Paheli and Boojho visited their grandparents’ village during summer break. After dinner, they went on to the roof of the house. It was a clear cloudless night. They were surprised to see a large number of bright stars in the sky. They had never seen such a beautiful scene in their city (Fig. 17.1). at a place where there are no bright lights and the atmosphere is clear. Look at the sky on a dark, clear night. You see the entire sky dotted with countless stars, some bright and some not so bright. Observe them carefully. Do all of them appear to twinkle? Do you find any star-like object which does Paheli wondered, why the village sky was so different from the night sky in big cities? Her grandfather explained that due to bright light, smoke and dust, sky in big cities is rarely clear. Night sky watching can be a fascinating experience not twinkle? The objects which do not twinkle are planets. The moon is the brightest object in the night sky. The stars, the planets, the moon and many other objects in the sky are called celestial objects. Are all celestial objects similar? Let us find out. 17.1 The Moon Activity 17.1 Observe the moon continuously for several nights, preferably from one full moon to the next. Make a sketch of the moon every night in your note book and note from the day of the full moon. Also note everyday the part of the sky (east or west) in which the moon is seen. Is there a change in the shape of the moon everyday? Are there days when the shape of the moon appears to be perfectly round? Are there days when the moon cannot be seen at all even if the sky is clear? The day on which the whole disc of the moon is visible is known as the full moon day. Thereafter, every night the size of the bright part of the moon appears to become thinner and thinner. On the fifteenth day the moon is not visible. This day is known as the new moon day. The next day, only a small portion of the moon appears in the sky. This is known as the crescent moon. Then again the moon grows larger every day. On the fifteenth day once again we get a full view of the moon. The various shapes of the bright part of the moon as seen during a month are called phases of the moon (Fig. 17.2). The time period between one full moon to the next full moon is slightly longer than 29 days. In many calendars this period is called a month. Why does the moon change its shape every day? Fig.17.2 : Phases of the moon Let us try to understand why phases of the moon occur. You have studied in Chapter 16 that the moon does not produce its own light, whereas the Sun and other stars do. We see the moon because the sunlight falling on it gets reflected towards us (Fig. 17.3). We, therefore, see only that part of the moon, from which the light of the Sun is reflected towards us. Activity 17.2 Take a big ball or a pitcher. Paint half of it white and half black in colour. Go out into the playground with two of your friends. Draw a circle of radius about 2 m on the ground. Divide the circle into eight equal parts as shown in Fig. 17.4. Stand at the centre of the circle. Ask a friend to hold the ball at different points of the circle. Ask her to keep the white portion of the ball always towards the Sun. If you are performing this activity in the morning then the white portion of the ball should be kept towards the east. If the activity is being performed in the afternoon then the white portion of the ball should be kept towards the west. In each case the line dividing the white and black portions is kept vertical. Standing at the centre of the circle observe the visible white portion of the ball while your friend stands at the points on the circle marked earlier. Draw the shape of the white portion as you see it. Compare your drawings with the different phases of the moon as shown in Fig. 17.5. Remember that the moon revolves around the Earth. The Earth along with the moon, revolve around the Sun (Fig. 17.6). Can you now guess the relative positions of the Sun, moon and the Earth on the day of the full moon and on the day of the new moon? Sketch these positions in your notebook. In which part of the sky would you look for the full moon? The size of the illuminated part of the moon visible from the Earth increases each day after the new moon day. After the full moon day, the sunlit part of the moon visible from the Earth decreases in size every day. friend in such a manner that your face always remains towards him. Can your friend see your back? How many rotations did you complete in one revolution? The moon revolves around the Earth in a similar manner. The moon completes one rotation on its axis as it completes one revolution around the Earth. The Moon’s Surface The moon is a fascinating object for poets and story-tellers. But when astronauts landed on the moon, they found that the moon’s surface is dusty and barren. There are many craters of different sizes. It also has a large number of steep and high mountains (Fig. 17.7). Some of these are as high as the highest mountains on the Earth. We learnt in Chapter 13 that sound cannot travel when there is no medium. Then, how can we hear any sound on the moon? bright? Are they of the same colour? In fact, stars emit light of their own. The Sun is also a star. Why does it appear so