GRADE 7 NATURAL SCIENCES TERM 4 PLANET EARTH AND BEYOND - mr adams

GRADE 7 NATURAL SCIENCES

TERM 4

PLANET EARTH AND BEYOND

TOPIC 1

UNIT 1

THE SUN

TOPIC 2

EARTH¡¯S AXIS

UNIT 2

EARTH¡¯S SEASONS

SOLSTICE AND

DIRECT AND OBLIQUE RAYS

EQUINOX

OF THE SUN

TOPIC 3

UNIT 3

TOPIC 4

GRAVITY

UNIT 4

PLANETS

TIDES

SHORELINE ECOSYSTEMS

HISTORICAL DEVELOPMENT

OF ASTRONOMY

MODERN DEVELOPMENTS

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Contents

UNIT 1 ................................................................................................................................. 3

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 ................................................................................................................................. 6

SOLSTICE AND EQUINOX................................................................................................. 6

Activity 1: ........................................................................................................................ 7

UNIT 3 ................................................................................................................................. 9

GRAVITY ............................................................................................................................. 9

FACTORS THAT AFFECT AN OBJECT¡¯S GRAVITY .................................................... 9

LET¡¯S REVISE: PLANETS .............................................................................................. 9

PHASES OF THE MOON .............................................................................................. 10

Activity 2: ...................................................................................................................... 10

Activity 3: ...................................................................................................................... 11

THE EFFECT OF MASS OR DISTANCE ON GRAVITY ............................................... 11

UNIT 4 ............................................................................................................................... 12

TIDES ................................................................................................................................ 12

HIGH AND LOW TIDE ................................................................................................... 13

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

Activity 4: Revision ...................................................................................................... 13

SHORELINE ECOSYSTEMS ........................................................................................ 14

Activity 5: Revision ...................................................................................................... 15

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

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

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

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

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, 5? 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 ? 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.

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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.

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.

A Globe of the Earth

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.

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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