# Movements of the Earth and planets

• If it is nighttime in South Africa, is it also nighttime in Brazil?
• Do the other planets also take a year to revolve once around the Sun? Is their year the same length as our year on Earth?
• Why do we get day and night?
• What is the difference between revolution and rotation?

## Rotation (Earth)

For a long time people believed that the Earth stood still and the Sun moved around the Earth. In this chapter we will find out what really happens. We'll start by thinking about day and night.

### Day and night

During the day, it appears as though the Sun moves across the sky as it rises (comes up) in the morning and sets (goes down)in the evening.

East and West are two directions you must know. If you point at the Sun when it appears to rise in the morning, you are pointing in the direction of East. If you point at the place where the Sun sets, you are pointing direction of West. Have a look at the picture of Sophie. Sophie loves to get up early and watch the sunrise. She is standing with her arms stretched out, pointing in the directions East and West.

# Find East and West

MATERIALS:

• aplace where you can stand in the early morning

INSTRUCTIONS:

1. Look at the picture of Sophie at sunrise and answer the questions.

QUESTIONS:

Which is Sophie's right hand? Show her right hand with your finger.

Why do we see a shadow of Sophie on the ground?

The Sun is casting the shadow as Sophie is blocking the light from the Sun which reaches that part of the ground.

In which direction is her shadow pointing?

West

If Sophie looks straight in front of her, in which direction is she looking?

North

Now find the direction of East in your classroom. You must point to the place where you see the Sun come up in the morning. In the classroom, stretch out your arms and point in the direction of East and West. Clean the floor and stick some insulating tape on the floor so that everyone can remember which direction is East and which direction is West.

Help your learners understand that East is not really a place. Wherever you are on Earth, you can point towards East and if you keep walking East you will see the sunrise in front of you every morning. This idea can be a little confusing because we do talk about places like Mpumalanga, the place where the Sun rises. But if you actually go to Mpumalanga's eastern border, then you see the Sun rise over Mozambique. In Mozambique, the Sun rises over the sea, and so on.

So the difference between the words "place" and "direction" is important. We see this if we ask the question "How far must I walk to reach the East?" The answer is "You can never reach it; the East will always be in front of you."

This becomes practically significant when we tell the learners that the Earth rotates towards the East. You cannot find a place on the globe that is marked "the East".

What does the word "direction" mean? You can walk in a direction towards a place. If the wind blows, you see leaves moving in the same direction as the wind. Remember, a direction is not a place that you can reach!

Introduce what it means to discuss something with someone else. You can also introduce the concept of debating, which is a more formal discussion where two opposing (opposite and different) views are put forward on a topic and each group argues their case. Allow learners a few minutes to discuss the following question with their partner. Then ask for some feedback on the answers.

Let's have a discussion. To discuss something is to talk about it and your ideas with someone else or a group of people. Turn to your classmate next to you and discuss the following question. Write your answer down, then write your partner's answer down.

Where does the Sun go at night? Why do we get day and night?

NB: The Sun does not actually "go" anywhere at night! We may talk about the movement of the Sun across the sky in everyday language, but this is INCORRECT. The Sun does not move - the Earth rotates on its axis. In the rest of this section, make it clear to learners that the Sun may appear to move across the sky, but it is actually the Earth that is spinning which makes it look like the Sun moves.

In this next section we will find out these answers!

### Does the Sun really move, or does the Earth move?

When you ride in a bus, you may see houses outside the bus. It looks as if the houses move past your window.

Why do the houses seem to move past your window? Are the houses really moving? Discuss this question.

Don't accept an answer "No, they are not moving"; ask why they seem to move. Learners must realise that the houses seem to move but it happens because the person in the bus is moving. We want learners to put this idea into words for themselves.

So it is really the person in the bus who is moving; it looks to the person as though the houses are moving, but they are not moving.

