Most students don't have any concept of the vast distances and empty spaces of the solar system. I recently asked a group of high school seniors how close to the earth the moon was, compared to the sun, and students answered anywhere from a tenth to a hundredth the distance. The true answer was a real surprise to them; the sun's distance from the earth is about 400 times the distance from the earth to the moon.

Why do students have a bad sense of the scale of things in the solar system? The answer is that nobody creates "true" scale models of the solar system. When creating diagrams of the solar system, people will either keep the sizes to scale while ignoring the real distances, or they will show the real distances while ignoring the relative sizes. Why? Because it is nearly impossible to keep both items to scale and fit the model on a standard sheet of paper. Actually, it's nearly impossible to do on a giant sheet of poster board. If you make the sun small enough that all the orbits will fit on the page, the inner planets are so small they are tinier than the smallest dot you can make with a pencil.

The following is a suggestion for helping students get a general feel for the relative sizes and distances of the planets in the solar system.

Step One: Scale Model of Sizes
For this project, students will need a sheet of paper, a pencil, a ruler, and a compass. Their goal is to, on a single sheet of paper, create a diagram that shows the relative sizes of the planets.

To start out, you can give them a table like the following, or you can require them to research the numbers themselves. This table shows the radii of the solar system's planets.

Mercury 2,440,000 m
Venus 6,051,000 m
Earth 6,378,000 m
Mars 3,397,000 m
Jupiter 71,492,000 m
Saturn 60,268,000 m
Uranus 25,559,000 m
Neptune 24,764,000 m

Students will first need to determine which planet is the largest (Jupiter) and find a scaling factor which will allow them to draw the planet on a sheet of paper. Assuming they want a quarter-inch margin on each side, this means Jupiter can be no more than 8 inches tall, or 4 inches in radius. Thus, a good scaling factor would be 71 million meters = 4 inches, or (roughly) 18 million meters = 1 inch. You can help them determine this scaling factor, or you can ask them to figure it out themselves, depending on their math ability level.

Once they have the scaling factor, they can begin calculating the scaled sizes of all the other planets, and drawing them (using the ruler and the compass!) on the page. A typical result will have Jupiter centered on the page, with the other gas giants shown as concentric circles inside it, and the inner planets drawn in the corners, or anywhere else on the page there is free space.

Remind students to print their scale on their diagram.

Step Two: Scale Model of Distances
For this part of the project, ask the students to research the distances of the planets from the sun, or give them the following table:

Mercury 0.4 AU
Venus 0.7 AU
Earth 1 AU
Mars 1.5 AU
Jupiter 5.2 AU
Saturn 9.5 AU
Uranus 19.2 AU
Neptune 30.1 AU

This shows the distances from the sun in astronomical units.

This time students will need to determine which planet is the furthest from the sun, and how much they need to scale the drawing to make that distance fit along the length of the page (about 10 inches, including margin). The simplest scale to use is 10 AU = 1 inch. Explain to students that they can simply mark the planets and the sun as dots on the page, rather than trying to draw the sizes to scale.

Step Three: Scale Model on a Map
The final step in this project is very useful in helping students grasp the extraordinary amount of empty space in the solar system. For this project you will need a map of your home state (and possibly even the entire country), a compass, and a pencil.

This time, we'll include the sun (695,500,000 meters in radius) in our model. Begin by scaling the size of the sun down to the size of something easily located on your map, like your school. Once you have this scale factor, calculate the relative sizes of all the planets, and use the same scale factor to calculate the distances.

Now you'll need to take those scaled distances and convert them to map distances, using the map's scale factor. Measure out these distances and draw a circle on the map using the compass. Now you can scan that circle to find a recognizable landmark, so you can say "If the sun is at our school, then the earth is X inches wide and it's in the town of Y!"

Suddenly students begin to see just how empty the universe really is!

When I did this in my physical science class, the first two steps were done as homework assignments. I helped the students figure out the scale in class, and they did the drawings at home. The third step was done in class, in groups of four students. Depending on the students and their ability level, it may be better to do this as a "demonstration," rather than a student project.