The Earth and the Sun

Why does the Earth move in a circle around the Sun?
Why must 365 days pass between my birthday and my next birthday?
What is the solar system?

Moving around the Sun

Four months ago, in the middle of the year, the days were short. The nights were cold and the Sun was low in the middle of the day. We are in the fourth term now. The days are longer and the Sun is much higher in the middle of the day. Soon we will have summer. Then winter will come again. The seasons repeat every year We say that the seasons repeat in a cycle.

A cycle is when things happen the same way, again and again. For example, do you remember learning about the water cycle in Term 2 in Matter and Materials? This is a cycle. Water evaporates from the oceans, lakes and rivers and becomes water vapour. Then the water vapour condenses and becomes liquid water again.

Scientists explain why the seasons change in a cycle. They find that the Earth is moving around the Sun. Earth needs a year to go around the Sun once and come back to the same place. As the Earth moves around the Sun, we experience different seasons.

When it is summer in South Africa…http://www.flickr.com/photos/foolstopzanet/3205760827/

…it is winter in England. http://www.flickr.com/photos/12684466@N06/4251142658/

The learners do not have to know about the tilt of the Earth's axis. The axis is tilted at 23 ½ degrees from vertical. As a result, South Africa gets more sunshine when the Sun shines mostly on the southern hemisphere and less sunshine when the Sun shines mostly on the northern hemisphere. More sunshine means we have summer and less sunshine means we have winter.

Name the four seasons and place them in the correct order starting with summer.

summer, autumn, winter, spring

Make a model of the Earth moving around the Sun

This is a demonstration, using two learners at a time. One learner must run with the ball to get it moving in a circle. You need plenty of room; at least a clear 10 metre diameter circle. The long string helps to give learners the correct idea that the Earth's orbit is at a very great radius from the Sun. Plan this to happen at the start or end of a period, because the learners take time to move outside.

MATERIALS:

strong string about 5 metres long
a ball in a plastic bag
four thick rubber bands

INSTRUCTIONS:

Join the bag to the string, with four rubber bands.
Someone must run in a circle with the ball in the bag to help you get it going.
Then swing the ball around as fast as you can, on the end of the string. The rest of the class must watch the plastic bag carefully to see whether the rubber bands stretch.
You see the learner swinging the ball around himself. The learner represents the Sun and the ball represents the Earth. If you look carefully at the rubber bands, you can see that the ball is pulling on the learner, and the learner is pulling on the ball.
Take turns to swing the ball; feel how hard you need to pull on the ball to keep it going around.

The ball orbits in a circle path around the learner.

QUESTIONS:

What do you feel as you swing the ball?

You will feel the string pulling on your hand.

If the ball in its bag could feel, what would it feel?

The ball will feel an equal force of the string pulling on it.

If the string breaks, in what direction will the ball carry on travelling? Point with your hand to show your answer.

Give learners time to think about this. The answer is that the ball will continue travelling in the direction it was going at the moment the string broke. Learners can test this answer by letting the string go.

Why can you not see a circle in the picture above? Does the ball really move in a circle when you swings it?

Skill: interpreting a diagram. Learners must understand that the ball is actually moving in a circle but from the side it looks like an ellipse. We must teach learners how to "read" diagrams.

The ball represents the Earth. You swings it quite fast, but how long does the Earth really need to go once around the Sun?

one year

The planet Earth orbits around the Sun in 365 ¼ days and we call that one year. As Earth moves to new positions around the Sun, we have four seasons: summer, autumn, winter and spring, and then summer comes again.

Now in Space, the Earth keeps on going around the Sun at more than 100 000 kilometres per hour. But there is no string pulling on the Earth, so what pulls on the Earth?

Gravity force pulls the Sun and the Earth towards each other.

There is no string in Space between the Earth and Sun! The Sun pulls on the Earth and the Earth pulls on the Sun with the forces of gravity. The pull is so strong that it works at a distance of 150 million kilometres! Like the string, gravity force keeps the Earth moving in its orbit around the Sun, year after year.

The Earth and other planets

Some of the bright things we see in the sky at night are not stars, they are planets. Venus is the easiest planet to find because it is big and bright. You can find it in the evening just after the Sun has set, or in the morning just before the Sun rises. African names for Venus are iKhwezi and Naledi ya masa.

We can see the planet Venus near the sunset.

The differences between stars and planets

Stars are balls of very hot gas and they make their own light. Planets do not make their own light; they reflect light from the Sun. Planets are a long way from Earth but stars are much further away.

