# Planet Earth

• Why does the Sun appear to move across the sky?
• How long does it take the Earth to move around the Sun?
• How long does it take Earth to spin around once on its axis?

How to introduce this topic

Remind them of the lessons in the last term of Gr. 4, when they learnt about the Earth, Sun, Moon and planets. Use Figure 1 to start them thinking about what is on the surface of the Earth and under the surface of the Earth.

Start "Activity: Begin to make soil" (making compost), in the first week of the term, because the compost needs time to break down You need compost out of the column by week 3 or week 4.

You will need a globe in the classroom and atlases that show South Africa in large enough scale to find names of towns. The cognitive development issue is that children need the ability to move mentally from the physical place where they are to another place on the planet, and that means being able to handle models of those places in the form of globes and maps. Also read through all the "What you will need" sections today, because some of the things take time to collect. The learners can help with collecting many of the items you will need.

## The Earth moves

In Gr. 4 we learnt that the Earth moves in two different ways. The Earth orbits the Sun and the Earth also spins on its own axis. Let's revise these concepts again.

### The Earth spins on its axis, and so we have day and night

In Gr. 4 you learnt that the Earth spins on its axis. But what does this mean? Imagine an orange with a pencil stuck through it. Look at the following picture. If you hold the pencil in your fingers, you can spin the orange around. The pencil is the axis of the orange.

The Earth does not really have a pencil through it, but it does spin around. We can imagine a big pencil through the middle of the Earth.

We are on the Earth. Let us imagine we are at the point where you see the red "X" on the orange:

Use an orange and do this as a demonstration. You can set up a mirror or a big sheet of white paper, outside, to shine the Sun into the classroom.

• The Sun shines on the Earth and so we, at X, see the Sun. We call that daytime.
• But the Earth never stops spinning. So we, at X, move around into the shaded part of the Earth. Then we cannot see the Sun any longer and it is nighttime for us at X.
• The Earth spins right around in 24 hours, so it will take 24 hours for us to come around to the same position you see in the picture.
• We call the 24 hours a day. When we say "a day" we really mean a day and night; together they last 24 hours.

If we are at position X, we move past the Sun. But to us, it looks as if the Sun is moving. The Sun seems to move from the East to the West. The Sun seems to come up (rise) in the East, move across the sky during the day and go down (set) in the West. But the Sun does not actually move.

# Who is having daytime?

What you need

• the classroom globe of the Earth
• the photographs of the Earth below

INSTRUCTIONS:

1. There are two images of the Earth.
2. Look carefully at these pictures and use them and the globe to answer the questions.

QUESTIONS:

Picture A

You are in South Africa. Find South Africa on the globe.

Find South Africa in Picture A.

Was it daytime in South Africa, when the spacecraft took the photo?

Yes, the Sun is shining on South Africa.

Was is daytime in Saudi Arabia? Hint: Use your globe to find Saudi Arabia and then find it in Picture A to see if it is day or night.

Yes, there is sunshine on Saudi Arabia.

Was it day time in Argentina when this photograph was taken? Use the globe to help you locate Argentina. Explain your answer.

No, it was night time in Argentina; in the photo, Argentina is on the darkened side of the Earth, and there are lights showing.

Now use the globe to locate Brazil. In Picture A, the tip of Brazil is in the sunlight. Is it morning or afternoon in Brazil? Why?

It is morning, because Argentina is moving towards the east and into the part of the Earth that is in the light.

Picture B

Look at Picture B. What part of the Earth is this picture showing?

Europe and the top of Africa is shown.

Can you see the lights on in Italy? When it is dark in Italy, is it still light in Spain?

Yes

Is it late afternoon or morning in Spain in Picture B?

It is late afternoon.

Now that we have looked at some photographs of the Earth as it changes from day into night as the Earth rotates, let's make a model of Earth using our heads to explain this.

# Your head can be a model of the Earth

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. Let's say that your nose represents Africa. Have a look at the following picture.
1. 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.
4. When you have turned your back to the Sun, you cannot see the bright light any more. That is like nighttime in Africa.
5. Turn further to your left and you will see the Sun "rise" over your left cheek. That is like sunrise in Africa.

Teacher note: They must move their whole bodies round, not just their heads!

QUESTIONS:

Which of your cheeks (left or right) represents west? That is where the Sun appears to go down.

The learner's right cheek; that is where she saw the Sun disappear.

Which cheek represents east, where the Sun comes up?

Their left cheeks are like the east. You can chalk an "E" on their left cheeks to remind them.

We see the Sun rise and move across the sky every day, but the Sun does not really move. It only seems that way to us. Actually, Earth is spinning around and around, and we are moving around with the Earth. The Earth takes 24 hours to complete one full rotation.

The Earth moves in an orbit around the Sun

The Earth moves around the Sun. While the Earth orbits the Sun, it is also spinning on its own axis. It spins round 365 times while it completes one orbit of the Sun. That means 365 days pass and we call that a year.

The Earth is a planet. There are 7 other planets also moving around the Sun. You can see one of the other planets on most evenings, or early in the morning. This planet is called Venus or iKhwezi or Naledi ya Masa. It is not a star.

Venus also moves around the Sun but its orbit is a smaller circle than Earth's orbit. Venus takes 225 Earth days to go once around the Sun.

