GRADE 7 NATURAL SCIENCES TERM 4

PLANET EARTH AND BEYOND

Contents

UNIT 1: THE SUN ....................................................... 3

SOLAR ENERGY AND THE EARTH’S AXIS ................................................................... 3

THE EARTH’S SEASONS ............................................................................................... 4

DIRECT AND OBLIQUE RAYS OF SUNLIGHT ............................................................... 5

UNIT 2: SOLSTICE AND EQUINOX .......................... 6

ACTIVITY 1 ...................................................................................................................... 6

FACTS: FOR YOUR INFORMATION ............................................................................... 7

UNIT 3: GRAVITY ....................................................... 8

FACTORS THAT AFFECT AN OBJECT’S GRAVITY ...................................................... 8

LET’S REVISE: PLANETS ............................................................................................... 8

PHASES OF THE MOON ................................................................................................ 9

ACTIVITY 2 ...................................................................................................................... 9

ACTIVITY 3 .................................................................................................................... 10

THE EFFECT OF MASS OR DISTANCE ON GRAVITY ................................................ 10

UNIT 4: THE TIDES .................................................. 11

HIGH AND LOW TIDE ................................................................................................... 12

SPRING TIDES .............................................................................................................. 13

ACTIVITY 4: REVISION ................................................................................................. 13

SHORELINE ECOSYSTEMS ......................................................................................... 13

ACTIVITY 5: REVISION ................................................................................................. 14

HISTORICAL DEVELOPMENT OF ASTRONOMY ........................................................ 15

MODERN DEVELOPMENTS ......................................................................................... 15

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ACTIVITY 6: REVISION ................................................................................................. 16

FAMOUS ASTRONOMERS IN HISTORY ..................................................................... 16

CAREER FOCUS- INFORMATION................................................................................ 16

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UNIT 1: THE SUN

The Sun of the almost billion stars in the Milky Way Galaxy, is by far the most important to

life on Earth. It is the main source of energy on Earth and it sustains life on Earth. Different

parts of the Earth receive different amounts of energy from the sun, because the earth

rotates on its own axis, which is slightly tilted. It is this tilt that leads to the four seasons in

the southern and northern hemisphere.

SOLAR ENERGY AND THE EARTH’S AXIS

The Earth rotates on its own axis once per day. The Earth’s axis is an imaginary line that

starts at the North Pole, goes through the centre of the Earth, and ends at the South Pole.

The Earth’s axis is not vertical. It is tilted at an angle of 23, 5º from the vertical and it

maintains this tilt as it moves around the sun. The side of the Earth that is facing the sun

experiences daytime and the side of the Earth facing away from the sun experiences night

time. As the earth turns on its own axis, the light falls on different parts of the world.

The earth is further divided using imaginary lines. The equator runs horizontally across the

middle of the Earth, at 90 º to the Earth’s axis. The half of the Earth above the equator is

known as the northern hemisphere and the half of the Earth below the equator is the

southern hemisphere. The Earth is further divided by the imaginary lines known as the

Tropic of Cancer and the Tropic of Capricorn, which run parallel to the equator. The Tropic

of Cancer lies north and the Tropic of Capricorn lies south.

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An easy way of remembering which hemisphere of the earth is at the top and which is at

the bottom is to remember the following:

a) People get corns on their feet-the bottom half of their body. The tropic of Capricorn is

the bottom hemisphere of the planet.

b) Many people get cancer of the lungs-the top half of the body. The tropic of Cancer is the

top hemisphere of the planet.

THE EARTH’S SEASONS

The Earth revolves around the sun in a movement that is known as an orbit. The earth

takes one year to do a complete orbit around the sun. Different parts of the earth will

receive different intensities of solar energy depending on where the Earth is in its orbit of

the Sun.

This is the reason for the different seasons of summer, autumn, winter and spring. As the

Earth revolves around the sun, it maintains its tilt on its own axis. When the southern

hemisphere is tilted towards the sun, it will receive the most intense solar energy (at the

Tropic of Capricorn) and so it will be summer in the southern hemisphere. At the same

time, the northern hemisphere will be tilted away from the sun and so will receive less

intense solar energy, which means that it will be winter in the northern hemisphere.

