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Question 1

Why do we classify the sun as a star?

Sun is classified as a star because it
  • has its own source of energy.
  • is continuously emitting huge amount of heat and light.
  • has a life period. It was born 5 billion years ago and is expected to glow for another 5 billion years.
  • Question 2

    What are celestial bodies? Name some of these.

    The heavenly bodies revolving in some fixed orbits in space are called celestial bodies. For examples, the stars, the sun, the moon, the planets and shooting stars.

    Question 3

    Why do celestial objects appear to move across the sky from east to west?

    The celestial objects appear to move across the sky from east to west because the earth is moving about its axis from west to east. It takes nearly 24 hours for the earth to complete one revolution about its axis from west to east, but to us it appears that our earth is stationary and the various objects in space are moving from east to west.


    Question 4

    Stars are not visible during the day. Explain.

    Stars are far away from us compared to the star called sun. Because of the distance other stars appear to us like a point and are not visible during the day because of the glare of bright sunlight.


    Question 5

    Define the unit in which astronomical distances are measured.

    The size of stars and the distance between them is enormous. To express such large distances, a unit called light year is commonly used. One light year is equal to the distance traveled by light in one year. Light travels 3 × 105 km in one second. A year has 365 × 24 × 60 × 60 = 3.1536 × 107 seconds. Thus, light will travel a distance of 3 × 105 × 3.1536 × 107 km in one year. This is equal to 9.46 × 1012 km. Thus one light year is equal to the distance of 9.46 × 1012 km.


    Question 6

    What distinguishes Pole Star from other stars? Explain.

    Pole Star (or ‘Dhruva Tara’) appears stationary in the celestial sphere unlike other stars which seem to be moving.

    Pole Star has been one of the most familiar stars to travelers in earlier times to find directions at night. It appears stationary because it is situated in the north direction along the axis of rotation of the earth.

    Question 7


    How many constellations are identified? How are they named? Give examples.

    Until now about 88 constellations have been identified i.e., each of these eighty-eight bunches have been identified to have definite boundaries. Each constellation has been assigned a name according to its resemblance either to a human being, an animal or some other known object. For example, the cluster of seven stars (Saptarishi) arranged so as to resemble a plough is called Ursa Major and is a part of the Great Bear. The arrangement is as shown in Fig. Seven bright stars form the body and tail of a big bear. The head and paws are formed from other faint stars not shown in the figure.


    Saptarishi (Great bear, plough)


    Another constellation is also very striking and is called orion (vyadha) the pattern is as shown in Fig.

    (b) Orion means hunter. Seven bright stars are arranged to form the body of hunting man. The head and limbs of the hunter are formed from other faint stars not shown in figure.


    Question 8

    In which part of the sky and at what time of year are the following stars and constellations seen: great bear, the pole star and the orion.


    As the earth orbits the sun, different constellations seem to rise and set like the sun in the sky. Therefore, different constellations are visible at a given position on the earth at different times of the year.

    Great Bear: (saptarishi) constellation can be distinctly seen during April in summer in the Northern skies.

    Pole Star: (Dhruva Tara) is the star which appears to be stationary near the Northern horizon. As the star is located in equatorial plane at a very large distance, it appears to be stationary. It can also be located by a line joining the pointer stars, of the great bear, and extending it in the north direction, as shown below in Fig.  It is seen throughout the year in night sky exactly above the north pole of the earth.

    Orion: This constellation is visible during winter in the northern skies.

    Question 9

    In which season of the year, the following constellations are seen?


    (i) Orion

    (ii) Ursa major (great bear).

    (i) Orion Constellation: Visible during winter season in the northern skies.

    (ii) Ursa major (great bear) Constellation: Visible in April in the northern part of the sky. 


    Question 10

    Two constellations are seen in the sky having seven prominent stars in each, Name the one which appears during winter. Draw its diagram showing the position of stars.


    Two constellations having seven prominent stars are: Ursa major (Great bear) and Orion. Orion appears in winter and appears as is below in figure.



    Question 11

    Give an account of phases of moon.


    Moon does not emit by light itself. It reflects the light of sun falling on it. Thus, that part only of the moon appears bright which is towards us. The moon along with earth is revolving around the sun. As a result the relative positions of the earth and the moon are continuously changing. Thus, when the earth lies in between the moon and the sun, the full face of the moon is visible to us. This day is called ‘Purnima’ or Full moon day.