The days referred to here are Earth days; Venus has its own day-length but it's very much longer than an Earth day.

Mars is another planet you can see on some nights. Mars appears as a small, orange dot in the sky. Mars takes 687 Earth days to go once around the Sun.

You can also see Mercury, Jupiter and Saturn in the sky, but they are harder to see than Venus and Mars. All the planets seem to move along the same path that the Sun and Moon seem to move.

Explain to learners using the picture above that all the planets lie on the same plane - as if they were all lying on a plate. The "plane" does not mean an aeroplane; it means a flat surface like the top of your table or a big flat plate. Also, make sure they understand the next concept of this diagram not being drawn to scale - the orbits of the outer 4 planets are much larger than shown here. But, this is difficult to represent using an image in a book as if it were to be drawn to scale, the outer rings would not fit on the page.

The orbits of the outer planets are actually much bigger than what is shown in this image. But, if we tried to draw the orbits to scale, they definitely would not fit on this page!

In the image of Sophie, her model of the solar system should have all planets on the same plane to be correct. Perhaps ask your learners what Sophie should do to make the model correct if they compare it to the previous diagram of the solar system? She can rotate the rings so that they all lie in the same plane.

Let's make a scale model of the solar system using our bodies to understand what it means to orbit the Sun!

# Ascale model of the solar system

This activity is useful to help learners understand the structure and scale of the solar system, especially how far apart the planets are from the Sun. The activity requires a large open space, such as a school field. The model only uses 9 learners at once, so swap the learners so that each one gets a chance to be one of the celestial bodies. The teacher can be the Sun in the middle and the learners can be divided into 8 groups, each group assigned to a planet. The teacher should note that as each learner/planet revolves around the Sun, he/she also rotates! This is even trickier to get right, but let the learners attempt to do so.

MATERIALS:

• 100 m heavy string
• 9 pieces of heavy cardboard
• scissors
• permanent marker

INSTRUCTIONS:

1. Learners are divided up into 8 groups and each group is assigned a planet.
2. Each group must cut a piece of string to represent the distance of their planet from the Sun, using the lengths indicated in the table below. The actual distance of the planets from the Sun is given in millions of kilometres and the length of the string is in metres (m). 1 million kilometres is known as 1 gigametre (Gm). 1 kilometre is 1000 metres. Therefore, 1 gigametre is 1 000 000 000 metres.
 Planet Actual distance from Sun (millions of km) Length of string (m) Mercury 58 0.4 Venus 108 0.7 Earth 150 1.0 Mars 228 1.5 Jupiter 779 5.2 Saturn 1434 9.6 Uranus 2873 19.2 Neptune 4495 30.0
1. Each group must cut a circle out of the cardboard and write the name of their planet and the actual distance from the Sun on it.
2. Make a hole at one edge of the cardboard and tie the length of string to it.
3. Now it is time to go outside to a big open space, like the school field!
4. Your teacher will be the Sun in the centre. She does not move because the Sun does not move.
5. One member from each group must hand the 'Sun' the end of their length of string and then stretch out their length of string.
6. Do this one at a time starting from Mercury and going out to Neptune. you do not all need to be in a straight line but can be in different positions around the 'Sun'.
7. Place the strings on the ground, all stretched out in different directions.
8. Walk around so that you can all see the scale model of the solar system.

Explain to learners that on this scale, the nearest star (other than the Sun), would still be $$\text{2748}$$ km away!

1. Now comes the tricky part - making the planets orbit the Sun.
2. Select one learner from each group to be the planet.
3. He/she must pick up the planet and walk in a circle around your teacher, all going in the same direction. Try and walk at the same speed.

Hold all the ends of the strings in one hand above your head so that you are like a maypole and the learners will revolve around you.

1. Swap with other learners in your group so that you each have a turn to be a planet orbiting the Sun.

Make sure to point out to learners about the length of time it takes each learner to orbit the Sun. When Neptune has completed one revolution, Mercury will have completed many more as it is closer to the Sun. Point out the big gap between Mars and Jupiter - the space between the inner and outer planets. Mention to learners that there is actually a ring of asteroids (giant rocks) that also orbit the Sun in this space.

• The Earth spins on its axis. This is the reason we have day and night.
• The Earth also moves through Space, around the Sun.
• The Earth's path through Space around the Sun is called its orbit.

How many hours are there in a day?

24 hours

How many hours pass from sunrise until the next sunrise?

24 hours

How many days pass between your 10th birthday and your 11th birthday?

365 days

Teacher note: The answer is actually 365 ¼ days. When a day is defined in terms of the spinning of the Earth learners will be able to appreciate the quarter of a day as well, and thus an 'extra' day during leap year. '365 days' makes things manageable only for short term.

How many times must the Earth spin around between your birthdays?

365 times

Which planets have smaller orbits than Earth?

Mercury and Venus.

Write out the whole paragraph and complete it using some of the words/phrases in the word box. You do not need all the words/phrases.
 Word box: the orbit of Mars the orbit of the Earth 687 Earth days 365 days Sun Earth

If I lived on Mars, I would have to wait much longer for my birthday. The reason is that _____ is much bigger than _____, and Mars takes _____ to go around the _____ once.

If I lived on Mars, I would have to wait much longer for my birthday. The reason is that the orbit of Mars is much bigger than the orbit of the Earth, and Mars takes 687 days to go around the Sun once.