NS term 4

Module 16: Relationship of the Sun to the Earth

16.1  Solar Energy and the Earth's Axis  (p. 142–143)

Key Word

Solar Energy

•      The Sun transfers its heat energy to Earth by radiation. Radiation is transmitted from the energy source in all directions — imagine the Sun as a giant ball radiating heat and light energy in all directions.

•      The Earth receives the heat and light energy transferred by the Sun, known as solar energy.

The Earth's Axis

•      The Earth rotates on its 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 facing the Sun experiences daytime, and the side facing away from the Sun experiences nighttime. As the Earth turns on its axis, the light falls on different parts of the world.

The Equator, Hemispheres and Tropics

•      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 the northern hemisphere; the half below the Equator is the southern hemisphere.

•      The Earth is further divided by 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 of the Equator, and the Tropic of Capricorn lies south of the Equator.

16.2  The Earth's Seasons  (p. 144–146)

Key Words

The Earth's Orbit and the Seasons

•      The Earth rotates around its own axis and revolves around the Sun in a path known as an orbit.

•      The Earth takes one year to complete one full orbit around the Sun.

•      Different parts of the Earth receive different intensities of solar energy depending on where the Earth is in its orbit around the Sun — this is the reason for the different seasons: summer, autumn, winter and spring.

•      As the Earth revolves around the Sun, it maintains its tilt on its axis.

•      When the southern hemisphere is tilted towards the Sun, it receives the most intense solar energy (at the Tropic of Capricorn), so it is summer in the southern hemisphere.

•      At the same time, the northern hemisphere is tilted away from the Sun and receives less intense solar energy, so it is winter in the northern hemisphere.

•      The Earth continues orbiting the Sun, and six months later (in June) it is on the opposite side of the Sun.

•      The northern hemisphere is then tilted towards the Sun, so the Tropic of Cancer receives the most intense Sun, and it is summer in the northern hemisphere.

•      The southern hemisphere is then tilted away from the Sun, meaning it is winter in the southern hemisphere.

•      In March and September, neither hemisphere is tilted towards or away from the Sun, and each hemisphere receives an equal intensity of solar energy. This accounts for the seasons of spring and autumn.

Length of Day and Direct/Oblique Rays

•      The length of the day also depends upon the season and where the Earth is in relation to the Sun.

•      In summer, the days are longer than in winter. This is because the parts of the Earth tilted towards the Sun have a longer path to travel while in the Sun.

Direct and Oblique Rays of Sunlight

•      The Sun's solar energy is more intense in the hemisphere tilted towards it — the energy is focused over a small area. During summer we receive direct rays from the Sun.

•      The other hemisphere, which has winter, receives oblique rays spread over a large area, so the solar energy is less intense.

•      The direct and oblique rays of sunlight rotate as the seasons change.

•      In March and September, the Sun's direct rays land on the equator, so the northern and southern hemispheres each receive a similar intensity of the Sun's rays — accounting for the seasons of autumn and spring.

16.3  Solar Energy and Life on Earth  (p. 147)

Plants Absorb Light Energy from the Sun

•      Plants contain food, such as sugar and starch.

•      Plants make their own food using light energy from the Sun.

•      During photosynthesis, plants use chlorophyll to absorb light energy from the Sun and carbon dioxide from the air, and convert them into sugar and starch for food. Solar energy is an essential ingredient for plants to make food.

•      Animals cannot make food substances for themselves; they can only get it from eating plants, or by eating animals which have eaten plants. All animals depend on plants for food.

•      The Sun's energy sustains all life on Earth: animals depend on plants to survive, and plants depend on the Sun's light energy to make food.

16.4  Stored Solar Energy  (p. 148–149)

Key Word

Formation of Fossil Fuels

•      When plants and animals die, they can, under the right conditions and over millions of years, form fossil fuels like oil, natural gas and coal.

•      Millions of years ago, plants absorbed energy from the Sun. Animals ate these plants. The plants and animals died and fell into swamps, lakes or seas, and were soon covered by a layer of sediment.

•      The layer of sediment protected the dead remains so that they didn't decay normally. The dead matter formed a substance called peat.

•      Over time, more layers of soil, mud and rock formed above the peat, increasing the pressure and pushing it further underground, where it was much hotter.

•      The heat from the Earth's centre and the pressure changed the peat into fossil fuels. Plant substances mainly became coal. Natural gas and oil formed from a combination of dead plant and animal matter.

Use of Fossil Fuels

•      Fossil fuels are an essential source of energy for cars, for generating electricity, and for many other purposes. They essentially store the energy that plants absorbed from the Sun millions of years ago.

•      Humans release the potential energy stored in fossil fuels every day by burning coal to generate electricity, driving in cars, and heating homes.

•      We are using fossil fuels at a much faster rate than they take to form. This is why fossil fuels are non-renewable, and why we have to find other sources of energy to reduce our reliance on fossil fuels.