large compared to the other stars? Which appears bigger, a football placed near you, or a football placed at a distance of 100 m? The stars are millions of times farther away than the Sun. Therefore, the stars appear to us like points. The Sun is nearly 150,000,000 kilometres (150 million km) away from the Earth. The next nearest star is Alpha Centauri. It is at a distance of about 40,000,000,000,000 km from the Earth. Can you read this distance in kilometres conveniently? Some stars are even further away. Such large distances are expressed in another unit known as light year. It is the distance travelled by light in one year. Remember that the speed of light is about 300,000 km per second. Thus, the distance of the Sun from the Earth may be said to be about 8 light minutes. The distance of Alpha Centauri is about 4.3 light years. In fact, the stars are present in the sky during the day-time also. However, they are not visible then because of the bright sunlight. Observe some prominent star or a group of stars in the sky for about two hours or more. What do you find? Do you find any change in the positions of stars in the sky? You will find that the stars appear to move from east to west. A star which rises in the east in the evening, sets in the west in the early morning. Why do stars appear to move from east to west? Let us find out. Activity 17.4 Stand in the centre of a big room and start rotating. In which direction will the objects in the room appear to move? Do you see them moving in the direction opposite to your motion? Paheli recalls that when she is in a moving train the nearby trees and buildings appear to move in the backward direction. If the stars appear to move from east to west, could it mean that the Earth, rotates from west to east? Now I understand why the Sun appears to rise in the east and set in the west as the Earth rotates from west to east on its axis. There is actually a star, the pole star, which is situated in the direction of the earth’s axis. It does not appear to move (Fig. 17.10). 17.3 Constellations Look at the sky for some time. Can you see some stars forming groups with shapes like those in Fig. 17.11. The stars forming a group that has a recognisable shape is called a constellation. Constellations were devised by ancient people to be able to recognise stars in the sky. The shapes of constellations resemble objects familiar to those people. You can easily identify some constellations in the night sky. For this, you should know how a particular constellation looks like and where to look for it in the night sky. One of the most famous constellations which you can see during (a) Great Bear (b) Orion (c) Cassiopeia (d) Leo Major Fig. 17.11 : Some constallations in the night sky summer time in the early part of the Activity 17.6 night is Ursa Major [Fig.17.11 (a)]. It is also known as the Big Dipper, the Great Bear or the Saptarshi. There are seven prominent stars in this constellation. It appears like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl (Fig. 17.12). Observe this constellation for a few hours. Do you find any change in its shape? Do you find any change in its position? You will observe that the shape of the constellation remains the same. You will also find that the constellation appears to move in the sky from east to west. Activity 17.7 This activity should be performed on a clear moonless night during summer at about 9.00 pm. Look towards the northern part of the sky and identify Ursa Major. You may get help from elders in your family. Look at the two stars at the end of Ursa Major. Imagine a straight line passing through these stars as shown in Fig. 17.13. Extend this imaginary line towards the north direction. (About five times the distance between the two stars). This line will lead to a star which is not too bright. This is the Pole Star. Observe the Pole star for some time. Note that it does not move at all as other stars drift from east to west. Activity 17.8 During a summer night, observe Ursa Major 3-4 times at an interval of 2 to 3 hours. Also locate the Pole Star each time. Does Ursa Major appear to move from east to west? Does it appear to revolve around the Pole Star? Compare your observations with those in Fig. 17.14. Pole star Fig. 17.14: Ursa Major moves around the Pole Star. In fact, all the stars appear to revolve around the Pole Star. Note that the Pole Star is not visible from the southern hemisphere. Some of the northern constellations like Ursa Major may also not be visible from some points in the southern hemisphere. Orion is another well-known constellation that can be seen during winter in the late evenings. It is one of the most magnificent constellations in the sky. It also has seven or eight bright stars [Fig. 17.11(b)] Orion is also called the Hunter. The three middle stars represent the belt of the hunter. The four bright stars appear to be arranged in the form of a quadrilateral. The star Sirius, which is the brightest star in the sky, is located close to Orion. To locate Sirius, imagine a straight line passing through the three middle stars of Orion. Look along this line towards the east. This line will lead you to a very bright star. It is Sirius. (Fig. 17.15). Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter W or M [Fig. 17.11(c)]. Did You Know? A constellation does not have only 5-10 stars. It has a large number of stars (Fig. 17.16). However, we can see only the bright stars in a constellation with our naked eye. All the stars which make up a constellation are not at the same distance. They are just in the same line of sight in the sky. 17.4 The Solar System The Sun and the celestial bodies which revolve around it form the solar system. It consists of large number of bodies such as planets, comets, asteroids and meteors. The gravitational attraction between the Sun and these objects keeps them revolving around it. The Earth, as you know, also revolves around the Sun. It is a member of the solar system. It is a planet. There are seven other planets that revolve around the Sun. The eight planets in their order of distance from the Sun are: Mercury, Venus, Earth, Mars , Jupiter, Saturn, Uranus and Neptune. Figure 17.17 shows a schematic view of the solar system. Did you know? Till 2006 there were nine planets in the solar system. Pluto was the farthest planet from the Sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto does not fit this definition. It is no longer a planet of the solar system. Let us know about some members of the solar family. The Sun The Sun is the nearest star from us. It is continuously emitting huge amounts of heat and light. The Sun is the source of almost all energy on the Earth. In fact, the Sun is the main source of heat and light for all the planets. The Planets The planets look like stars, but they do not have light of their own. They merely reflect the sunlight that falls on them. Can you distinguish between planets and stars? The simplest method of identifying planets from stars is that stars twinkle, whereas planets do not. Also the planets keep changing their positions with respect to the stars. A planet has a definite path in which it revolves around the Sun. This path is called an orbit. The time taken by a planet to complete one revolution is called its period of revolution. The period of revolution increases as the distance of the planet increases from the sun. Activity 17.9 Go out into the playground with four or five of your friends. Draw four circles of radii 1m, 1.8m, 2.5m and 3.8m, all having a common centre (Fig. 17.18). Ask one of your friends to stand in the centre and represent the Sun. Your other four friends may represent Mercury, Venus, Earth and Mars. Ask your friends to move around the Sun in anti-clockwise direction in their own orbits (Fig. 17.18). Do they collide with one another? Besides revolving around the Sun, a planet also rotates on its own axis like a top (Fig. 17.19). The time taken by a planet to complete one rotation is called its period of rotation. Some planets are known to have moons/satellites revolving round them. Any celestial body revolving around another celestial body is called its satellite. The Earth can be said to be a satellite of the Sun, though generally we call it a planet of the Sun. We use the term satellite for the bodies revolving around planets. Moon is a satellite of the Earth. There are many man-made satellites revolving round the Earth. These are called artificial satellites. Mercury (Budh) The planet mercury is nearest to the Sun. It is the smallest planet of our solar system. Because Mercury is very close to the Sun, it is very difficult to observe it, as most of the time it is hidden in the glare of the Sun. However, it can be observed just before sunrise or just after sunset, near the horizon. So it is visible only at places where trees or buildings do not obstruct the view of the horizon. Mercury has no satellite of its own. Venus (Shukra) Venus is earth’s nearest planetary neighbour. It is the brightest planet in the night sky. Sometimes Venus appears in the eastern sky before sunrise. Some times it appears in the western sky just after sunset. Therefore it is often called a morning or an evening star, although it is not a star. Try to locate Venus in the night sky. Activity 17.10 Find out from some newspaper or from an almanac the time when Venus is visible in the sky. You can easily recognise Venus by its brightness. Remember that Venus cannot be seen very high in the sky. You must try to observe Venus either 1-3 hours before sunrise or 1-3 hours after sunset. Venus has no moon or satellite of its own. Rotation of Venus on its axis is somewhat unusual. It rotates from east to west while the Earth rotates from west to east. If you get a chance, try to observe Venus through a telescope. You will observe that Venus shows phases just like the moon (Fig. 17.20). The Earth The Earth is the only planet in the solar system on which life is known to exist. Some special environmental conditions are responsible for the existence and continuation of life on the Earth. These include just the right distance from the Sun, so that it has the right temperature range, the presence of water and suitable atmosphere and a blanket of ozone. From space, the Earth appears blue-green due to the reflection of light from water and landmass on its surface. The axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth. The Earth has only one moon. Mars (Mangal) The next planet, the first outside the orbit