What it shows
The Moon turns once on its axis in the same time it takes to go once round Earth, so the same side always faces us. You can act this out with two people to feel why.
The science
When you walk round a chair and keep facing it, your body turns through one full turn by the time you finish a circle. That is the Moon’s motion. If you did not turn, you would show all four sides to the chair, so the Moon does not just orbit without rotating. Far side is a better name than dark side, because it gets sunlight as the Moon moves.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A chair or stool to stand for Earth
- A friend as the Moon
- A ball with a smiley face marked on one side
- Chalk to draw a circle on the floor
- A stopwatch
- A notebook
Safety
For this project: Walk slowly in a clear space and keep chairs out of the path.
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.
Step 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.
Draw a circle of radius one metre around the chair on the floor.
Hold the ball with its smiley face turned towards the chair.
Walk slowly round the circle while keeping the face always towards the chair.
Notice where you are facing at the start, quarter, half and three-quarter points.
Count how many times you and the ball turn during one circuit; it should be exactly once.
Walk the circle again without turning your body and note which sides face the chair.
Compare the two walks and write what the second shows.
Draw the diagram with arrows to show the two cases.
What you should see
You find that keeping the face towards the chair requires one full turn per circuit, which shows why the same side of the Moon always faces us.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01How long does the Moon take to spin once?
About 27.3 days, the same as its orbit.
02What do we call this match of spin and orbit?
Tidal locking.
03Is there a dark side of the Moon?
No, the far side gets sunlight too; we just cannot see it from Earth.
04Which side do we see?
The near side, with the large dark patches called maria.
Take it further
Look up which space probes have photographed the far side and compare it with the near side.
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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