Globe of the Earth

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The Earth will continue orbiting the sun and six months later (in June) it will be on the

opposite side of the sun. This means that the northern hemisphere will be tilted towards

the sun, so the Tropic of Cancer will receive the most intense sun and it will be summer in

the northern hemisphere.

In March and September, neither hemisphere will be tilted towards or away from the sun

and each hemisphere will receive an equal intensity of solar energy. This accounts for the

seasons of spring and autumn.

DIRECT AND OBLIQUE RAYS OF SUNLIGHT

In December, the southern hemisphere will receive the sun’s direct rays at the Tropic of

Capricorn and it will be summer in the southern hemisphere. The solar energy will be

focussed over a small area, so it will be very intense. At the same time, in the northern

hemisphere, the sun’s rays will fall at an extreme angle on the Earth. These rays are called

oblique rays. The oblique rays have to spread over a large area and the solar energy will

be less intense.

In June, the Earth will be on the opposite side of its orbit of the sun, which means that the

northern hemisphere will receive the sun’s direct rays at the Tropic of Cancer. The

southern hemisphere will receive the oblique rays. It will be summer in the northern

hemisphere and winter in the southern hemisphere.

Watch the following video clips of the Earth rotating around the Sun.

1. http://www.youtube.com/watch?v=r2lp146ka5A

2. http://youtu.be/op6vsLNf3WY

3. http://youtu.be/WLRA87TKXLM

Interesting Facts about the

Tropic of Capricorn

The Tropic of Capricorn runs through the Namib

Desert in Namibia, the Kgalagadi Desert in

Botswana, the Limpopo Province in South Africa and

the Gaza and Inhambane Provinces in Mozambique

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UNIT 2: SOLSTICE AND EQUINOX

In the southern hemisphere, we have our summer solstice on 21 or 22 December. This is

also known as the longest day because we will experience the most time in the light of the

sun on this day. On 20 or 21 June, it will be the winter solstice in the southern hemisphere.

This is also known as the shortest day, because we will have the shortest amount of

exposure to the sun’s rays on this day. On 20 March and 22 or 23 September, the world

experiences what is known as equinox, when the length of the day and the night are

exactly equal.

ACTIVITY 1

Answer the questions, after looking at a demonstration of the Earth’s orbit around the sun.

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Questions:

1. What is the Earth’s axis? (1)

2. Explain how earth experiences day and night. (2)

3. Describe the Earth’s orbit around the sun. (3)

4. Explain why the southern hemisphere experiences summer in December and the

northern hemisphere experiences winter in December. (1)

5. Explain why the days are longer in summer. (1)

6. What is the equator? (1)

7. Explain how the sun sustains all life on earth. (1)

Total: 10

FACTS: FOR YOUR INFORMATION

Fact 1: Humans are omnivores because we eat food from plants and animals. Omni is a

Latin word meaning “all” or “every”. Some people choose to not eat meat from animals or

animal products for cultural, religious, ethical or personal health reasons. We call these

people Vegetarians.

Fact 2: The sun radiates heat and light energy which takes about eight minutes to reach

Earth. Earth is the third planet from the sun, which is the perfect distance to sustain plant

and animal and human life. Any closer to the sun and we would burn up. Any further away

and we would freeze.

As the Earth is revolving around the sun, the moon revolves

around the Earth. We say that the Earth is in the sun’s orbit

and the moon is in the Earth’s orbit. The diameter of the

moon is roughly one quarter of the diameter of the Earth.

The moon lies 384 400 km away from the earth. If you use

the diameter of the Earth as a measure, you’d need the

total diameter of about 30 Earths to get to the moon.

Figure 1 - demonstrates the difference in size between the earth and the moon, but it does

not represent the distance between the Earth and the moon.

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The relative sizes of the Earth and the moon.

Watch this video for a good explanation of the relative sizes of the Earth, moon and sun:

https://youtu.be/FjCKwkJfg6Y.