    On the other hand, when the sunlight falls on that side of the moon which is on the other side of the earth, moon is not visible to us. This day is called ‘Amavus’ or New moon day. The next day only a crescent of moon gets illuminated and thus a crescent of the moon becomes visible in the western horizon immediately after the sun set. The size of the illuminated portion of the moon goes on increasing till on the fifteenth day, the full moon appears. Then again the bright portion of the moon gradually decreases till on the fifteenth day, it becomes new moon day. The different bright portions of the moon visible to us are called phases of moon.

    Question 12

    Give important features of Moon.


  • The moon is a natural satellite of the earth.
  • It revolves round the earth on a definite regular path.
  • The moon is about one-fourth the size of the earth and its weight is about eighth that of the earth.
  • Its surface is covered with hard and loose dirt, craters and mountains.
  • On the moon, days are extremely hot and nights are very cold.
  • The moon has no light of its own.
  • The silvery moon light is actually the light of the sun which is reflected by the moon’s surface.
  • Question 13

    Write two distinguishing features of a constellation.

    Constellation has two following characteristics:
  • A constellation is a group of a few stars.
  • A constellation appears to resemble an animal or a human being or some other known object in shape
  • Question 14

    Why do stars appear to be permanent to us even though these are born, grow and then die?
    The stars appear to be permanent because the physical characteristics of a star like brightness, temperature, colour and size change very very slowly with time as compared to our life span. It takes millions of years for a noticeable change to occur in a star whereas a man stays on earth for 70-75 years on an average. Therefore, a star appears to be permanent even though it is slowly dying.

    Question 15

    Which planet of the solar system is (a) largest and (b) smallest in size?

    (a) Largest planet: Jupiter,

    (b) Smallest planet: Mercury.

    Question 16


    Name the planets that were known to ancient astronomers.

    Planets known to ancient astronomers are:

    (a) Budh (Mercury)

    (b) Shukra (Venus)

    (c) Prithvi (Earth)

    (d) Mangal (Mars)

    (e) Brihaspati (Jupiter)

    (f) Shani (Saturn)

    Question 17

    Name the planet that was predicted before it was actually observed.


    Neptune-U.J. Levessier predicted the existence of planet, called neputne, beyond Uranus in 1846 that was later observed.


    Question 18

    What are planets? How is a planet different from a star?

    Planets are celestial bodies revolving round the sun. They do not have their own light. They reflect sun’s light. Out of nine planets of solar system, only five are clearly visible to the naked eye. Planets keep changing slightly day after night with respect to the stars.

    Difference between stars and planets can be summed up as follows:




    1. Stars have their own light.

    1. Planets do not have their own light. They just reflect the light of the star to which they being.

    2. Stars twinkle
    2. Planets do not twinkle.
    3. Their number is infinite in the celestial sphere.
    3. Their number is only nine in the solar system.
    4. The relative distance between stars remains constant.
    4. The relative distance between the planets changes with time.
    5. Stars are very big in size. They appear small as they are very far off.
    5. Planets are relatively smaller.
    6. a star is a huge mass of extremely hot gases and its temperature is very high.
    6. A planet is made of rocks and metal and its temperature depends on its distance from the sun.
    7. Stars are stationary relative to planets, though a star along with its solar system is moving through million of stars.
    7. Planets move round the sun from west to east.

    Question 19

    State the physical conditions of different planets.

    Physical conditions of different planets

    (i) Mercury (Budha)

    1. It is the closest planet to the sun. Its distance from sun is 57 ´ 106 km. Since it is very close to the sun, most of the time it is hidden in the glare of the sun. It can be visible before the sun rise in the east and after the sun set in the west. It appears quite bright and correspondingly it is termed as ‘morning star’ and ‘evening star’. It is termed as star because it appears very bright in the sky.


    2. It is of the same size as moon.

    3. It revolves round the sun in 88 days and takes 58 days to complete one rotation about its own axis.

    4. Life cannot exist on mercury due to lack of atmosphere and extremes of temperature [340° C ] (-150° C) and it has no protective blanket around it to save it from harmful radiations.

