When tides collide

Before you begin

• Check out this info from National Geographic to get a quick overview of the topic of tides.
• Anyone working towards stage four of the Sailing Staged Activity Badge could take part in this activity run alongside Wild wind and Rules of the nautical road to check their knowledge and meet the badge requirements. 
• If you want to learn about the neap and spring tides ahead of this activity, check out Learn how tides work

Act it out

1. Explain that as a group you’ll now be making a moving picture of the tides. Everyone should get into a big circle and join hands. Ask for a volunteer from the group to stand in the middle to act as ‘Earth’.
2. Explain that the rest of the group will act as the sea. The further away from the Earth they are, the higher the tide is on that side of the Earth.

You could ask everyone to hold onto a length of rope tied into a loop, instead of holding hands.

3. When the person leading the activity calls ‘flow’, everyone should step as far away from the ‘Earth’ as they can, still keeping hold of the person next to them. This is the high tide.
4. Call ‘ebb’ and everyone should step as close to the ‘Earth’ as they can – this is the low tide. Have a couple of goes until everyone understands what we mean by tidal ebb and flow.

In reality, the tide doesn’t ebb and flow at the same time all around the world. It’s also affected by where the moon is in the sky.

5. Everyone should start in a circle, at an equal distance from the ‘Earth’. Ask another person to volunteer to be your ‘moon’ and stand at one end of the space. Explain that the moon’s gravity will affect the tide on Earth. Ask everyone on the side closest to your ‘moon’ to take two big steps towards it, still holding hands. The Earth should also take one step towards the moon.

The moon’s gravity pulls the sea towards it, and it also affects the Earth. The sea on the other side of the Earth is too far away, so it isn’t affected and doesn’t move. This means there should be a ‘high tide’ in the sea closest to the moon, and on the opposite side of the Earth.

6. See what the tides look like now and where the sea is highest and lowest. It won’t be the same on different sides of the earth. Ask the ‘Earth’ to spin around on the spot, like it would over the course of a day. They should call out whether they see high or low tide and see how the tide ebbs and flows from high to low and back again. As the Earth rotates during one day, it should see two high and two low tides.
7.  Now ask the ‘moon’ to circle the space and orbit your ‘Earth’. Everyone holding hands and acting as the sea should move with the moon to show how the tide would ebb and flow during a month.
8. The sun’s gravity also affects the tide, so ask for one more volunteer to stand at one end of the space and act as your ‘sun’. To start, make sure the ‘sun’ and ‘moon’ are in line with each other.
9.  Explain that like the moon, the sun’s gravity also affects the tides, but it’s only about half as strong. Repeat the action from step 5 to show the moon’s effect, then ask everyone closest to the sun to take an extra half a step forward.

You should notice the high tides get higher as the moon and sun’s effect is added together. This is known as the spring tide.

10. Try again with your ‘sun’ at a right angle to your ‘moon’. Get everyone to show the effect of the moon and sun’s gravity again. This time, you should notice that the high tide is made lower by the effect of the sun, which is pulling the sea in another direction. This is known as the neap tide.
11.  Have some fun and see if everyone can follow the movement of both the ‘moon’ and ‘sun’ as they move around the space. See if the group can work out the best positions for the sun and moon to make the highest high tide, the lowest low tide, or the smallest difference between high and low tide.