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THE SOLAR SYSTEM 1.1 THE SOLAR SYSTEM 1. The Solar System consists of nine planets and the Sun. 2. The other members of the Solar System are natural satellites, asteroids, comets and meteors. 3. The nine planets in the Solar System beginning with the nearest to the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. 4. The relative of the Sun and its nine planets are shown in the figure below. 5. All the nine planets are moving around the Sun each following its own oval path called orbit. 6. The time taken for a planet to complete one revolution around the Sun is different from the time taken for another planet to do so. 7. Mercury takes the shortest time to complete one orbit, while Pluto takes the longest. 8. This is due to the fact that Mercury is the nearest planet to the Sun as shown in the table below: 9. The relative sizes of the planets are shown in the figure below: A. Mercury 1. Mercury is the closest planet to the Sun. 2. Since Mercury is closest to the Sun, its temperature can be over 300 degree Celcius. 3. Mercury rotates on its own axis very slowly, making one full rotation every 59 Earth days. B. Venus 1. Venus is the second planet from the Sun. 2. The size of Venus is about the same as that of the Earth. 3. The planet is always covered by a very thick cloud of carbon dioxide and acid vapour. 4. Thus, the temperature of Venus is extremely high, even though Venus is twice as far from the Sun as Mercury and receives only quarter as much light. 5. As a result, the surface temperature can be above 450 degree Celcius. 6. Venus takes about 225 days to revolve around the Sun. 7. Venus rotates on its own axis once every 243 days. 8. Its gravity is the same as that of the Earth. C. Earth 1. Earth is the third planet from the Sun. 2. It is the fifth largest planet. 3. It is the only planet with oxygen and water. Thus, it is the only planet on which living things can survive and grow. 4. Earth takes about 365 days to revolve around the Sun and rotates on its own axis once every 24 hours. 5. Earth has a natural satellite (the Moon) revolving around it. D. Mars 1. Mars is the fourth planet from the Sun. 2. It also known as the Red Planet because its surface is being covered with red dust. 3. Mars is a cold and lifeless planet. 4. One revolution of Mars around the Sun takes 687 days. E. Jupiter 1. Jupiter is the largest planet in the Solar System. 2. It is the fifith planet from the Sun. 3. Most of Jupiter's volume is made up of gases and ice with only a small rocky core. 4. The atmosphere is composed of mainly hydogen in gaseous and liquid forms together with helium, methane and ammonia. 5. Jupiter has sixteen natural satellites or moons and takes about 12 years to revolve once around the Sun. 6. Jupiter rotates so fast, once every tens hours, that it creates strong winds and great storms which persist over most of the planet. 7. Therefore, Jupiter is the planet which has the shortest day in the Solar System. F. Saturn 1. Saturn is the sixth planet from the Sun. 2. It is surrounded by a system of rings which make it the most striking and beautiful planet when viewed through a telescope. 3. Saturn is the second-largest planet in the Solar System. 4. However, it is lighter than a body of water of the same time. 5. Saturn is thought to have a small, rocky high temperature core surrounded by ice and ammonia, mixed with grit and gases. 6. The time taken for Saturn to revolve around the Sun is around 29 years. 7. Saturn rotates on its own axis once every 10.7 hours. G. Uranus 1. Uranus is the seventh planet from the Sun in the Solar System. 2. This planet is blue because it has cold gases, mainly made up of hydrogen and helium. 3. Uranus also has a ring around it but the ring cannot be seen clearly because it is too thin and dim. 4. It has five moons, and it rotates on its own axis slightly more slowly than Saturn. 5. The time taken for Uranus to revolve around the Sun is around 84 years. H. Neptune 1. Neptune, the eighth planet from the Sun is almost the twin of Uranus. 2. It is a little heavier than Uranus and is very slightly bigger. It is bluish-green. 3. Neptune consist mainly of ice with some rocks and gases, and has less atmosphere. 4. Neptune has two moons and takes about 165 years to revolve once around the Sun. I. Pluto 1. Pluto is the ninth planet from the Sun. 2. It is the smallest planet in the Solar System, and is made of rock and thin ice . 3. Pluto is a dark and cold planet. Its surface temperature can be as low as -230 degree Celcius. 4. This planet can be seen only through a telescope because it is so far away from Earth. 5. Pluto takes about 247.7 years to revolve once around the Sun. A. Natural Satellites 1. Natural satelltes are the natural objects which orbit around planets. 2. The Moon is the Earth's only natural satellite. 3. All the planets in the Solar System have natural satellites except Mercury and Venus. 4. The number of natural satellites is shown in the table below: B. Asteroids 1. Besides the nine planets, there is also an asteroid belt. 2. The asteroid belt lies between the orbits of Mars and Jupiter. 3. Asteroids are small pieces of rocks mixed with metals orbiting round the Sun. 4. Asteroids vary in size. Some can be as small as dust particles while some can be a few hundred kilometres in diameter. C. Comets 1. Comets are small pieces of ice and dust that orbit around the Sun, but their paths are not like those of the planets. 2. Most comets spend much of their time out of the Solar System, beyond Pluto. However, they can enter new orbits that bring them into the Solar System where they become visible from Earth. 3. The most famous comet is called Halley's comet which returns to the centre of the Solar System every seventy-six years. 4. Halley's comet was last seen in early 1986. 5. It will not be seen again until its return in 2061. 6. Halley was the first astronomer to show that some comets followed orbits that keep returning them to the vicinity of the Sun. 7. Every comets has a head and a long tail. The small solid part at the centre of the head is called the nucleus. 8. The nucleus contains ice and the heat of the Sun causes the ice to melt and give out gas and dust. 9. The gas and dust given out from the head of the comets forms the tail. The gas gives out light. 