UNIT 3: GRAVITY

Gravity is the tendency of objects to attract or pull each other. All objects have gravity-

even your own body. However, we won’t notice our own gravitational pull because our

mass¹ is too small. For gravity, to be noticeable, the object that is attracting or pulling other

objects needs to be massive.² We are pulled towards the ground because of the Earth’s

gravitational pull, which pulls us and everything else on earth towards its centre. This is

one of the reasons that we can live on a planet that is shaped like a ball.

Vocabulary

Mass: the amount of matter in an object

Massive: having a great mass.

FACTORS THAT AFFECT AN OBJECT’S GRAVITY

The strength of an object’s gravitational pull depends on the mass of the object and the

distance between the object and the object it is pulling.

Mass is a measure of the amount of matter in an object and its measured in kilograms.

The mass of an object stays the same whether the object is on earth, on the moon, or

anywhere else in space.

LET’S REVISE: PLANETS

Listen to the planet songs on the following you tube video links:

https:// www.youtube.com/watch?+mQrLgH97v94

https://www.youtube.com/watch?v=BZ-qLUIj_AO

https://www.youtube.com/watch?v=7t099KIWVVS

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https:www.youtube.com/watch?v=noiwY7kQ5NQ

The names of the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and

Neptune. The dwarf planet Pluto was thought to be a planet, but in 2009, Pluto’s status

was downgraded to a dwarf planet, because it did not meet all the criteria that were

required for it to be classified as a planet. Although gravity on the moon is less than that on

Earth, the moon still exerts a gravitational pull, pulling objects towards itself and pulling the

waters of the earth.

PHASES OF THE MOON

Moons are small bodies that travel around a planet. The diameter of the moon is roughly

one quarter of the diameter of the Earth. Altogether there are about 167 moons travelling

around the planets in our Solar system. Earth’s moon is a small, rocky body that does not

make its own light. It shines by sunlight that is reflecting off its surface. The moon revolves

around the Earth in an anti-clockwise direction. At the same time, Earth is in orbit round

the Sun. It takes the moon 29,5 days, almost a month, to orbit Earth once.

Look at the Phases of the moon by using the following website and links.

https:// www.youtube.com/watch?v=HklrWpsnuQ

https://www.youtube.com/watch?v=C4zpRmUdu7I

https://www.youtube/watch?v=AQRNzepe4wl

ACTIVITY 2

After watching the videos and singing the songs from the links, paste in the worksheet of

the phases of the moon and label them to show the differences.

1 New moon

2 Waxing crescent

3 First quarter

4 Waxing Gibbous

5 Full moon

6 Waning Gibbous

7 Last quarter

8 Waning crescent

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ACTIVITY 3

A. Complete the sentences below in a way that makes the meaning of the

underlined word clear.

1. A moon is a … (1)

2. Name one phase of the Moon and explain it (2)

3. Gravity is… Explain. (2)

4. Mass means… (2)

5. An astronaut is… (2)

6. A geologist is… (2)

7. Tides are… (2)

8. At high tide, the water level is… (1)

9. Spring tide is the tide when high tide is … and low tide is … (2)

B. Choose the correct option from each statement, and write out the statement

correctly.

1. Objects of large mass exert more / less gravitational pull on other objects of

small mass. (1)

2. Earth’s gravitational pull / rotation keep the moon revolving around Earth. (1)

3. There is a high tide at the same place about every twelve / six hours. (1)

4. Spring tides are experienced when the Sun, Earth and Moon are aligned with /

at right angles to each other. (1)

Total: 20

THE EFFECT OF MASS OR DISTANCE ON GRAVITY

An object or body with a great mass will exert a stronger gravitational pull than objects with

smaller masses. The distance between two objects will also affect their gravitational pull.

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The gravitational pull of the Earth to the moon, keeps the moon in the Earth’s orbit. Gravity

in space is weaker than on Earth because the astronauts are further away from the Earth

and so its gravitational pull is less. The gravitational pull of the moon itself is less than the

gravitational pull of the earth, which is why if you watch a video clip of astronauts walking

on the moon, they will appear to be bouncing.

Watch a short clip of the moon landing here: https://www.youtu.be/RMINSD7MmT4.