You saw how big the Sun is when we compare the Earth and the Sun. Go back to the beginning of this term's work to find the picture of the Sun and the Earth. That great big Sun does not move around the Earth.

The Earth is turning around and that is why we see the Sun move past us. We are like Sophie in the bus. She is in the bus and she is moving past the houses. The Sun is like the houses; they are not moving. It looks to us as though the Sun is moving, but it's really the Earth that is turning around.

You can think of the Earth as an orange with a pencil through it.

If you twist the pencil, the whole orange spins around. This is like the Earth spinning around. The pencil is called the axis of the orange. In the same way, we can think of an axis that goes through the Earth. The axis is a line that we can imagine, it is not a real thing. Earth spins around that axis. We say the Earth rotates (spins).

We say that the Earth has an axis that it rotates on. This axis runs from the North Pole to the South Pole as you can see in the picture.

Other objects also have axes of rotation. For example, an ice skater has a vertical axis that she rotates on when spinning on a spot during her performance.

A log which is floating in water can also roll around. It will have a horizontal axis of rotation.

# Make a model of the Earth in daytime and nighttime

MATERIALS:

• aglobe of the Earth or a balloon with the shapes of the continents drawn on it
• string to hang a globe or balloon
• a large mirror

INSTRUCTIONS:

1. This picture shows you how to set up the equipment.
2. Set up a mirror outside the room so that it reflects bright sunlight onto the Earth globe. Your globe must be able to spin around.
3. Everyone must look at the globe from the same side.
4. Find Africa on the globe. Turn the globe so that the Sun's light falls on South Africa.
5. One side of the globe is in shadow. Find the shadow in the picture. Write the word "shadow" next to it.

This is to make sure your learners are following the explanation.

1. Find where Durban and Cape Town are on the globe.

Make small people out of Prestik and stick their feet on the globe at Durban and Cape Town.

1. Look at the globe in the picture. If you were in Durban, would it be daytime or nighttime?

2. If you were in Cape Town, would it be daytime or nighttime?

3. Now turn the globe so that Africa moves to the right. That is, you turn the globe towards the East. You will see Durban becoming dark and moving into the shadow. When Durban is in nighttime, Cape Town will still be in sunlight.
4. Keep on turning the globe towards the East. Now Cape Town will go into the shadow. That is nighttime for the people in Cape Town.
5. Which city will come back into daytime first, Cape Town or Durban?

Skill: Predicting from the pattern they can see. Answer: Durban. We know that the Sun rises earlier in Durban than in Cape Town.

1. Keep on turning the globe to the East, and Durban will come back into daytime. You have to move to the other side of the model to see Durban move into sunlight.
2. How many hours pass for the Earth to turn around once?

24 hours. People did not discover how long an hour is; they decided how long an hour would be, and they divided up the day-night into 24 hours.

We see the Sun appear to rise and move across the sky every day. But the Sun does not really move; it only seems to move! Earth is spinning round and round, and we are moving around with the Earth. The Earth takes 24 hours to complete one full rotation.

MATERIALS:

• yourself
• sunlight coming from one side

INSTRUCTIONS:

1. This model will help you to understand why we see the Sun move across the sky. Do this in the early morning when the Sun is still low.
2. We will say that your nose is Africa. You are on Africa. Look at picture below.
1. Now stand so that bright light from the Sun shines across your right cheek.
2. Turn slowly to your left. Turn your eyes towards the bright place where the Sun is. You will see the Sun move to your right while you move to the left.
3. Move your feet and turn further; you will see the Sun "go down" over your right cheek.

They must move their whole bodies around, not just their heads!

1. When you have turned your back to the Sun, you cannot see the bright light any more. That is like night time in Africa.
2. Turn further to your left and you will see the Sun "rise" over your left cheek. That is like sunrise in Africa.

NB. Make sure to emphasise to learners that they are moving their head around which is like the Earth rotating, whilst the light source (the Sun) does not move.