    5. The surface features of mercury resemble those of moon more than those of the earth.

    6. It has no moon or satellite of its own.

    (ii) Venus (Shukra)

    1. Its distance from the sun is 108 × 106 km.

    2. It completes its orbit round the sun is 225 days.

    3. It has almost the same radius, density and mass as that of earth. Thus, it is called twin of earth.

    4. It is the brightest planet and appears as a morning and evening star.

    5. The surface temperature of venus is about 450° C and it is covered by thick blanket of cloud made up of CO2, H2, O2, N2. No life is possible on this planet because of high temperature, absence of water and sufficient oxygen.

    6. It has no moon or satellite of its own.

    (iii) The Earth (Prithvi)

    1. Its distance from the sun is 149 × 106 km.

    2. It has plenty of water, oxygen in the atmosphere and is neither too cold nor too hot, making life possible on this planet.

    3. It takes days to complete one revolution round the sun and 24 hours to complete one rotation round its axis.

    4. It has thick blanket of ozone layer high up in its atmosphere to save life from harmful effects of ultraviolet radiations coming from the sun.

    5. It has one satellite called moon.

    (iv) Mars (Mangal)

    1. Its distance is 227 × 106 km from sun.

    2. It takes 687 days to complete one revolution round the sun and 24 hours to complete one rotation round its axis.

    3. It has slightly reddish appearance.

    4. It has two natural satellites or moons named Phobos and Deibos.

    5. Unlike Mercury and Venus it can be seen in any part of the night sky.

    6. The day temperature varies from 5° C to 15° C and there is no evidence as yet of life on Mars.

    7. It has no protective blanket to protect it from harmful solar radiations.

    (v) Jupiter (Brihaspati or Guru)

    1. Its distance from sun is 778 × 106 km.

    2. It takes 12 years to complete one revolution round the sun.

    3. It is the largest planet and is more massive than the combined mass of other planets of the solar system.

    4. It has a dozen satellites or moons. Four of them are quite large and bright and can be seen with a low power telescope.

    5. There is a faint ring consisting of extremely small particles around Jupiter.

    (vi) Saturn (Shani)

    1. After Jupiter, Saturn is the second biggest planet of the solar system.

    2. It looks like a large yellow star to the naked eye.

    3. It possesses well-developed set-of rings around it. These rings consist of particles whose sizes vary from tiny specks to rocks measuring a few kilometers in diameter.

    4. It is at a distance of 1427 × 106 km from the sun.

    5. It takes about 29.5 years to complete one revolution round the sun.

    6. It is said to have 30 satellites or moons.

    (vii) Uranus (Arun)

    1. This is the seventh planet from the sun and is 2870 × 106 km away from sun.

    2. It takes 84 years to complete one revolution round the sun.

    3. It has 21 satellites or moons.

    4. It rotates about its axis from east to west in contrast to other planets which rotate from west to east.

    5. Its atmosphere contains hydrogen and methane.

    (viii) Neptune (Varun)

    1. It is the eighth planet in terms of its distance from the sun.

    2. It has 8 satellites revolving around it.

    3. Its distance from sun is 4505 × 106 km.

    4. It takes 165 years to complete one revolution round the sun. 


    Question 20

    State the features of moon and mercury that have close resemblance.


    Moon and mercury resemble each other in the following respects:

    1. Both are nearly similar in size and mars.

    2. Both reflect light of sun.

    3. Both have no atmosphere.

    4. Both have rocky and mountainous surfaces.

    Question 21

    Name a celestial body which is not a star but is called a star. Explain.

    Venus is a planet. It is called as morning star or evening star due to its appearance as a very bright dot just before sunrise for about eight weeks in the eastern sky and for about eight weeks in the western region of the sky immediately after sunset.


    Question 22

    Name the brightest celestial body that appears at night. Explain.

    Venus appears brightest of all celestial bodies including all planets, the moon, the stars which are visible to us at night. This is because venus has a lot of cloudy atmosphere which helps it to reflect almost 80% of the sunlight that falls on it.


    Question 23

    Describe the changes in season on the earth due to its rotation around the sun and the tilting of its axis of rotation.

