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Notes from the field ·

How Far Away Is the Sun? Build a Scale Model of the Sun, Earth and Moon

By Ahmed, founder

Textbook diagrams of the solar system are wrong in a very particular way: they squeeze the distances so everything fits on a page. A scale model on a school field shows how much emptiness there really is.

The model

If the Sun is an exercise ball about 65 cm (26 in) across:

  • Earth is a peppercorn about 6 mm across (the Sun is about 109 Earth-widths across).
  • The Moon is a grain of salt, about 1 mm across.
  • Earth and Moon are about 18 cm (7 in) apart (about 30 Earth-widths).
  • The Sun and Earth are about 70 m (roughly 75 yards) apart.

Check: Earth is about 12,700 km across, the Sun about 1.39 million km, the Moon about 3,500 km, Earth–Moon about 384,000 km and Earth–Sun about 150 million km. Divide each real size by 1.39 million km and multiply by 65 cm, and you get the numbers above.

Do it

You need: an exercise ball or a large ball, a peppercorn, a grain of salt (or a tiny bead), a tape measure or trundle wheel, and a field.

  1. Put the ball down. That’s the Sun.
  2. Hold the peppercorn 18 cm from a grain of salt, and ask: “Is that the Earth and Moon?”
  3. Pace out 70 m from the ball. One student stands there with the peppercorn.
  4. Look back at the Sun from “Earth.” How big does it look? Notice how much empty space lies between.

If your field is shorter, use a longer measuring tape to the end, or choose a smaller Sun (a basketball) and recalculate everything. Keep the ratios consistent.

What students realise

  • The Sun is enormous: about 109 Earths would fit across it.
  • Earth is a tiny speck, and the Moon is tinier still.
  • Space is mostly empty.
  • The nearest other star on this scale would be thousands of kilometres away.

A model has limits

Ask: “What does this model show well? What’s missing?” It shows sizes and distances but not motion, gravity or light. And on this scale, Earth’s orbit would be a circle 140 m across, which you can’t draw on the field easily.

Extension

  • Light travel time: light from the Sun takes about 8 minutes to reach Earth. How long would it take to cross your model, at your walking speed?
  • Planets: Look up where Jupiter and Neptune would stand. Neptune would be about 2 km away.

The full lesson is How far away is the Sun, really? in Sun, Earth and Moon. Sign up for three lessons free.