Module 17: Relationship of the Moon to the Earth

•      This module covers: the relative positions of the Earth and the Moon, the concept of gravity, factors that influence gravitational pull, the Sun's, Earth's and Moon's gravitational pull, the causes of tides, high and low tides, spring tides, and the effect of tides on shoreline ecosystems.

17.1  The Relative Positions of the Moon, Earth and Sun  (p. 152)

Key Word

•      As the Earth revolves around the Sun, the Moon revolves around the Earth. 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. Using the diameter of the Earth as a measure, you'd need the total diameter of about 30 Earths to get to the Moon.

•      The Sun's diameter is about 109 times the diameter of the Earth.

•      The distance between the Sun and the Earth is about 107 times the diameter of the Sun.

17.2  Gravity  (p. 153–154)

Key Words

What is Gravity?

•      Gravity is the tendency of objects to attract or pull each other. All objects have gravity — even your own body — but you won't notice your own gravitational pull because your mass is too small.

•      For gravity to be noticeable, the object attracting other objects needs to be massive.

•      We are pulled towards the ground because of the Earth's gravitational pull, which pulls us and everything else on Earth toward its centre. This is one of the reasons we can live on a planet shaped like a ball.

•      Mass is a measure of the amount of matter in an object, measured in kilograms. The mass of an object stays the same whether it is on Earth, on the Moon, or anywhere else in space.

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.

The Effect of Mass on Gravity

•      An object or body with a great mass will exert a stronger gravitational pull than objects with smaller masses.

•      Planet A and Planet B have the same mass, so the gravitational pull on their moons is equal. Planet C has a smaller mass than the other two, so its gravitational pull on its moon is weaker.

The Effect of Distance on Gravity

•      The distance between two objects also affects their gravitational pull. Objects that are closer together exert a stronger gravitational pull on each other than objects that are further apart, even if they have the same mass.

•      To summarise: more massive objects exert a stronger pull than smaller objects over the same distance; and for objects of the same mass, the closer they are to each other, the stronger the pull of gravity between them.

Pull of Gravity of the Sun, Earth and Moon

•      The Sun is so massive that it holds all the bodies in the solar system in its orbit because of its gravitational pull. There are eight planets in the solar system, as well as a range of dwarf planets, asteroids and other space material.

•      The Moon is held in its orbit around the Earth by the pull of the Earth's gravity. The Moon also has its own gravity.

•      Astronauts on the Moon seem to bounce around rather than have their feet fixed firmly on the ground. This is because the pull of gravity on the Moon is weaker than on Earth.

•      The Moon is still massive enough to have some gravitational pull, and its effects can be noticed on Earth. One of the most significant effects of the Moon's gravitational pull on Earth is the tides in the Earth's oceans and seas.

17.3  Tides  (p. 156–159)

What Are Tides?

•      Tides are the predictable, repeated rise and fall of 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 a 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 happens every day, and we can predict how high or low the tides will be throughout the month.

The Role of the Moon in Causing the Tides — the Tidal Bulge

•      The main cause of tides on Earth is the gravitational pull of the Moon on the Earth. The Moon pulls on the water in the seas and oceans as it orbits the Earth. It takes about a month for the Moon to do a full orbit of the Earth.

•      The Moon's pull creates a tidal bulge, which forms on both sides of the Earth. This tidal bulge is responsible for high tide.

•      The bulge forms on both sides because as the Moon pulls on the Earth and its waters, the Earth pulls back. This pushes the waters on the opposite side of the Moon outward too, so the tidal bulges on both sides of the Earth are equal.

High Tide and Low Tide

•      The Moon takes a month to orbit the Earth, but most coastal areas experience two high tides and two low tides every day. This happens because the Earth rotates on its axis once every 24 hours, so a coastal town or city will pass through both tidal bulges on the Earth in one day.

•      For example, if it is midday (12:00) and low tide in Durban, as the Earth rotates, Durban heads towards a tidal bulge and experiences high tide just after six o'clock that evening. By about 00:30 that night it is low tide again, and by about 6:40 the next morning it is high tide again — its second high tide for the day.

Spring Tides

•      About once a month, the Earth experiences a spring tide. A spring tide means that the high tide is very high and the low tide is very low. Waves get higher and the sea gets rougher during a spring high tide.

•      A spring tide happens when the Moon's orbit aligns with the Sun — this happens at full moon and new moon. The Sun's gravitational pull adds to the Moon's gravitational pull, which means the tidal bulges on the Earth get bigger.

Shoreline Ecosystems  (p. 159)

Key Word

•      The area on the shoreline between the low tide and high tide levels is home to unique animals and plants that have adapted to the rapidly changing conditions caused by the tides — the shoreline ecosystem. The shoreline is divided into a high tide zone and a low tide zone.

The High Tide Zone

•      The high tide zone is exposed for most of the day and gets flooded by seawater at high tide. Plants and animals here must be adapted to live in water, withstand beating waves and rough seas, and withstand high temperatures so 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 grow well due to more sunlight) and sea animals such as sea anemones, crabs and starfish. These 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

•      The low tide zone is usually underwater, but is exposed at low tide. The water provides protection against predators and protects the ecosystem from excessive heat from the Sun.