UNIT 4: THE TIDES

Who has never been at the ocean or to the sea? If you have not visited a coastal area you

might not have seen the sea at all. Here are some images of the ocean and waves

breaking on a beach.

Tides are the predictable, repeated rise and fall of the sea and ocean levels. You can see

the effect of the tides in the waves on the sea. During high tide, the sea level rises and the

waves bring the seawater further up the beach, or raise the sea level in the harbour.

During low tide, the water level drops and you will be able to see much of the material that

was previously underwater.

Tides are predictable because the rise and fall of the tides happen every day and we can

predict how high or low the tides will be throughout the month. Tides are caused by the

gravitational pull of the moon on the Earth. The sun also has a role to play, but because it

is further away from the Earth than the moon, its effect is less noticeable.

The moon pulls on the water in the seas and oceans of the Earth as it orbits the earth. It

takes about a month for the moon to do a full orbit of the Earth. As the moon pulls on the

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waters of the Earth, it creates a tidal bulge which is responsible for high tide. Why does the

tidal bulge form on both sides of the Earth?

This tidal bulge forms on both sides of the Earth as the Earth pulls back, it pushes its

waters that are on the opposite side of the moon, so that the tidal bulges on both sides of

the earth are equal.

HIGH AND LOW TIDE

The moon takes a month to orbit the Earth, but most coastal areas will experience two

high tides and low tides every day. How does this happen? Remember that the Earth

rotates on its axis once every 24 hours. This means that a coastal town or city will “pass

through” the two tidal bulges on earth in one day.

Take the coastal city of Durban in Kwa Zulu- Natal for example.

Imagine it’s about midday (12:00) and low tide on the beach. As the Earth rotates on its

axis, Durban heads towards the tidal bulge of the Earth. Just after six o’ clock that evening,

it will be high tide.

The Earth continues rotating on its axis and by half past twelve at night, Durban will

experience its second low tide for the day. The Earth then rotates towards the other tidal

bulge and by about 06:40 the next morning, Durban will experience its second high tide for

the day. The image below shows the low tide and high tide water line.

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SPRING TIDES

About once a month, the Earth will experience a spring tide. A spring tide means that the

high tide is very high and the low tide is very low.

People who live near the coast, might notice that the waves get higher and the sea gets

rougher during a spring tide. The reason for a Spring tide, is that the moon’s orbit aligns

(lines up) with the sun. This happens during full moon and new moon.

ACTIVITY 4: REVISION

1. Explain the orbit patterns of the moon, earth and the sun in relation to each other. (2)

2. Explain why the sun can hold all the bodies in the solar system in its orbit. (2)

3. What is gravity? (1)

4. Explain why astronauts appear to be bouncing on the moon’s surface. (2)

5. What is spring tide and how does it happen? (2)

6. Why does a tidal bulge form on both sides of the earth? (3)

7. Give two examples of animals in the low tide zone that have developed to suit their

environment. (2)

Total: 14

SHORELINE ECOSYSTEMS

The area on the shoreline between low tide and high tide levels is home to a number of

unique animals and plants that have adapted to living in the rapidly changing conditions

caused by the tides in the shoreline coastal system.

The shoreline can be divided into a high tide zone and a low tide zone. The high tide zone

is exposed for most of the day and gets flooded by seawater at high tide. The plants and

animals in the high tide zone need to be adapted to live in water and withstand the beating

waves and rough seas when the tide comes in. They also have to withstand high

temperatures so that they can survive on the rocks in the sun when the tide goes out.

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The high tide zone is home to plants such as green algae, which grows well because they

are exposed to more sunlight in this zone. Sea animals such as sea anemones, crabs and

starfish live in this zone.

The plants and animals rely on the tide to bring oxygen, food and other nutrients into their

tide pools at every high tide. The low tide zone is usually underwater, but will be exposed

at low tide. The water provides the plants and animals in the zone with some protection

against predators, because of the wave action of the tides. The water also protects this

ecosystem from excessive heat from the sun. The plants and animals in the low tide zone

have developed adaptations like suckers to cling to the rocks so that they are not washed

out to sea. Many animals also have shells or hard body coverings to prevent drying out

when they are exposed to sunlight.

