Universal Zoom is a neat iPad app designed to show users scales of size and distance. The app features more than 100 images of objects in the universe. The objects are shown next to each in a progressive scale. Tap an object to see it compared with other objects in the app. For example, I tapped on the image of Mount Everest to see it in comparison to the Eiffel Tower and Angel Falls. Tapping on the name of the object will reveal the Wikipedia entry about it.
Universal Zoom currently costs $1.99. At that price I’m not sure that I would I buy it for every iPad in a classroom, but I might buy it for a shared iPad for students to explore. One of these five free online resources could accomplish a similar goal.