Approve by CDC Nepal Modern Concept Science and Technology Book 6 301energy. Therefore, all plants and animals would die in the absence of the sun.FACTS WITH REASONSThe sun looks almost same in size to the moon while looking at them from the earth, why?The sun and the moon seem about same sized while looking from the earth because objects look smaller when they are observed from far. The sun is very big but it is very far from the earth. The moon is very small but comparatively very close to the earth.PlanetsThe heavenly bodies that revolve around the sun on their own elliptical orbits are called planets. There are eight planets in the solar system. They are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. There are also a number of small, dwarf planets in the solar system. Some of the dwarf planets are Pluto, Eris and Ceres. Planets revolve around the sun as well as rotate around their own axis. Planets do not emit their own light like stars but reflect the light of the sun. This makes the planets shine in the dim sky.FACTS WITH REASONS1. Planets do not have their own light but they shine. Give reason.Planets shine and become visible because they reflect the sunlight that falls on them.2. Earth is a planet. Why?Earth is a planet because it is a huge, round heavenly object revolving around the sun, a star.a) MercuryMercury is the smallest planet of the solar system. It is closest to the sun. The part of the Mercury that faces the sun is extremely hot and other part is extremely cold. Mercury has no atmosphere, hence life cannot exist there.b) VenusVenus is the brightest and hottest planet of the solar system. It is the closest planet to the earth. The atmosphere of the Venus mainly consists of carbon dioxide. Life cannot exist on the Venus.Mercury is made up of mostly rock and heavy metals.
302 Modern Concept Science and Technology Book 6 Approve by CDC Nepal FACTS WITH REASONS1. Venus is the hottest planet, why?Venus is the hottest planet because it has the highest average temperature among all the planets due to the presence of lots of greenhouse gases like carbon dioxide.2. Venus is called evening star. Why?Venus is called evening star because it is seen in the western sky of the earth in the evening.ACTIVITY 51. Mercury can be seen just before the sunrise in the months of September and October. Similarly, it can be seen just after the sunset in March and April. Observe Mercury and show it to your family members too.2. Venus can be seen just before the sunrise and just after the sunset. Observe Venus and show it to others also. Please note that Venus is the brightest heavenly body at night after the moon.c) EarthEarth is the third nearest planet from the sun, on which we live. This planet has life supporting gases, i.e. oxygen and carbon dioxide, water and suitable temperature for living beings. Therefore, life exists on the earth’s surface. The earth has one natural satellite, i.e. moon. Earth is the only planet which has all the conditions essential for the existence of life. The earth takes 365.25 days to complete one revolution around the sun. It takes 24 hours to complete one rotation on its own axis. It causes day and night to occur on the earth.Differences between the sun and the earthS.N. Sun S.N. Earth1. The sun is a mediumsized star, which is the main member of the solar system.1. The earth is a planet, which revolves around the sun in an elliptical orbit.
Approve by CDC Nepal Modern Concept Science and Technology Book 6 3032. The sun is too hot and does not support life.2. The earth has suitable temperature and supports life.FACTS WITH REASONS1. Earth is called a planet, why?Earth is called a planet because it is a big, sphere-like heavenly body, which revolves around the sun.2. Earth is also known as the only living planet. Why?Earth is also known as the only living planet because it is the only planet where life is present due to water, appropriate temperature and suitable atmosphere.d) MarsMars is the fourth planet from the sun. It can be seen with our naked eyes. It looks like a bright orange-red star due to the presence of a red mineral called limonite. It is almost half the size of the earth. The atmosphere of the Mars mainly consists of nitrogen gas. Mars has two moons.FACTS WITH REASONS1. First four planets are called inner planets or terrestrial planets. Why?First four planets are called inner planets or terrestrial planets because they are made up of rocks and soil.2. Mars is called a red planet. Why?Mars is called a red planet because of its reddish appearance due to the limonite rocks.e) JupiterJupiter is the largest planet of the solar system. It is the third brightest object at night after the moon and the Venus. The Jupiter looks like a bright star with our naked eyes. It consists of bands of clouds and a big red spot. This planet has four large moons and 67 small moons.Jupiter is the fastest rotating planet in our solar system. Its days are only 10 hours long.
304 Modern Concept Science and Technology Book 6 Approve by CDC Nepal f) SaturnSaturn is the second largest planet and the third farthest planet of the solar system. This planet is surrounded by the rings of dust and rocks. It has 62 moons.FACTS WITH REASONSSaturn is called a giant planet. Why?Saturn is called a giant planet because it is massive like Jupiter.g) UranusUranus is the second farthest planet of the solar system. It is a very cold planet. Uranus too has rings around it. This planet has 27 moons.h) NeptuneNeptune is the farthest planet of the solar system. It cannot be seen with our naked eyes. It is the coldest planet. It has 14 moons.ACTIVITY 61. Make a colourful chart of the solar system with the help of your textbook or your science teacher.2. Name the eight planets of the solar system. Also, write important features of each planet in your exercise book.As the fifth brightest object in our solar system, Saturn can be seen with the naked eyes and by using binoculars.Uranus is the first planet to be discovered by the use of a telescope.Neptune is not visible with naked eyes.
