Eighth Grade · Science · week 32

Scale of the solar system

Use consistent ratios to build a correct proportional scale model of solar-system sizes and distances.

The sizes and distances in the solar system are hard to picture because they are so large. The Sun's diameter is about 1,400,000 km, and Earth's is about 12,700 km -- the Sun's diameter is about 110 times Earth's. The distance from Earth to the Sun is about 150,000,000 km, defined as one astronomical unit (AU).

Building an accurate scale model means using the SAME scale factor for every size and distance, not a different scale for planets than for the space between them, a common flaw in classroom posters. If the Sun is shrunk to a 1-meter ball, Earth becomes about 1 m / 110, roughly 0.9 cm (a pea) -- and that Earth would need to sit about 107 meters away, roughly a football field's length, to keep the ratio accurate too.

📋 For the teacher
Teach it (10–15 min):
  • Do the arithmetic together slowly with a calculator -- the huge numbers make it easy to lose track without writing each step.
  • If space allows, measure out a real scaled distance using a chosen small object as the Sun, to feel how empty the space really is.
  • Point out that the vast empty space in an accurate model is not a flaw -- it is one of the most important things the model teaches.
Talk about it:
  • Why is it important to use the same scale factor for both sizes and distances in a model?
  • What surprised you most about how small Earth is compared to the Sun, or how far apart they are?

Mastery looks like: They compute a consistent scaled size and distance for Earth given a chosen model size for the Sun, using one scale factor throughout.

Common stumbles: Using one scale for planet sizes and a different, more convenient scale for distances between them.

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