Yes, my ds9 will shake his hands and whole body when he's really excited about something.
We did a similar solar system thing at one point, using marbles, talking about which ones looked most like the planets, the various moons, how big the sun would have to be, etc.
Have you considered binoculars or a telescope? Telescope might be a bit much for a 3-4 year old, binoculars give a pretty good view of the moon and some clusters of stars, and I think you might even see the moons of Jupiter if you try. Binoculars are easier for younger kids to see with, too.
