tables, 2 graphs, 1 photograph, 1 figure, and 1 note.)