of the Earth is Mars. It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites. Jupiter (Brihaspati) Jupiter is the largest planet of the solar system. It is so large that about 1300 earths can be placed inside this giant You are familiar with the equator of the Earth. The plane of the equator is called the equatorial plane (Fig. 17.21). The plane in which the Earth revolves round the Sun is called the orbital plane of the Earth (Fig. 17.21). These two planes are inclined to each other at an angle of 23.5º. This means that the axis of the Earth is inclined to its orbital plane at an angle of 66.5º. planet. However, the mass of Jupiter is about 318 times that of our Earth. It rotates very rapidly on its axis. I have an idea! If you take a large ball which can accommodate about 1300 peas, then the ball will represent Jupiter and one pea will represent the Earth. Jupiter has a large number of satellites. It also has faint rings around it. You can easily recognise Jupiter as it appears quite bright in the sky. If you observe it with the help of a telescope, you can also see four of its large moons (Fig. 17.22). Saturn (Shani) Beyond Jupiter is Saturn which appears yellowish in colour. What makes it unique in the solar system is its beautiful rings. These rings are not visible with the naked eye. You can observe them with a small telescope. Saturn also has a large number of satellites. One interesting thing about Saturn is that it is the least dense among all the planets. Its density is less than that of water. Uranus and Neptune These are the outermost planets of the solar system. They can be seen only with the help of large telescopes. Like Venus, Uranus also rotates from east to west. The most remarkable feature of Uranus is that it has highly tilted rotational axis (Fig. 17.24). As a result, in its orbital motion it appears to roll on its side. The first four planets, Mercury, Venus, Earth and Mars are much nearer the Sun than the other four planets. They are called the inner planets. The inner planets have very few moons. The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are much farther off than the inner planets. They are called the outer planets. They have a ring system around them. The outer planets have large number of moons. 17.5Some Other Members of the Solar System There are some other bodies which revolve around the Sun. They are also members of the solar system. Let us learn about some of them. Asteroids There is a large gap in between the orbits of Mars and Jupiter (Fig. 17.25). This gap is occupied by a large number of small objects that revolve around the Sun. These are called asteroids. Asteroids can only be seen through large telescopes. Asteroids Fig. 17.25 : Asteroid belt Comets Comets are also members of our solar system. They revolve around the Sun in highly elliptical orbits. However, their period of revolution round the Sun is usually very long. A Comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail of a comet is always directed away from the sun (Fig. 17.26). Many comets are known to appear periodically. One such comet is Halley’s comet, which appears after nearly every 76 years. It was last seen in 1986. Can you tell, when Halley’s comet will be visible again? Superstitions about the Comets Some people think that comets are messengers of disasters, such as wars, epidemics and floods. But these are all myths and superstitions. Appearance of a comet is a natural phenomenon. We have no reason to be afraid of it. Meteors and Meteorites At night, when the sky is clear and the moon is not there, you may sometimes see bright streaks of light in the sky (Fig. 17.27). These are commonly known as shooting stars, although they are not stars. They are called meteors. A meteor is usually a small object that occasionally enters the earth’s atmosphere. At that time it has a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates quickly. That is why the bright steak lasts for a very short time. Some meteors are large so that they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a meteorite. Meteorites help scientists in investigating the nature of the material from which the solar system was formed. Meteor Showers When the Earth crosses the tail of a comet, swarms of meteors are seen. These are known as meteor showers. Some meteor showers occur at regular intervals each year. You can find the time of their appearance from a scientific magazine or from the internet. Artificial Satellites You must have heard that there are a number of artificial satellites which are orbiting the Earth. You might wonder how artificial satellites are different from the natural satellites. The artificial satellites are man-made. They are launched from the Earth. They revolve around the Earth much closer than earth’s natural satellite, the moon. India has built and launched several artificial satellites. Aryabhatta was the first Indian satellite. Some other Indian satellites are INSAT, IRS, Kalpana-1, EDUSAT, etc. (Fig. 17.28). Artificial satellites have many practical applications. They are used for forecasting weather, transmitting television and radio signals. They are also used for telecommunication and remote sensing. ISRO KEYWORDS WHAT YOU HAVE LEARNTARTIFICIAL SATELLITES