Models help students understand how things work. Creating a model of the solar system helps students understand how the light from the sun interacts with the earth, moon and other planets. Creating a mobile in which the sun and other planets suspend from a series of supports gives the illusion that the planets are independent and unsupported.
Creating a mobile that shows the sun, earth and moon in proportion can be done, but it would be a large and unwieldy project. The sun is approximately 1,392,000km in diameter, while the earth is only 12,756km in diameter. To put that into perspective, if you made a model of the sun is 1m in diameter, then the earth would be just over 1cm in diameter, and the moon would be 25mm. There is also the problem of distance. According to NASA, the earth is approximately 147,300,000 to 152,100,000km from the sun, depending on the season of the year. To keep an accurate scale with a 1mm earth and a 1m sun, they would have to be more than a kilometre apart. This would keep any mobile out of the classroom. Most mobiles focus on functionality rather than scale.
It is neither necessary nor convenient to use the same materials for the sun and all the planets. The sun, as the largest body in our solar system, should be made out of a lightweight material such as paper mache. A large rubber or plastic playground ball also makes an appropriate sun, but make sure the ball isn't inflated or making a hole for light will deflate the object. When designing the earth, look for something that is roughly one-tenth the size of the model sun. If a playground ball is used, try a super ball or a ping-pong ball. The moon is about one-quarter the size of the earth so try a marble or even a gumball.
Painting the Mobile
Spray all of the objects in the solar system with a light coat of primer. This helps the paint adhere to the objects uniformly. Drill a 1/16-inch hole through the earth and moon objects. Cut a hole in the sun large enough to place a small flashlight. Paint the sun a bright mixture of yellow, orange, red and even a little white. For realism, consult NASA photographs and copy the large sunspots and solar storms that erupt across the face of the star. Paint the earth blue and green with outlines of the continents. Paint the moon grey. Add speckles to the moon to approximate craters and to give the tiny model depth. Paint a 6-cm and 50-cm length of dowel black.
Putting the Mobile Together
Thread a black string through the earth and the moon. Tie the string to either end of the 6-cm dowel. Balance the centre of the dowel on your finger. Slide the strings closer or farther from the centre until the dowel balances on your finger. Glue the string in place. Tie a third string in the centre. Tie the other end of the string to one end of the 50-cm dowel. Thread a black string through the top of the sun. Rest the other end of the 50-cm dowel on top of the sun and tie the string around the dowel. Tie the hanging string between the earth and the sun. Slide the string with the earth and moon and the string with sun until the mobile is balanced. Glue the strings into place.
Using the Mobile
Turn on the small flashlight in the sun. Observe how the earth and moon are lit on only one side. Rotate the moon around the earth and observe how the lit portion of the moon seems to change depending on the relationship between the earth and moon. Observe how, in some positions, the moon blocks the rays of the sun from the earth to form an eclipse.