Approve by CDC Nepal Modern Concept Science and Technology Book 6 305SatelliteA satellite is an object that revolves around the planet in a fixed orbit. There are two kinds of satellites. They are:i) Natural SatelliteA satellite which is naturally formed and revolves continuously around a planet is called a natural satellite. This satellite is primarily formed during the formation of the solar system. There are more than 173 natural satellites in the solar system. Some of them are the Moon of Earth, Ganymede of Jupiter, Titan of Saturn, Triton of Neptune, etc.ii) Artificial SatelliteThe satellites which are made by humans and launched in the space to revolve round a planet are called artificial satellites. There are more than 8,000 artificial satellites in the solar system. For example, Geostationary satellite.AsteroidsThe millions of rocky and metallic heavenly bodies that range from few kilometres to hundreds of kilometres in diameter are called asteroids. They lie in between Mars and Jupiter. Thus, the rocky and metallic objects which are present in between the orbits of Mars and Jupiter that revolve around the sun are called asteroids or minor planets. The asteroids have two kinds of shape: nearly spherical which are now known asminor or dwarf planets and irregular. Some of the irregular asteroids are Juno, Vesta, Pallas, Iris, Flora, Victoria, Eros, etc. On the basis of size, Pallas is the largest asteroid.CometsComets are tailed flying bodies that can be observed in the sky. Halley’s Comet is one of the most clearly visible comets from the earth.They are small bodies in the solar system which are made up of ice, dust, earth and asteroidHalley’s Comet is one of the most clearly visible comets from the earth.
306 Modern Concept Science and Technology Book 6 Approve by CDC Nepal rocks and trapped gases. Thus, a broom-shaped shining object made of ice with a bright head is called a comet. There are about 3,296 known comets. A person may not see a particular comet twice in his/her lifetime. This is because it has a highly elliptical orbit around the sun due to which it takes many decades to revolve around the sun. A comet is generally a ball of ice, dust and gases. But, when it approaches the sun, a tail is produced. The tail of the comet increases on approaching the sun due to the solar radiation and solar wind.FACTS WITH REASONS1. Why do comets produce a tail on approaching the sun?When a comet approaches the sun, some of its frozen matter begins to evaporate into gases due to the heat of the sun. These gases rush out due to the effect of solar wind and high speed of the comet. In this way, a long tail is formed when a comet goes near the sun.2. The tail of the comet becomes shorter on going farther from the sun, why?The tail of the comet becomes shorter on going farther from the sun. It is because the heating effect of the sun increases on approaching the sun which causes more ice and gases to become vaporized. But, when the comet goes far away from the sun, the heating effect decreases and thus the tail of the comet becomes shorter.3. Why do comets have a longer revolution period? Comets have a longer revolution period because their path around the sun is highly elliptical, due to which they take a long period of time to revolve once around the sun.Meteors (falling stars or shooting stars)A meteor is a space rock ranging from the diameter of an atom to several metres. It floats freely inside the solar system. Scientists believe that they are the leftovers of the formation of the solar system. Thus,the pieces of stone or metal which enter the earth’s atmosphere are called meteors. Halley's cometmeteor shower
Approve by CDC Nepal Modern Concept Science and Technology Book 6 307Meteors drag themselves in the atmosphere and produce a bright light. So, they are also known as ‘shooting stars’, falling stars’, or ‘fireball’. The meteor that does not burn completely in the atmosphere and reaches the earth surface is called a meteorite.Key terms and terminologies of the unitSolar system: The family of the sun which consists of eight planets and their satellites, comets, asteroids, etc. is called the solar system. Sun: The sun is a medium-sized star closest to the earth. Planet: The heavenly bodies that revolve around the sun on their elliptical orbits are called planets. Satellite: A satellite is an object that revolves around the planet in a fixed orbit. Natural satellite: A satellite that is naturally formed and revolves around a planet is called a natural satellite.Artificial satellites: The satellites which are made by humans and launched in space to revolve around a planet are called artificial satellites. Asteroids: The rocky and metallic objects which are present in between the orbits of Mars and Jupiter that revolve around the sun are called asteroids.Comet: A broom-shaped shining object with a bright head is called a comet.Meteors: The pieces of stone or metal which enter the earth’s atmosphere are called meteors.Meteorite: The meteor that does not burn completely in the atmosphere and reaches the earth's surface is called a meteorite. 3 STEPS 3 STEPS EXERCISE EXERCISESTEP11. Select the best answer from the given alternatives.a. What is our home planet?i. Nepal ii. solar system iii. Earth iv. satellites b. Which one is the largest planet? i. Sun ii. Earthiii. Neptune iv. Jupiter c. Why is comet called tailed star? i. It is in the tail of solar system. ii. A tail can be seen in it when it is near the sun. iii. It is in the tail of the Earth. iv. It is called shooting star.
308 Modern Concept Science and Technology Book 6 Approve by CDC Nepal d. What is a meteor called if it hits earth?i. meteoroid ii. meteor showeriii. asteroid iv. meteoritee. Which one is an asteroid? i. revolve around sun, very big and round ii. revolves around sun, small and irregular in shapeiii. revolve around the earth, big and round iv. revolve around jupiter, small and irregular STEP22. Answer the following questions in one word.a. Write the name of the largest planet. b. Which planet is called the brightest planet?c. Which planet is called the blue planet?d. Which heavenly object is called tailed star?e. Which heavenly object is called shooting star? 3. Write any two differences between:a. Planet and satellite b. Asteroid and comet4. Give reasons.a. Planets revolve around the sun. b. Mars is called the red planet. c. Tail of a comet changes its length while revolving around the sun. d. Meteor is called a shooting star.STEP35. Answer the following questions:a. Define solar system.b. Is life possible without the sun. Discuss.c. Enlist some characteristics of planets. d. What are meteors? e. Define meteorites.���