What it shows
Planets nearer the Sun take less time to go round it. The crank turns four arms at different rates so you can see Mercury race ahead of Earth.
The science
Kepler’s third law says that the square of a planet’s year is proportional to the cube of its distance. Mercury takes 88 days to go round, Venus 225, Earth 365 and Mars 687. In the model, pulleys of different sizes give each arm a different speed. It is only an approximation of real orbits, which are slightly elliptical, and the sizes and distances are not to scale.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Cardboard sheet 40 cm square
- Four bottle caps as planets, a yellow ball for the Sun
- Four rubber bands and different sized spools or card discs as pulleys
- Skewers as axles
- Straws as bearings
- Glue, tape and markers
Safety
For this project: An adult must cut the cardboard and skewers. Cover the ends of the skewers with clay.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Make a base board with a central Sun and four concentric circles drawn for orbits.
Fix four axles in the centre, one inside the other with straws, so each arm turns freely.
Attach an arm for each planet with its bottle cap at the end.
Fit pulley discs of different sizes on each axle under the base.
Link them with rubber bands to the crank wheel.
Turn the crank slowly and count the orbits of each planet in one turn of Earth.
Compare the counts with 4, 1.6, 1 and 0.5 for Mercury, Venus, Earth and Mars.
Colour and label each planet and write its year length.
What you should see
Mercury circles about four times while Earth goes round once, matching real year ratios roughly.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which planet has the shortest year?
Mercury.
2Why do outer planets move slower?
The Sun’s pull is weaker farther away.
3What is a year for a planet?
The time it takes to go round the Sun once.
4Is the model to scale?
No, sizes and distances are not to scale.
Take it further
Add Jupiter and work out how many turns of Earth give one of Jupiter.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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