## Revolution (Earth)

By now you know that all the planets revolve (travel) around the Sun. Each planet has its own pathway. This is called its orbit. We can also say planets orbit the Sun. Earth also moves in its own orbit around the Sun. This movement is called the revolution of the Earth around the Sun. We can also say that the Earth orbits around the Sun.

We have now come across two new words: rotation and revolution. Remember, these are not the same thing! Let's do an activity using our own bodies to understand the difference between revolution and rotation.

Most people, including adults, mix up the terms `revolution' and `rotation'. The simplest way to deal with this is to say that the Earth spins around its own axis, and it orbits around the Sun. We can use "orbit" as a verb. In the next activity, make sure to show the difference in the body movements when rotating and when revolving. Explain to learners when they are doing each one. This is also another way of making a model of the motions of the Earth. We did it before, with the ball swinging around on a long string.

# Make a model of the Earth revolving around the Sun

MATERIALS:

• room to move around

INSTRUCTIONS:

1. We are going to use our bodies to understand the difference between revolving and rotating.
2. First, the whole class must stand, and spread out. Now spin around with your arms out, staying on one spot. This is called rotation! The Earth rotates like this on its axis.
3. Now get into pairs. One learner must stand in one spot and the other learner must walk in a circle around the other person. This is revolution. The second learner is revolving around the learner standing still in the middle. The Earth revolves around the Sun like this.
4. Now, let's put both movements together! As the Earth rotates on its axis, it also revolves around the Sun. This might be tricky! Spin around (rotate) while also moving in a big circle around your partner (revolve). Look at the picture below.

QUESTIONS:

In this model, who represents the Sun and who represents the Earth?

Every 4 years we have a leap year, which is when there is one extra day in the year. This is because the Earth actually takes 365.25 days to revolve around the Sun, and not just 365 days. So every 4 years, we have an extra day to account for these quarter days.

The learner standing still is the Sun and the learner moving around is the Earth.

When you are spinning and walking in a circle around your partner, sometimes you face your partner and sometimes your back is to your partner. Which of these represents day for you and which represents night for you?

When you face your partner it is like day as the Sun shines on your face and when your back is to your partner, then it is like night.

You could spin around very quickly, but how many hours need to pass for the Earth to rotate once?

24 hours

You could also move around your partner quite quickly. In reality, how long does it take for the Earth to go once around the Sun?

365 days

• Wherever we are, we can find the East - West direction by the place where the Sun rises and sets.
• The Earth spins on its axis once in 24 hours. This spinning is called rotation.
• The part of the Earth that faces the Sun experiences daylight, and the part that is turned away from the Sun experiences night.
• The Earth travels in its orbit right around the Sun. This travel in orbit is called revolution.
• A complete travelling of the Earth around the Sun makes a year.

How can you find East?

Look towards the place where the Sun comes up in the morning.

How can you find North?

Stretch out your arms so that your right hand point East and your left hand points West. Then North is the direction in front of you.

Why does it look as though the Sun moves across the sky when we know that the Sun does not move?

The Sun seems to move because the Earth is rotating (turning). The Sun seems to move from East to West, but that is because the Earth is rotating from West to East.

Where is the Sun, when it is nighttime where we are?

The Sun is on the other side of the world.

When we are having night, is everybody in the world also having night?

No

The Earth spins on its own axis. What term refers to this?

Rotate/rotation

The Earth and the other planets travel in an orbit around the Sun. What term refers to this?

Revolve/revolution

How many hours does it take the Earth to rotate once?

24 hours

How many days does it take the Earth to revolve once around the Sun?

365.25 days

Do you think Mars would take more Earth days to revolve around the Sun than Earth does? Why?

Yes, it would take longer because it is further away and therefore has a longer/bigger pathway.

Bonus question: Why do you think we have a leap year every 4 years and not every 3 or 5 years?

This is because it is 365.25 days. As our calendar only has 365 days, four quarters make up an extra day every four years. This might be quite hard as it deals with fractions.