🌍 Sun-Earth-Moon Orrery: step-by-step build

Push the arm round: the Earth circles the Sun, spinning as it goes, with the Moon in tow.

An orrery is a clockwork model of space. Ours has a big yellow Sun in the middle, and an Earth that travels round it on a long arm. Here is the clever bit: the Sun gear is locked to the base, so it never moves. The Earth's gear rolls round it like a wheel round a roundabout, so the Earth spins by itself — 2⅔ times per trip round the Sun. Gears that orbit round another gear are called planetary gears. They are hidden inside electric screwdrivers, bikes and automatic car gearboxes!

  • Level 4 of 5 (tricky)
  • About 35 minutes
  • 12 steps, 31 parts
  • Powered by: Hand-crank
  • Series: Mechanical Wonders

What you will learn

Build these first

  • 📡 Spinning Radar: A motor-driven radar dish that sweeps round and round on a turntable — with a blinking light. (level 3 of 5, about 25 minutes, 15 steps)
  • 🌬️ Bevel-Gear Windmill: Turn the handwheel at the bottom — the sails spin at the top! (level 3 of 5, about 25 minutes, 16 steps)

Parts you need

  • 1 × Open frame 5×7
  • 4 × Black pin (grippy)
  • 6 × Half bush
  • 2 × Beam 5
  • 1 × Axle 10
  • 2 × Red axle-pin
  • 1 × Thin beam 2
  • 1 × Beam 9
  • 1 × Axle 6
  • 1 × Medium gear (24 teeth)
  • 4 × Bush
  • 1 × Thin beam 4
  • 1 × Knob wheel
  • 1 × Axle 5
  • 1 × Dark medium gear (24 teeth)
  • 1 × Giant gear (40 teeth)
  • 1 × Thin-tyre wheel
  • 1 × Yellow-hub wheel

Steps

  1. Lay the white 5×7 frame flat, pointing sideways.
  2. Push four black pins up into the long sides of the frame, just left of the middle: two on each side, next to each other.
  3. Stand a half bush on the table in the middle of the frame's opening. Press two 5-hole beams flat onto the pins, side by side, across the frame, and push the tall 10-long Sun axle down through the middle hole of the right beam into the half bush.
  4. Lock the Sun axle: push a red axle-pin into the middle hole of the left beam, then press a thin 2-hole beam down over the Sun axle and the axle-pin. Put a half bush on each.
  5. Earth arm: take a 9-hole beam. Put a half bush on a 6 axle and push it up through the beam's end hole (the half bush stays underneath to lock it).
  6. Slide the 24-tooth gear and a bush onto the Earth axle.
  7. Add the thin 4-hole Moon arm pointing outwards, then the knob wheel as the Earth, and a half bush on top so the Earth cannot fly off.
  8. Push a red axle-pin into the end of the Moon arm and put a white bush on it: the Moon!
  9. Mars: put a half bush on a second 5 axle and push it up through the other end of the arm (the half bush stays underneath to lock it). Add a dark 24-tooth gear and a bush on top.
  10. Drop the arm over the Sun axle from above, so it rests on the two half bushes. Give it a push: it turns freely round the still axle.
  11. Slide the giant 40-tooth gear down the Sun axle until it meshes with both planet gears, and push a bush down on top of it so it stays there. This is the Sun gear — it is locked, so it never turns.
  12. Put the thin red wheel on top of the Mars axle, and top the Sun axle with the yellow-hub wheel: the shining Sun! Hold the frame with one hand and push the arm round by its end with the other.

Play time

Put a sticker dot on one knob of the Earth. Push the arm round slowly and count: how many Earth spins in one trip round the Sun? Can you make the Moon point at the Sun?

Real-world inspiration

Orrery — https://en.wikipedia.org/wiki/Orrery ; epicyclic (planetary) gearing — https://en.wikipedia.org/wiki/Epicyclic_gearing