Abstract
Making astronomy more accessible and attractive to students is very important. In keeping with this objective, we present an activity on teaching planetary motion and the relative positions of the Sun, Earth and an inner or outer planet. We give special consideration to the phases of an inner planet and the gibbous appearance of an outer planet to motivate the interest of the students. This paper presents a simple study of the relative position and the phases of the planets. The contents are appropriate for university students, although not only for astronomy degree students. This presentation is suitable for students of other degrees, who are studying a basic course in astronomy. This activity could follow the general study of Kepler's laws and elliptic motion. Its aim is to explain observations using the general theory of elliptic motion, so that students can understand this theory in greater depth. They study an especially interesting phenomenon, despite not being very well known by ordinary people, which is the different appearance of the planets depending on whether they are inner or outer planets. A group of evocative images of Venus and Mars are used to introduce the study of elliptic motion. We take care to present the results obtained using simple and schematic diagrams to facilitate its understanding. Finally, we present a simple mathematical model for carrying out the calculations related to angles and relative distances and we compare the results obtained by this simplified process with the parameters obtained directly from the photographs, and with the values calculated by elliptic motion.