    Earth is rotating about its axis through its north and south pole. It takes 20 hours in one rotation. The day and night on earth occur due to its rotation. The axis of rotation of the earth is slightly tilted with respect to the plane of its orbit around the sun. Further the tilting of the rotation of the earth is always in same direction. The earth completes one revolution around the sun in 365.25 days. As a result, the tilting of the northern and southern hemispheres of the earth towards the sun keeps on changing throughout the year.

    (a) We experience summer when the northern hemisphere is tilted towards the sun.

    (b) We experience winter when the southern hemisphere is tilted towards the sun.

    (c) We experience autumn and spring when the earth is between above two extreme positions.

    Question 24

    What are the distinguishing features of the planet ‘Saturn’ or ‘Shani’?


    Saturn has two distinguishing features.

    1. There are three beautiful rings that encircle the planet.

    2. It has the 30 satellites or moons, largest amongst all the planets.

    Question 25

    Stars twinkle and planets do not twinkle. Justify.

    The stars are very far away from the earth and appear as dots or points. The point position of the stars vibrate due to disturbances by air currents and hence they appear to twinkle. Since the planets are much nearer and their discs position does not appear to vibrate due to air currents etc. in the atmosphere and hence planets do not twinkle.


    Question 26

    Why do stars appear as points while planets appear as discs?

    It is because the distance of the stars from us are much more than that of the planets.


    Question 27

    How does the motion of a satellite differ from that of a planet?

    A satellite revolves around some planet in a fixed orbit while a planet revolves around the sun in a fixed orbit. Planets revolve faster than satellites.


    Question 28

    Differentiate between ‘a star’ and ‘a shooting star’.



    Shooting Star

    1. A star is made up of hot gases like hydrogen. 1. A shooting star is made up of rock and metal.
    2. A star is self-luminous. 2. A shooting star has no light of its own.
    3. A star does not get destroyed due to friction. 3. A shooting star burns due to heat of friction when entering the atmosphere of the earth.
    4. A star is very big in size. 4. A shooting star is very very small.

    Question 29

    Mention the four independent motions of the earth.

    The earth has the following four independent motions:

    1. Revolution of the earth in elliptical orbit round the sun.

    2. Rotation of the earth about its own axis, once in 24 hours.

    3. Spinning of the earth about its axis.

    4. Motion of the earth along with the sun and other planets in space.

    Question 30

    Name different types of celestial bodies that are members of solar system.

    Sun is the head of the solar family. There are nine planets in our solar system. Their names are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Other members of solar family are: Comets, Asteroids and Meteros. The motion of all members of the solar family is governed mainly by the gravitational force of the sun.


    Question 31

    What is the difference between a meteor and a meteorite?

    Meteors: Meteors are stony or metallic bodies of very small size, traveling in interplanetary space and become visible when they travel through the earth’s surface. The meteors are also called shooting stars. Some meteors are the dust particles left behind by comets and others are pieces of asteroids which have collided. These are quite small, some may be as small as grain of sand.

    Meteorite: While traveling through the earth’s atmosphere, these fast moving bodies (meteors) get heated up to a very high temperature by air friction. The heat produced is so high that the meteor starts glowing and then burnt and lost. The path of the meteor appears as a streak of light in the night sky. Very large meteors are able to survive from such heat destruction and actually reach the earth’s surface. These meteors are called meteorites.

    Question 32


    What is the solar system? What are its constituents?

    The sun, planets revolving around it and all the objects bound to the sun due to gravity is called the solar system. The solar family consists of eight planets with their moons, asteroids and comets. Mercury is the planet nearest to the sun followed by Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

    The space between Mars and Jupiter has a belt of pieces of rocks of various sizes which somehow failed to form a planet. The largest piece is 700 km in diameter while smallest is about 100 m in size. This belt is called Asteroid belt.

    Question 33


    What is a comet? Name the prominent comet seen in early 1986.

    Comets are heavenly bodies. They are large, loose collections of material that penetrate into the inner regions of the solar system from outside. They have a bright head and a long tail. The head is made up of heavy particles and the tail is made up of gases like ammonia and methane and always points away from the sun. They move in elliptical orbits. They are small (of about 10 km diameter) when away from the sun but when they reach near the sun, the solar radiations heats up the material which expands and forms a head of 10,000 km diameter. Once in many years one gets a chance to see a comet with a naked eye.

