Selina Concise Solutions for ICSE Class 9 Geography Chapter 1 Earth as a Planet

Exercises

I. Short Answer Questions

Question 1. How can you prove that the earth is a sphere by looking at the Pole Star ?
Answer: The Pole Star can be seen at an angle of \( 90^{\circ} \) at the North Pole, as it lies directly at the line of axis of the earth, while its angle decreases towards the Equator i.e. it is \( 0^{\circ} \) at the equator. It is only possible in an quarter arc of a circle. It proves that the earth is a sphere with circular surface.
In simple words: If the Earth were flat, the North Star would stay in the same spot in the sky no matter where you moved. Because the star seems to sink lower as you move toward the Equator, it proves you are traveling along a curved surface like a ball.

📝 Teacher's Note: Use a globe and a laser pointer to show how the "angle" of the light changes relative to the surface as you move from the top to the middle. This visual aid makes the geometry clear.

🎯 Exam Tip: Remember to mention the specific angles: \( 90^{\circ} \) at the North Pole and \( 0^{\circ} \) at the Equator. These are the key data points examiners look for.

Question 2. Briefly describe the shape of the earth.
Answer: The earth has got a spherical shape, slightly bulging at the equator and flattened at the poles. The spherical shape is confirmed by the arrival of a ship seen first by its mast and later on the ship. The satellite picture of the earth, shadow of the earth on the moon as circular, and the views of circular horizons from the height etc. all these prove the spherical shape of the earth.
In simple words: The Earth is shaped like a slightly squashed ball—it is a bit wider around the middle and a bit flat at the top and bottom. We know this because of photos from space and how ships appear from the horizon.

📝 Teacher's Note: Introduce the term "Oblate Spheroid" or "Geoid" to students. Explain that the Earth is not a perfect marble but more like a ball someone is gently pressing down on.

🎯 Exam Tip: List at least three proofs in your answer, such as satellite photos, the circular shadow on the moon, and the ship's mast experiment.

Question 3. What is the earth’s mean temperature ? State its one advantage.
Answer: The earth’s mean temperature is \( 17^{\circ}\text{ C} \), which is ideal to support the survival of the life-system.
In simple words: The Earth's average temperature is about \( 17^{\circ}\text{C} \). This is perfect because it is not too freezing like Mars and not boiling hot like Venus, allowing things to grow.

📝 Teacher's Note: Contrast this with the extreme temperatures of other planets. This helps students understand how narrow the "Goldilocks zone" for life really is.

🎯 Exam Tip: Always include the unit "Celsius" and mention that this temperature allows for the "survival of life."

Question 4. Why is the earth called a watery planet ?
Answer: The earth is called a watery planet, as its major part i.e. \( 70\% \) is covered by water or hydrosphere. The earth is seen blue in the satellite pictures due to the majority of water.
In simple words: Earth is called the watery planet because most of its surface is covered by oceans. This makes it look like a bright blue marble from outer space.

📝 Teacher's Note: Mention that while most of the water is salty, it is the cycle of this water that keeps our climate stable and provides fresh water through rain.

🎯 Exam Tip: Use the statistic "\( 70\% \)" and the term "hydrosphere" to make your answer more scientific.

Question 5. What is ‘biosphere’ ?
Answer: Biosphere is the narrow zone of contact amidst lithosphere, hydrosphere and atmosphere.
In simple words: The biosphere is the thin layer on Earth where land, water, and air meet. This is the only place where living things like plants and animals can survive.

📝 Teacher's Note: Think of the biosphere as the "skin" of an apple—it is very thin compared to the whole Earth, but it is where all the life happens.

🎯 Exam Tip: The keyword "narrow zone of contact" is essential. Make sure you list the three other spheres: lithosphere (land), hydrosphere (water), and atmosphere (air).

Question 6. Name the conditions necessary for life on earth.
Answer: The conditions necessary for life on earth, are ideal temperature range with an average between \( 10^{\circ}\text{C} \) and \( 35^{\circ}\text{C} \), solar radiation, humidity, wind, water, land etc.
In simple words: To have life, a planet needs the right temperature, sunlight, water, soil, and breathable air. Earth provides all of these perfectly.

📝 Teacher's Note: Ask students to imagine if one of these was missing (like water or sunlight) and what would happen to life. This encourages critical thinking about ecology.

🎯 Exam Tip: Include both the "ideal temperature range" and physical elements like "water and solar radiation" in your list.

Question 7. Give any two features of the earth that make it a ‘Unique Planet’.
Answer: The earth is so for the only planet which consists of life- system. It is due to its ideal average temperature of \( 17^{\circ}\text{C} \) to support the life and provide with atmosphere and hydrosphere to impart air and water essential for life.
In simple words: Earth is unique because it is the only known home for living things. This is possible because it has a protective atmosphere and plenty of liquid water.

📝 Teacher's Note: Emphasize that "Unique" means one-of-a-kind. While other planets have atmospheres, Earth's is the only one with the right mix of oxygen for us.

🎯 Exam Tip: Focus on the "life-system" and the "ideal temperature" as the two most important points.

Question 8. Why is the planet Venus hotter than the planet Mercury?
Answer: Venus is hotter than Mercury because the atmosphere around Venus is mainly composed of carbon dioxide. The carbon dioxide produces Greenhouse effect on the surface of Venus. Thus, the temperature on its surface remain very high.
In simple words: Even though Mercury is closer to the Sun, Venus is hotter because its thick clouds of carbon dioxide act like a heavy blanket, trapping heat inside and not letting it out.