10. The tail of the comets always points away from the Sun. D. Meteors 1. Meteoroids are small pieces of rock or metal floating in outer space. 2. When meteoroids float near to the Earth, they may be drawn into the Earth's atmosphere due to gravity. 3. Meteoroids which enter the Earth's atmosphere are called meteors. 4. Most meteors will burn up when they enter the Earth's atmosphere due to friction. 5. Thus, streaks of light can be seen in the sky at night in the dark. They leave behind fiery tails and are called shooting stars. 6. Meteors which do not burn completely and reach the Earth's surface are called meteorites. 7. Meteorites which fall to the ground will make huge craters. One such crater is found in Arizona, the United State of America. 8. Many of the planets and their natural satellites are covered with meteorite craters. However, the Earth does not have many meteorite craters since it is protected by its atmosphere. 1.2 RELATIVE SIZE AND DISTANCE BETWEEN THE EARTH, MOON AND SUN 1. The Earth is in the shape of a sphere. 2. Its diameter is about 12 760 kolimetres. 3. Its circumference is about 40 000 kilometres. 4. If our national car, the Proton, is used for measuring the circumference of the Earth then 9 million Proton cars will be needed. 5. It take about 3 years for a person to walk round the Earth once. 6. The air surrounding the Earth is called the atmosphere. 7. The part of the Earth covered with water is called the hydrosphere. 8. About 70% of the Earth is covered by the hydrosphere. This includes the lakes, oceans and rivers. A. Proof that the Earth is Spherical 1. In the olden days, people believed that the Earth was flat and circular in shape just like a big coin. 2. The spherical shape of the Earth is proved by the images obtained from artificial satellites. 3. The following examples shows that the Earth is round. The observation of a ship sailing towards land will confirm that the Earth is round. (a) At first,smoke is observed from the ship. (b) Next, as the ship approaches the beach the funnel of the ship is observed. (c) Finally, the whole ship is observed. B. The Gravity of the Earth 1. If you drop an object down a tall building, it will fall to the ground. 2. This is caused by the gravitational pull of the Earth. 3. Therefore, the gravitational pull causes objects to be pulled towards the centre of the Earth. 4. The weight of an object is affected by the pull of the Earth's gravity on the object. 5. Without the Earth's gravity objects will float on the Earth. 1. The Moon is smaller than the Earth. It is also in the shape of a sphere. 2. The Moon is about 50 times smaller than the Earth. 3. Its diameter is about 1/4 that of the Earth. 4. A grain of sago and a marble can be used to represent the Moon and the Earth. 5. However, at night we observe that the Moon seems to be of the same size as the Sun. 6. This is because the Moon is much closer to the Earth compared to the Sun. A. Surface of the Moon 1. The Moon is the natural satellite of the Earth. 2. The Moon has no atmosphere and no water too. 3. Therefore, no living things can be found on the Moon. 4. Rocky highlands and deserts are the typical features of the landscape on the Moon. 5. As the Moon has no atmosphere, it is extremely quite, since no sound can travel through a vacuum. 6. The surface of the Moon has many huge craters up to hundreds of kiometres wide. 7. These craters are produced by meteorites. You have just learned this in the prevoius unit. B. Gravity of the Moon 1. Since the Moon is much smaller than the Earth, its pull of gravity is much less than that of the Earth. 2. Therefore, if you stand on the surface of the Moon, your weight will be only 1/6 of your weight on the Earth. 3. Therefore, you will be floating on the surface of the Moon. 1. The Sun is a star and not a planet. 2. The Sun is located at the centre of the Solar System. 3. The Sun is the closest star to the Earth. 4.The Sun looks bigger and brighter than all the other stars because it is the closest star to the Earth. 5. The Sun is very hot. It is glowing ball of gas. 6. The diameter of the Sun is about 1 400 000 kilometres. 7. This is about 100 times the size of the Earth. 8. The circumference of the Sun about 400 times bigger than the circumference of the Earth. 9. The Sun is about 15 000 000 kilometres away from the Earth. 10. The distance of the Sun from the Earth is 400 times the distance of the Earth from the Moon. A. The Earth as Compared with the Other Planets 1. Most planets are not conducive for living things. 2. This is because they are either too hot (too close to the Sun) or too cold (too far from the Sun). 3. The following examples show why the other planets are not suitable for living things. (a) The surface temperature of Mercury is very high. It is about 300 degree Celcius and this temperature is hot enough to melt lead. (b) Venus is extremely hot, and its surface temperature can be above 450 degree Celcius. (c) Venus is always covered with a very thick cloud of carbon dioxide and acid vapour. Therefore Venus experiences the greenhouse effect. (d) Other planets such as Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are extremely cold and lifeless. (e) The atmosphere of Jupiter is made up of hydrogen, mainly in gaseous and liquid forms, together with helium, methane and ammonia. (f) Saturn is surrounded by a mantle of ice and ammonia, mixed with grit and gases. (g) Uranus is covered with cold gas, mainly hydrogen and helium. (h) Neptune consists mainly of ice with some rock and gases. Pluto is made of rock and thin ice which explains why its dark and cold. B. Why the Earth is Ideal for Living Things 1. The Earth is the only planet with a plentiful supply of oxygen and water. 2. Therefore, it is the only planet on which living things can survive and grow. 3. If the Earth is located closer to the Sun, then all living things will die, since of the temperature of the Sun is too high (6000 degree Celcius). 4. However, if the Earth is located much further from the Sun, living things will freeze and be covered with ice since the temperature is extremely low. 5. Thus, our Earth is the most suitable for living on.