Contents

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

2 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, 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.

Globe of the Earth

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.

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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.

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

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. 2. 3.

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