G8 Universe / Scale, Distance and Old Light

Shrink the Solar System to a Street and Feel the Emptiness

Diagrams lie about space by drawing the planets far too large and far too close.

Every classroom poster of the solar system tells the same convenient falsehood. The planets are drawn as generous coloured balls, spaced neatly across a page, close enough that the eye takes in the whole family at once. Nothing about that arrangement is honest. If the planets were drawn at true relative size on that page they would mostly be invisible specks, and if they were drawn at true relative spacing the page would need to be longer than the building. The poster is a labelling diagram, not a picture, and the mismatch quietly ruins most people's intuition for the rest of their lives.

The useful correction is to walk it. Pick a scale where the sun is a beach ball at your front gate. On that scale the inner planets are grains of sand or peppercorns scattered along the footpath in front of a few neighbouring houses, and the largest planets are marbles well down the street. The outermost worlds are further still, out past the shops. Between all of them there is nothing at all, which is the point of the exercise. The overwhelming property of the solar system is not its objects but the emptiness they are suspended in.

Then extend the same scale to the nearest other star and the model breaks, in the most instructive way possible. At beach ball scale that neighbouring sun is not down the road or in the next suburb. It is a continent away. This is why interstellar travel is a genuinely different problem from crossing the solar system, and why the sky is dark rather than crowded with light. It also explains why astronomy is almost entirely a science of observation rather than visiting. The emptiness is not incidental. It is the main feature of the terrain.