The differences between stars and planets

INSTRUCTIONS:

Complete the following table.
Choose sentences from the box and write them in the table under the heading 'Planets'.

Choose the best sentence to write in the table.

Planets orbit around our Sun.
Planets are not as far away as stars.
We can see only 7 other planets in our solar system.
Planets do not make their own light; they reflect the light from the Sun.

Stars	Planets
Stars are hot balls of gas that shine brightly and give out light and heat.	Planets do not make their own light; they reflect the light from the Sun.
We can see thousands of millions of stars with a telescope.	We can see only 7 other planets in our solar system.
Stars are very, very far away from us.	Planets are not as far away as stars.
Stars do not orbit around our Sun.	Planets orbit around our Sun.

Eight planets move in orbits around the Sun

Mars is another planet you can find on some nights. It has an orange colour.

The names of the planets are:

Mercury,
Venus,
Earth,
Mars,
Jupiter,
Saturn,
Uranus, and
Neptune.

We used to call Pluto a planet, but not anymore. It has been decided that, strictly speaking, Pluto is not a planet. Now there are only 8 planets and Pluto is known as a 'Dwarf Planet'. The decision to reclassify Pluto was made after a long debate. Some of the facts which made scientists decide that Pluto is not a planet is that it is much smaller than any of the other planets. Unlike the other planets, Pluto also does not have a regular orbit around the Sun.

Here is a tip! To remember the names of the planets, say this rhyme: My Very Eager Mom Just Served Us Nachos. This is what each word stands for:

Rhyme	Planets
My	Mercury
Very	Venus
Eager	Earth
Mom	Mars
Just	Jupiter
Served	Saturn
Us	Uranus
Nachos	Neptune

The planets all move around the Sun in orbits. The path of the orbits are shown in the diagram. The Sun is the star at the centre of our solar system.

Drawing of the planets orbiting around the Sun. The planets are really much further away and much smaller than you see here.

The Sun and planets are called the solar system. A system is a set of parts that work together or push and pull on each other. The Sun and all the planets pull on each other as the planets move around the Sun.

The planets of the solar system

INSTRUCTIONS:

Look at diagram of the solar system again.
Answer the questions that follow.

QUESTIONS:

Why do the planets all keep on moving in orbits around the Sun?

The gravity force between the Sun and each planet keeps them moving in their orbits. The Sun is so big and heavy that it can cause a gravity force that pulls even the furthest planet, Neptune, into its orbit.

Which planet is the closest to the Sun?

Mercury

Is Venus or Earth closer to the Sun?

Venus

Write the names of the planets in order, beginning from the one that is closest to the Sun.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune

Which planet is the coldest, do you think?

Neptune

What is the reason that planet is the coldest?

It is the furthest from the Sun

The Sun and life

There are eight planets in the solar system. People often wonder if the other planets have creatures called aliens living on them. Perhaps you have seen movies about creatures from other planets. In this section, you will learn why Earth is the only planet we humans can live on. Part of the reason is that plants can grow on Earth, and we need plants to survive.

The following investigation leads on from what was done in the first term in Life and Living. Remind the learners that they have already investigated what plants need to grow. For this experiment, you can use a pot plant in the classroom, or you can choose a nearby tree to take the learners to.

What happens to a plant that grows without light?

AIM (What do you want to find out?):

PREDICTION (What do you think will happen?):

APPARATUS (What you will need):

agrowing bean plant in a pot
a small box with a lid that you can close.
a bottle of water for the plant.

METHOD:

The bean plant must be growing well. Put the plant in a place where it gets light and where you can watch it every day.
Make a cut in the lid of the box and put the box over one branch with leaves. The box must be dark inside.
Give the plant a tablespoon of water every day and keep the plant healthy.
After a week, open the box and look at the leaves that were growing inside.
Compare the leaves that grew in the dark with the leaves that grew in the light.

Put the small box over some leaves and make sure that light cannot reach those leaves

RESULTS (What you observed):

Draw two drawings of the plant. one drawing must be of the leaves that were covered in the box. the other drawing must be of the leaves that were exposed to the sunlight. Give your drawings a heading and labels.

CONCLUSION (What you learnt):

Write your conclusion from this investigation below.

How could you do this investigation better?

The light from the Sun helps plants on the Earth to grow. Look at the photo below here. All those leaves came from the same plant.

These leaves came from the same plant. The leaves on the top branch grew in light, but the leaves at the bottom branch grew without light.