    The name of the prominent comet seen in early 1986 is Halley’s comet.

    Question 34

    What are Asteroids? Name the largest seen Asteroid.

    Asteroids: They are small lumps of rocks orbiting round the sun between Jupiter and Mars. They are considered to be remains of a much larger planet which broke up due to gravitational effect of Jupiter. These are small in size. The size of an asteroid may vary from barely a kilometer to a few hundred kilometers. Ceres is the largest known asteroids. Its diameter is 633 k and it was discovered in 1801. There are about 100,000 asteroids.

    Question 35

    Name the periodic comet. Why is it so called?

    Halley’s Comet is periodic comet. It is called periodic comet because it appears after regular interval of time. A comet becomes visible when it approaches the sun because the sun’s rays makes the gas glow. Halley’s Comet has a period of 76 years, i.e., it is seen after every 76 years.


    Question 36

    Why cannot we draw the solar system to scale?

    We cannot draw the solar system to scale because he planets are very big and their distances is thousands of kilometers. If we take sun such as a ball of 10 cm in diameter, the earth would e 11 metres away on the paper.


    Question 37

    What are meteorites?

    Sometimes large objects called asteroids break up into smaller pieces when they approach the sun, and then move towards the earth. Most of the fragments burn up due to friction during the fall through our atmosphere. Sometimes pieces which are not able to burn up completely actually reach the surface of the earth. These objects are called meteorites.


    Question 38

    Why does the tail of a comet always point away from sun?

    Since the tail of a comet is caused due to light radiations from the sun, hence it always points away from it.


    Question 39

    Explain why the number of meteors striking the moon’s surface is quite large whereas very few reach the earth’s surface?

    The number of meteors striking the moon’s surface is quite large because moon has no atmosphere (air) to burn the falling meteors by frictional heat. On the other hand, the earth has an atmosphere containing air, so many of the meteors burn up completely on entering the earth’s atmosphere due to heat produced by friction between falling meteors and air.


    Question 40

    Why are meteors commonly called shooting stars?

    When the meteors enter the atmosphere of the earth at high speeds, these are heated up. The heat produced is so high and the meteors start glowing and eventually evaporate within a short time. The glowing meteor bursts leaving behind a streak of light. The meteors are, therefore, called shooting stars.


    Question 41

    Name the two planets between which lie the orbits of asteroids.

    Jupiter and Mars.


    Question 42

    Name the bodies of the solar system that have failed to assemble into a planet.



    Question 43

    Classify the following into planets, satellites, stars and constellation.

    Moon, Alpha Centuri, Neptune, Orion, Mercury, Ursa Major, Sun, Phobos

    Planets: Neptune, Mercury

    Stars: Alpha Centuri, sun.

    Constellations: Orion, Ursa Major.

    Satellite: Moon, Phobos.

    Question 44

    Which of the following is not a star?

    (i) Alpha century

    (ii) Pole Star

    (iii) Shooting Star

    (iv) Sun

    (iii) Shooting Star.

    Question 45

    Name any two artificial satellites.

    (i) INSAT 3E or Kalpana Chawla I for communication purposes.

    (ii) Remote sensing satellite IRS series.

    Question 46

    What is the difference between a natural and an artificial satellite?


    Natural satellites are:
    (i) heavenly bodies revolving around a planet in their fixed orbits.

    (ii) These are comparatively bigger in size and are made up of rocky materials or gases. Artificial satellites are

    (i) man made bodies fitted with sophisticated instruments and cameras and are

    (ii) made to rotate around their planets in pre-fixed orbits. These are much closer to their planet. E.g., earth.

    Question 47

    Why is moon not suitable as a communication satellite?


    Moon cannot be used for communication because

    (i) it does not rotate in the equatorial plane of the earth.

    (ii) It is not geostationary.

    (iii) Distance between moon and earth is very large.

    Question 48

    Name the planet that is known to have largest number of natural satellites.


    Saturn has 30 satellites.


    Question 49

    Name all the planets of the solar system in order of their distance from Sun.

    Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are in order of increasing distance from the sun.


    Question 50

    Name the planets that were discovered only after the invention of telescopes.

    Uranus, Neptune and Pluto could be observed only after telescopes became available.

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