The website below offers interactive science activities that allow students to "make something happen" then gives the students the reason it happened. This is valuable for 3rd grade because often times the students learn by doing. I would use this site as an additional resource for student understanding. While navigating this site, I learned that students can actually go onto the "Earth and Moon Rotation" portions and get a visual of how the Earth orbits around the sun, how the moon orbits around the Earth, and learn how long each takes.
http://www.uen.org/3-6interactives/science.shtml