Start Retrieval See–Think–Wonder Planet Sizes Scale Calculator Practical Prep Summary
Y7 Science · The Solar System

How Big Is
Space?

Every diagram you've ever seen of the solar system is lying to you. Today we find out the truth — and it's going to blow your mind.

Describe the scale of the solar system
Calculate planet sizes for a scale model
Explain why diagrams are misleading
▼

⏱ Retrieval Starter · 5 minutes

What do you already know?

Five quick-fire questions. No peeking — this is about what's already in your brain.

👁 See–Think–Wonder · 5 minutes

Textbook vs Reality

Look at these two versions of our solar system. One is how textbooks draw it. The other is closer to reality.

Typical Textbook Diagram

Planets look almost the same size, neatly spaced out.

Closer to Reality
← Earth

If the Sun were this size, Earth would be that tiny dot. And it should be 12 metres away on this scale.

👁 See
What do you notice? What differences stand out?
🧠 Think
What does this make you think about?
✨ Wonder
What questions does this raise?

🪐 Core Activity · 10 minutes

Rank the Planets by Size

Click the planets in order from smallest (1st) to largest (8th). How well do you really know them?

💡 Click each planet in order — smallest first, largest last. Click a selected planet to deselect it.

💡 Key Concept: Two Kinds of Scale

There are two ways to scale the solar system: by size (how big the planets are) and by distance (how far apart they are). The problem? You can't do both on the same model — if you make the sizes correct, the distances would stretch across an entire city. If you make the distances correct, most planets would be invisible specks. That's why every textbook diagram is a compromise — and a lie.

🧮 Guided Calculation · 15 minutes

Build Your Scale

Choose an object to represent the Sun. We'll calculate how big each planet would be on the same scale — with a little less help each round.

Step 1 · Choose Your Sun

Pick an object from your home to represent the Sun. This sets the scale for everything else.

Step 2 · Partial Support

Same object: . Now you calculate the scale factor.

Scale Factor = Object Size (cm) ÷ Sun Diameter (km)
cm
1,392,700 km
Round 3 · Minimal Support

Final challenge: calculate the model sizes for the remaining planets. The formula is at the top for reference — but the rest is up to you.

Model Size = Real Diameter × Scale Factor ()

🏠 Practical Prep · 10 minutes

Homework: Find Your Planets

Before next lesson, you need to find household objects that match each planet's scaled size. Use the checklist below and take a photo of each one.

📋 Your Checklist

  • Find or make a Sun object (your chosen scale object)
  • Find objects matching each planet's scaled size
  • Take a photo of all your planets together
  • Write down which object represents which planet
  • Note anything surprising about the sizes

💡 Size Finder Tips

Not sure what matches? Here are everyday objects for common sizes:

< 1 mm — grain of sand, pencil tip mark
1–3 mm — peppercorn, sesame seed, pin head
3–5 mm — lentil, small bead, match head
5–10 mm — pea, blueberry, pencil eraser
1–2 cm — marble, grape, coin
2–5 cm — ping pong ball, walnut, golf ball
5–10 cm — tennis ball, orange, apple

🚀 Lesson Summary

What We Learned Today

🤯
Scale Is Deceptive
Textbook diagrams dramatically mislead us about both the sizes and distances in our solar system.
📐
Two Kinds of Scale
You can model size OR distance accurately — but never both at once on the same model.
🧮
Scale Factor
Dividing your model size by the real size gives a scale factor you can apply to everything in the system.

COMING NEXT LESSON

Building Your Scale Solar System

"If Earth is 1 metre from the Sun… where would Neptune be?"

Bring your planet objects and get ready to head outside. We're going to discover just how much empty space there really is.

🎉 Lesson Complete