ACTIVITY 5: REVISION

1. Draw a diagram showing the earth, moon and sun when it is spring tide. Include the

tidal bulges and show the gravitational pull of the moon and the sun. (4)

2. Explain why a beach like Coffee bay in the Eastern Cape or Amanzimtoti in Kwa Zulu

Natal will experience two tides per day. (2)

3. How are plants in the high tide zone adapted to their environment? (2)

4. Explain how the high tide and low tides affect the ecosystem looking at what damage

the high tide zone can cause. (2)

5. The high tide zone is home to plants like….. (2)

6. What do the animals in the tidal pools rely on? (1)

Total: 13

Starfish Sea anemone (say an eh moan ee)

Crab

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HISTORICAL DEVELOPMENT OF ASTRONOMY

From our position on Earth, it seems that the sun, moon and stars move in predictable

patterns. Early communities used these patterns to measure the passing of time and to

record time in the form of calendars. This helped them to set times for planting and

harvesting their crops, and times for holy days and seasons.

The location of the stars in the sky also provided people with a reference point for finding

direction. The various indigenous groups like the Bushmen or San used to tell stories to

explain patterns and the phases of the moon. They used to hunt at full moon and at night,

because there was enough light for them to see the animals they were hunting. They kept

track of the phases of the moon to help them plan their hunting.

Ancient people noticed that the celestial objects moved in ways that could be predicted.

Our early ancestors identified these objects with gods and spirits. They also related these

objects and their movements to natural phenomena, such as rain, drought, the seasons

and the tides.

One of these earliest observations of predictable patterns form Earth is the ecliptic. This is

the path across the sky that the Sun appears to travel over the course of the year. For the

early observers it appeared that the Sun moved on this elliptical belt.

The zodiac is a circle of twelve constellations that can be seen only at certain times of the

year. They always seem to follow the path taken by the Sun and the Moon on the ecliptic.

The zodiac was used as a star map or calendar to measure direction and time.

MODERN DEVELOPMENTS

Everything that we have learnt about the Earth and its relationship to the sun and the

moon had to be discovered by dedicated scientists who specialised in the field of

astronomy. Copernicus, a Polish monk, was the first astronomer to suggest that the sun

was at the centre of the solar system.

Galileo Galilei is known for the improvements that he made in the design of the telescope.

These improvements meant that he could observe the moon and the planets with greater

accuracy.

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In 1610, Galileo observed that the planet Jupiter had at least four moons that orbited

around this massive planet. Galileo was also able to observe the Earth’s moon, using his

telescope. He discovered that the moon was not a perfectly round ball, but that it had

mountains and valleys on its surface.

Answer the questions in your workbooks.

1. What did the location of the stars help earlier people with? (1)

2. Why did the Bushmen or San tell stories to their children? (2)

3. What did our earlier ancestors identify objects with? (2)

4. What is the ecliptic? (2)

5. What is the zodiac? (2)

6. Explain what the Meerkat is? Explain. (2)

Total: 11

FAMOUS ASTRONOMERS IN HISTORY

1. Johannes Kepler, a German astronomer, who used mathematics and geometry in 1906

to prove that the planets orbited the sun in elliptical path, rather than in a perfect circle.

2. Isaac Newton, who discovered gravity in 1687, he described the force of gravity which

means that all objects attract or pull to each other.

3. William Herschel and his sister Caroline Herschel

discovered the planet Uranus and built very advanced

telescopes for the time. They also put together a map of

the universe, which was called the model of the night

sky, including 90 000 stars and it showed that the solar

system was part of a disc-shaped galaxy, called the

Milky Way.

CAREER FOCUS- INFORMATION

There are many scientists and engineers already working on the MeerKat project and

other projects in South Africa that are advancing the field of astronomy. If you were to

ACTIVITY 6: REVISION

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consider a career in astronomy, either as a researcher, an engineer or a computer

programmer, you could aim to work on one of the exciting projects related to the SKA and

maybe even make a new astronomical discovery someday. You will need to get good

marks in Mathematics and Physical Science in Matric so that you can do a Bachelor of

Science and Engineering degree, with specialisation in astronomy.

The MeerKat telescope used for projects