However, as suggested, the users could be anyone, of any age who find the concepts difficult to understand. The need for graphics and animation is very evident - if the dynamic relationship between the earth, sun and moon are to be explained. There is also a need for graphics showing the properties of light generally - source, shadows, reflection and so on. Text and voice over would seem to be essential too. You might include music - at least in intro.
Organisation There are at least three subsections.
Clearly the users need to understand light first. Then tackle the astronomy. All need to integrated, of course. You will need to have pretests and posttests for each section. The user won't understand earth-moon relationship unless they understand about shadows first.