First, you definitely need to show this TED-Ed 4 min video to explain it all
Then, view Apple EDU’s tweet with model eclipse animation
To recreate with Keynote, view this how-to example of step-by-step recreation from Koen Marien
Or here is a simpler, 30 sec animation that students more easily recreate with Keynote’s Magic Move
This was my first attempt at trying it, but now I know a little more I could add. Challenge your students to create something better than mine!
September update: Here is just one example of an O'Neill High School 8th grade student example created from this concept. Congratulations!