First off...I stole this project from Norm Stelfox! No claim of originality here ... just some modifications!
Step 1: Teach the basics of scale and ratios. This is the worst part because with the new mathematics curriculum, the grade 9's have a heck of a time with ratios. I start with this:
Step 2: Talk about Astronomical units (AU). Since the distance from the Earth to the Sun is 1AU, its an easy starting point. You can say "if the Earth is 1AU away from the sun which equals 149,597,870 km then how far away is Neptune if it is 30.06AU? They will struggle with this for a bit but will eventually see how the ratio works.
Step 3: Use Google Maps (or Google Earth) to make a personal map of the solar system (this is their practice).
The school becomes the Sun and their house becomes the Earth. Every kid will have a different scale and every project will be unique to them! If they live close to the school, Neptune won't be that far away ...but if they live far away... the distances will be exaggerated to epic proportions!
Here is my simple Sun and Earth map to give the kids an idea where to start (as you see...I live at the Seniors Center)
View Solar System Map in a larger map
Now I don't share their maps because they have personal information (like where they live)
Step 4: The tester!
Now get them to repeat the process in another city with 2 great monuments (Paris, Rome, Istanbul, New York, etc...)
Here is I.M. Pei's Pyramid at the Louvre as the Sun and the Arc de Triomphe in Paris.
View France solar system model in a larger map
This is a fun little way to show scale and gives the kids an idea of distances in the solar system in a more manageable way. I always end with: