What it shows
The Moon's gravity pulls the oceans into a bulge on the side facing it and a second bulge on the opposite side. As the Earth turns under the two bulges, most coasts see two high tides and two low tides a day.
The science
Gravity weakens with distance. The Moon pulls the near-side water a little more than it pulls the solid Earth, and it pulls the Earth a little more than the far-side water. The result is that the water is stretched away from the centre on both sides, giving two bulges. The Earth spins once a day beneath them, so a coast passes through a bulge about every 12 hours and 25 minutes. When the Sun and Moon line up at new and full moon, their effects add and give larger spring tides.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A cardboard disc for the Earth, painted with a map
- A larger cardboard oval cut as the water layer
- A small ball for the Moon on a card arm
- A split pin at the Earth's centre
- A base board
- A marker and labels
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
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.
Cut the Earth disc about 15 centimetres across and paint a simple map on it.
Cut the water oval about 18 centimetres long and 15 centimetres wide from blue card.
Fix the oval to the base with the long axis pointing at the Moon position, and pin the Earth disc on top through the centre so it can turn.
Fix the Moon ball on its arm so it sits beyond one end of the oval.
Mark a dot on the Earth's edge for Mumbai.
Turn the Earth slowly and call out high tide when the dot reaches a bulge and low tide when it reaches a narrow side.
Count that one full turn gives two high tides and two low tides.
Add a Sun on the far side of the board, in line with the Moon, and explain spring tides; move it at right angles and explain neap tides.
What you should see
Turning the Earth disc shows two highs and two lows per turn, and the Sun position explains why tides are bigger at new and full moon.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why are there two bulges, not one?
The Moon pulls the near water more than the Earth, and the Earth more than the far water, stretching the water both ways.
2How many high tides a day?
Usually two, about 12 hours 25 minutes apart.
3When are spring tides?
At new moon and full moon, when the Sun and Moon pull along the same line.
4Why is the gap more than 12 hours?
The Moon moves along its orbit, so the Earth must turn a little extra to catch up.
Take it further
Look up a tide table for Mumbai or Kochi and check that high tides shift by about 50 minutes each day.
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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