ASTEROIDS CASSIOPEIA CELESTIAL OBJECTS COMETS CONSTELLATIONS LIGHT YEAR METEORITES METEORS NATURAL SATELLITES ORBIT ORION PHASES OF MOON PLANETS POLE STAR REMOTE SENSING SOLAR SYSTEM STARS URSA MAJORThe phases of the moon occur because we can see only that part of the moon which reflects the light of the Sun towards us. Stars are celestial bodies that emit light of their own. Our sun is also a star. It is convenient to express distances of stars in light years. Stars appear to move from east to west. The pole star appears to be stationary from the Earth, because it is situated close to the direction of the axis of rotation of the Earth. Constellations are groups of stars that appear to form recognisable shapes. The solar system consists of eight planets and host of asteroids, comets and meteors. A body revolving around another body is called a satellite. Moon is the natural satellite of the Earth. Some planets also have natural satellites. Venus is the brightest planet in the night sky. Jupiter is the largest planet of the solar system. The artificial satellites revolve around the Earth. They are much closer than the moon. Artificial satellites are used for weather forecasting, long distance communication and remote sensing. Exercises Choose the correct answer in Questions 1-3 : 1. Which of the following is NOT a member of the solar system? (a) An asteroid (b) A satellite (c) A constellation (d) A comet 2. Which of the following is NOT a planet of the sun? (a) Sirius (b) Mercury (c) Saturn (d) Earth 3. Phases of the moon occur because (a) we can see only that part of the moon which reflects light towards us. (b) our distance from the moon keeps changing. (c) the shadow of the Earth covers only a part of moon’s surface. (d) the thickness of the moon’s atmosphere is not constant. 4. Fill in the blanks: (a) The planet which is farthest from the Sun is ____________ . (b) The planet which appears reddish in colour is ____________ . (c) A group of stars that appear to form a pattern in the sky is known as a ____________ . (d) A celestial body that revolves around a planet is known as __________. (e) Shooting stars are actually not ____________. (f) Asteroids are found between the orbits of _________ and _________ . 5. Mark the following statements as true (T) or false (F): (a) Pole Star is a member of the solar system. ( ) (b) Mercury is the smallest planet of the solar system. ( ) (c) Uranus is the farthest planet in the solar system. ( ) (d) INSAT is an artificial satellite. ( ) (e) There are nine planets in the solar system. ( ) (f) Constellation Orion can be seen only with a telescope. ( ) 1. If possible, visit a planetarium. There are planetariums in many cities. In a planetarium you can see the motion of the stars, constellations and planets on a large dome. 2. On a moonless night observe the sky for a few hours. Look out for a meteor, which appears as a streak of light. September-November is a good time for observing meteors. 3. Learn to identify the planets visible to the naked eye and some prominent constellations such as Great Bear (Saptarshi) and Orion. Also try to locate the Pole Star and the star Sirius. 4. Select a convenient place on the roof of your house or in some playground from where you can watch sunrise clearly. Remember that you will have to do this activity for a few months. So, choose the place carefully. On a sheet of chart paper draw the outline of the eastern horizon indicating big trees, poles, etc. Mark the outline of the sheet so that you can place it in the same position every time. After every two weeks, note the position from where the Sun rises and mark this on your chart paper (Fig. 17.30). Also note the date of your observation. Repeat this for a few months. It is advisable that you start observations in the month of November or May. 5. Form a group of students. Prepare a model of the solar system showing the planets, and their relative sizes. For this take a large chart paper. Make spheres representing different planets according to their relative size (Use Table 17.1). You may use newspaper, clay or plasticine to make spheres. You can cover these spheres with paper of different colours. Exhibit your models in the class. Table 17.1 Name Approximate Approximate Period Period of Planet radius (assuming Earth as 1 unit) distance from the Sun (Assuming distance of the Earth as 1 unit) of revolution of rotation Mercury 0.40 0.39 88 days 59 days Venus 0.95 0.72 225 days 243 days Earth 1.00 1.00 365.25 days 24 hours Mars 0.55 1.50 687 days 24 hours 37 min Jupiter 11.00 5.20 12 years 9 hours 55 min Saturn 9.00 9.50 29.46 years 10.66 hours Uranus 4.00 19.20 84 years 17.2 hours Neptune 3.90 30.00 165 years 16.1 hours 6. Try to make a scale model of the solar system showing distances of the planets from the Sun (Use Table 17.1). Did you face any difficulty? Explain it. 7. Solve the following riddle and try to make similar riddles yourself: My first is in VAN but not in PAN My second is in EARTH and also in HEAVEN My third is in ONE and not in TWO My fourth is in BUN and also in FUN My last is in STAR but not in RADAR I am a planet that moves round the Sun. You can read more on the following websites : • • • •

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