Picture the solar system model almost everyone builds at some point: a styrofoam ball for the Sun, smaller balls for the planets sized roughly right relative to each other, arranged a few feet apart so it all fits on a table. It looks perfectly reasonable.
It's also quietly dishonest. The sizes might be in correct proportion to each other, and the distances might be in correct proportion to each other — but the two proportions almost never match, because a table only has so much room. Fix the sizes to something you can hold, and the real distances become far too enormous to fit anywhere nearby.
A scale factor is just a ratio: model size ÷ real size. Pick it once, using any convenient reference (Earth is the easiest, since everything else is usually measured relative to it anyway), and apply that exact same ratio to every other size and every other distance in the model.
Pick how big Earth is in your model. Everything else — the Sun, every planet, every distance — gets scaled by the exact same factor.
| Body | Model size | Model distance from Sun |
|---|---|---|
| Sun | 1.1 m | — |
| Mercury | 3.8 mm | 45.8 m |
| Venus | 9.5 mm | 84.5 m |
| Earth | 1.0 cm | 117.4 m |
| Mars | 5.3 mm | 178.5 m |
| Jupiter | 11.2 cm | 610.5 m |
| Saturn | 9.4 cm | 1.12 km |
| Uranus | 4.0 cm | 2.25 km |
| Neptune | 3.9 cm | 3.53 km |
You decide Earth will be 1 cm across in your model. How big should Jupiter be, and how far from the Sun?
Run the numbers at any scale where the planets are big enough to actually see, and the distances stop being manageable — this isn't a quirk of one particular scale choice, it's unavoidable at every scale that keeps the planets visible. A handful of real installations have leaned into this instead of fighting it:
| Real model | Where | Scale reality |
|---|---|---|
| The Voyage | National Mall, Washington, D.C. | Stretches nearly the length of the Mall — Earth is smaller than a grape |
| Sagan Planet Walk | Ithaca, New York | A walking tour through downtown, ending nearly a mile from the Sun |
Two workable approaches:
Physical, walking-tour style — choose a scale small enough that the planets are held or marked with small objects, and pace out the real distances outdoors (a schoolyard, a park, a neighborhood). This is honest by construction: you can't fake a walking distance.
Digital — build a spreadsheet or a drawn diagram (a long horizontal strip works well) where every size and distance is calculated from one consistent scale factor, clearly labeled. A digital model can't be walked, but it can show honest numbers even when the distances are too large to physically pace out.