GRADE 7 NATURAL SCIENCES TERM 4 PLANET EARTH AND BEYOND

Transcription

1 GRADE 7 NATURAL SCIENCES TERM 4 PLANET EARTH AND BEYOND TOPIC 1 UNIT 1 THE SUN EARTH S AXIS EARTH S SEASONS DIRECT AND OBLIQUE RAYS OF THE SUN TOPIC 2 UNIT 2 SOLSTICE AND EQUINOX TOPIC 3 UNIT 3 GRAVITY PLANETS TOPIC 4 UNIT 4 TIDES SHORELINE ECOSYSTEMS HISTORICAL DEVELOPMENT OF ASTRONOMY MODERN DEVELOPMENTS

2 2 Contents UNIT THE SUN... 3 SOLAR ENERGY AND THE EARTH S AXIS... 3 THE EARTH S SEASONS... 4 DIRECT AND OBLIQUE RAYS OF SUNLIGHT... 5 UNIT SOLSTICE AND EQUINOX... 6 Activity 1:... 7 UNIT GRAVITY... 9 FACTORS THAT AFFECT AN OBJECT S GRAVITY... 9 LET S REVISE: PLANETS... 9 PHASES OF THE MOON Activity 2: Activity 3: THE EFFECT OF MASS OR DISTANCE ON GRAVITY UNIT TIDES HIGH AND LOW TIDE SPRING TIDES Activity 4: Revision SHORELINE ECOSYSTEMS Activity 5: Revision HISTORICAL DEVELOPMENT OF ASTRONOMY MODERN DEVELOPMENTS... 15

3 FAMOUS ASTRONOMERS IN HISTORY: CAREER FOCUS- INFORMATION 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.

4 4 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.

5 5 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

6 6 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 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.

7 7 Activity 1: Answer the questions, after looking at a demonstration of the Earth s orbit around the sun. 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

8 8 FYI - (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 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. 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:

9 9 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. Keywords: 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: The names of the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

10 10 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. The diameter of the moon is roughly one quarter of the diameter of the Earth. Moons are small bodies that travel around a planet. Altogether there is about 167 moons in our solar system. Earth s moon is a small rocky body that doesn t 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. PHASES OF THE MOON Look at the Phases of the moon by using the following website and links Activity 2: After watching the videos and singing the songs from the links, draw and label the phases of the moon in your workbook, to show the differences. New moon Waxing crescent First quarter Waxing Gibbous Full moon Waning Gibbous Last quarter Waning crescent There are about 29 days between the time any phase is seen and when it appears again. The moon revolves around the earth once in a month.

11 11 Activity 3: Use the given worksheet to answer questions on Solar energy and the earth s seasons. (15 marks) 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. 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:

12 12 UNIT 4 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 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.

13 13 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. 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)

14 14 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 marks 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. 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. Starfish Crab Sea anemone (say an eh moan ee)

15 15 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 marks 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. 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. 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.

16 16 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 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 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