•      Plants and animals here have adaptations like suckers to cling to the rocks so they aren't washed out to sea, and many have shells or hard body coverings to prevent drying out when exposed to sunlight.

Module 18: Historical Development of Astronomy

•      This module covers: the development of different calendars based on the patterns of the Moon, Sun and stars from Earth; the time periods of a year, a month and a day; the use of patterns to mark significant times in a community; the passing on of knowledge through stories; and discoveries in astronomy, including Copernicus, Galileo, Kepler, Newton, and William and Caroline Herschel.

18.1  Early Indigenous Knowledge  (p. 162–164)

•      Early indigenous cultures did not know why the seasons or day and night changed, but they did observe how the Moon changed each night and noticed the changes in the seasons.

•      Indigenous groups told stories to explain patterns like the phases of the Moon. A group would often start recognising a pattern for practical reasons — for example, the San (Bushmen) used to hunt at night during a full moon because there was enough light to see the animals they were hunting, and kept track of the Moon's phases to plan their hunting.

•      The San recognised the length of a day as the period between one sunrise and the next. At night, they would gather to sing and do sacred dances.

The San and the Changing Seasons

•      The San tracked the changing seasons to plan their hunting, gathering and living arrangements.

•      In the dry winter, they gathered in a large group to collect foods like nuts and roots. In spring, hunting became more important as plant food was limited. In summer (the rainy season), the San lived in smaller groups and hunted in these groups, since gathering food was easier as plants had grown and produced fruit. In autumn, the small groups gathered back into a large group, and the cycle repeated.

•      As cultures like the San learned to predict the seasons and changes in the Moon, they attached religious significance to certain holy days, marking them in relation to the Moon's phases or the seasons. This knowledge was passed on through stories and songs so later generations could prepare for the seasonal changes ahead.

A San Story: The Formation of the Milky Way

•      The San told a story about a young woman whose husband went out hunting and did not return before dark. Worried that he would not find his way home in the dark, she threw the glowing coals from her fire up into the night sky. As they rose, the coals turned to white ash, and the hunter was able to use the sparkling light to find his way home. The San say that this shimmering white ash is what we know today as the Milky Way.

•      While this is not really how the Milky Way formed, stories like this were an important way of passing on knowledge about the stars. San hunters used the positions of the stars to help them find direction, and a memorable story would remind them to use the stars when lost at night.

Ancient Egyptian Calendars

•      For the ancient Egyptians in North Africa, farming was very important. Life centred on the Nile River, so they observed the changing positions of the stars to predict when the Nile would flood, and when to plant and harvest their crops.

•      They developed several calendars to track the passing of time. A lunar calendar divided the year into 12 months, each month being one complete cycle of the phases of the Moon. Another calendar divided the year into 365 days, but this eventually fell out of sync with the seasons, because the Earth actually takes 365,24 days to orbit the Sun.

•      The calendar used across the world today is the Gregorian calendar. It measures a year as the time it takes the Earth to orbit the Sun, divided into 12 months. Each month has 30 or 31 days, except February, which has either 28 or 29 days.

18.2  Modern Developments  (p. 165–166)

Key Word

Copernicus

•      Copernicus, a Polish monk, was the first astronomer to suggest that the Sun was at the centre of the solar system, and that the Earth revolved around the Sun. Until then, scholars thought the Earth was at the centre of the universe.

•      Copernicus said the spinning of the Earth on its axis as it rotated daily made the Sun and stars look as if they moved around the Earth. He also suggested the other planets moved around the Sun. Although he shared his ideas from about 1514, he only officially published his book in 1543 — the year he died.

Galileo Galilei

•      Galileo improved the design of the telescope, allowing him to observe the Moon and planets with greater accuracy.

•      In 1610, Galileo observed that the planet Jupiter had at least four moons orbiting it. This discovery confirmed Copernicus's theory that the Sun is the centre of the solar system.

•      Galileo also observed the Earth's Moon and discovered it was not a perfectly round ball — it has mountains and valleys on its surface.

Johannes Kepler

•      Kepler was a German astronomer. In 1609, he used mathematics and geometry to prove that the planets orbited the Sun in an elliptical path, rather than a perfect circle. Planets move faster when they get nearer to the Sun.

•      Kepler improved upon Copernicus's research and calculated the distances of the planets from the Sun with more accuracy — but he could not explain why the planets orbited or how the solar system was held together.

Isaac Newton

•      In 1687, Newton published a paper describing the force of gravity and his famous three laws of motion.

•      Newton suggested it was the gravitational pull of the Sun that held all the planets in their orbits in the solar system — supporting the findings of Copernicus and Kepler.

William and Caroline Herschel

•      William Herschel, together with his sister Caroline Herschel, discovered the planet Uranus.

•      Herschel built very advanced telescopes, allowing him to see further into the solar system and beyond. He and Caroline observed the stars over many years and put together a map of the universe as observed from Earth.

•      The Herschels' model of the night sky included 90 000 stars and showed that the solar system was part of a disc-shaped galaxy, called the Milky Way.