What is the difference between the leaves on each branch? Write two sentences about the leaves. Begin like this "The leaves at the top of the picture are ..."

The leaves at the top of the picture are dark green and strong/healthy. The leaves at the bottom of the picture are pale green/light green and they look weak/sick. The learners are using the skills of observing and describing.

Why do the leaves look different, do you think? Write one reason why the leaves look different.

Here the learners must make a hypothesis. We do not know the answer but we can make a hypothesis that the pale green leaves grew in the dark. Another hypothesis could be: some insects were sucking juice from that branch so the leaves were weak. The second answer is not wrong and you should praise learners who think of more than one hypothesis. You are developing their skill in hypothesising. However, the first hypothesis is a better one than the second hypothesis.

Why do most plants stop growing in winter?

In winter and summer, the grass, trees and other plants around you look different.

INSTRUCTIONS:

Complete this table. The answers under the heading 'summer' are done for you.

Questions	Summer	Autumn	Winter	Spring
In which months of the year do we have these seasons?	Late in November, December, January, February	March, April, May	June, July August	September, October, early November
Are most of the days cold, cool, warm or hot?	Most days are hot.	Days are warm or cool.	Most days are cold.	Most days are warm.
How high is the Sun at the middle of the day?	Almost over our heads	Not high and not low	Low down in the sky	Not high and not low
How long is the night? Short, long or medium?	short	medium	long	medium
What happens to plants during this season?	Plants grow well.	Plants stop growing.	Many plants die or the leaves fall off.	Plants begin growing again.
Draw a picture to show the season.

QUESTIONS:

In winter, plants stop growing. Many plants lose their leaves or they die. Why does this happen, do you think?

Ask the children to make hypotheses about why plants die. Ask them to look at the table they completed. When the Sun is low the nights are long and the air is cold.

In spring, plants begin to grow again. Why does this happen, do you think?

The air becomes warmer because the Sun is in the sky for longer. The concept for teachers is that the Sun gives both light and heat to plants.

Do you remember learning in Term 1 about what a plant needs to grow? Write these down below.

light, heat or warmth, water, air

How is the Sun providing warmth and rain for South Africa?

INSTRUCTIONS:

Look at the following photo of Earth. You have seen it before in this book.
Answer the questions about this picture.

This is the planet Earth; this photo was taken from a spacecraft far away from Earth.

QUESTIONS:

Is it daytime or nighttime in South Africa?

Day-time; we can see that all of Africa is in sunshine. Teach learners that they can interpret a photo.

Is the weather cloudy or sunny in South Africa?

In the photo, most of South Africa is under cloud.

Where does the rain come from, to give water to the plants, animals and people? Write two or three sentences.

Water evaporates from the sea; the water vapour condenses into clouds; and rain falls from the clouds. This links to Matter and Materials in Term 2 when the Water Cycle was done.

The Thunderbolt Kids had just learnt about Earth and what the Sun provides Earth with. Sophie was sitting after class wondering if there are any other planets like Earth where people could also live.

Do you think people could live on other planets? Give a reason for your answer.

Earth is the only planet in our solar system with the right temperature for us to live on; it is not too hot and not too cold. Earth is at exactly the right distance from the Sun to be the perfect temperature to support life.

Some of the planets are too close to the Sun and they are too hot for anything to live on. Two very hot planets are:

Mercury and Venus

Most of the planets are so far from the Sun that they are very cold. They are so cold that people could not live on them. Five examples of cold planets are:

Mars, Jupiter, Saturn, Uranus, Neptune

The Earth moves round the Sun.
The path of the Earth around the Sun is called the orbit of the Earth.
We have defined the time it takes the Earth to complete one orbit as a year.
Some of the bright things we see at night in the sky are planets, and not stars.
Earth is one of the eight planets in our solar system.
Earth is the only planet in our solar system that we could live on.

What is at the centre of our solar system?

The Sun

Name the 8 planets in our solar system.

Mercury, Venus, Earth, Mars, Saturn, Jupiter, Neptune and Uranus.

What do plants get from the Sun that they need to grow?

light and heat

What is the shape of the Earth's path around the Sun?

a circle, or very nearly a circle - an ellipse

What is the name of the Earth's path around the Sun?

its orbit

Why does the Earth move in a circle around the Sun?

The force of gravity between the Sun and the Earth pulls the Earth towards the Sun; otherwise the Earth would go in a straight line into Space, and move far away from the Sun.