📝 Teacher's Note: Use the analogy of a car parked in the sun with the windows rolled up to explain the Greenhouse effect. The heat goes in but cannot easily get back out.

🎯 Exam Tip: The term "Greenhouse effect" and the gas "carbon dioxide" are the most important keywords for this explanation.

Question 9. Name the two sources of heat in the interior of the earth.
Answer: Two sources of heat in the interior of the earth are the radioactive elements trapped inside and the immense temperature and pressure in the deep interior.
In simple words: The inside of the Earth is hot because of heavy metals that give off radiation energy and the massive crushing pressure from the weight of the planet itself.

📝 Teacher's Note: Remind students that the deeper you go into Earth, the hotter it gets—this is why volcanoes erupt with molten rock.

🎯 Exam Tip: List "radioactive elements" and "pressure" as your two primary sources.

Question 10. What is meant by Terrestrial Life ?
Answer: On the surface of the earth, there are continents and oceans. The life on the continents or on land is called ‘Terrestrial Life’.
In simple words: Terrestrial life refers to all the animals and plants that live on land instead of in the water.

📝 Teacher's Note: The word "Terrestrial" comes from the Latin word 'Terra,' meaning Earth or land. Examples include lions, trees, and humans.

🎯 Exam Tip: Differentiate clearly between land (terrestrial) and ocean (aquatic/marine) life to show complete understanding.

Question 11. Name the members of the Solar System.
Answer: Members of the solar system includes the Sun and everything that orbits it like planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), their satellites, asteroids, comets, interplanetary gas and dust.
In simple words: The Solar System is a big family with the Sun at the center. It includes the eight planets, moons, space rocks (asteroids), and icy comets.

📝 Teacher's Note: Have students recite the order of planets using a mnemonic like "My Very Educated Mother Just Served Us Noodles."

🎯 Exam Tip: Don't just list the planets; remember to mention the "Sun" and "asteroids/comets" as well.

Question 12. What is a planet ?
Answer: A planet is a rocky or gaseous mass that revolves around a star.
In simple words: A planet is a giant object in space that circles around a sun and doesn't produce its own light.

📝 Teacher's Note: Clarify that planets shine because they reflect light from their star, unlike stars which are burning balls of gas that make their own light.

🎯 Exam Tip: Define it as a body that "revolves around a star."

Question 13. What is meant by Inner Planets ? Name them.
Answer: The planets that lie within the asteroid belt that is Mercury, Venus, Earth and Mars are called the inner planets.
In simple words: Inner planets are the four small, rocky worlds closest to the Sun, separated from the bigger planets by a belt of rocks.

📝 Teacher's Note: Inner planets are generally smaller and have solid surfaces you could stand on.

🎯 Exam Tip: Associate "Inner Planets" with being "inside the asteroid belt."

Question 14. Why are the inner planets called the terrestrial planets?
Answer: The inner planets are called the terrestrial planets because their structure is similar to that of the earth.
In simple words: We call them "terrestrial" because, like Earth, they are made mostly of rock and metal.

📝 Teacher's Note: Point out that the word "Terrestrial" means Earth-like. This is why we don't call Jupiter terrestrial—it's made of gas!

🎯 Exam Tip: Mention the "similarity to Earth" as the reason for the name.

Question 15. What are Jovian planets ? Name them.
Answer: The planets whose structure is similar to that of Jupiter are – called Jovian planets. They have ring systems around and have a large number of moons. They are of gaseous origin. Saturn, Uranus and Neptune are Jovian planets.
In simple words: Jovian planets are the "gas giants." They are huge, have many moons, and usually have beautiful rings like Saturn.

📝 Teacher's Note: "Jovian" comes from 'Jove,' another name for the Roman god Jupiter. These planets are mostly made of hydrogen and helium gas.

🎯 Exam Tip: Note the key features: ring systems, many moons, and "gaseous origin."

Question 16. How big is the sun ? Describe its size with respect to the earth.
Answer: The sun is a star or a ball of hot gases. Its diameter is \( 1,392,000 \text{ km} \). i.e. \( 110 \) times larger than that of earth and its volume is \( 1.3 \) million times larger than the earth.
In simple words: The Sun is unimaginably huge. You could fit over a million Earths inside it, and it is more than \( 100 \) times wider than our planet.

📝 Teacher's Note: Use a tiny bead for the Earth and a large exercise ball for the Sun to show the scale difference. It helps students grasp the Sun's dominance in our system.

🎯 Exam Tip: Memorize the numbers "\( 110 \) times wider" and "\( 1.3 \) million times more volume."

Question 17. Distinguish the earth from the other planets in one important aspect.
Answer: Our earth is also called a ‘lonely-planet’ because of its one important aspect having extensive life system.
In simple words: Earth stands out from every other planet because it is the only one we know of that is teeming with life.

📝 Teacher's Note: The phrase "lonely planet" refers to Earth being the only known biological oasis in a vast, empty space.

🎯 Exam Tip: The keyword here is "extensive life system."

Question 18. How did the earth get its atmospheric blanket ?
Answer: In the beginning, at the time of evolution of the planets, the earth was also a body of burning clouds of gases and dust. After cooling process, the heavy elements subsided downwards and the gases floated upwards and formed the atmosphere around the earth as a blanket.
In simple words: When Earth was very young and hot, it was a ball of fire and gas. As it cooled down, heavy rocks sank to the center, and the lighter gases stayed on the outside to form our air.

📝 Teacher's Note: This process is called "differentiation." Gravity pulled the heavy stuff in and let the light gases escape to the surface.

🎯 Exam Tip: Describe the "cooling process" and how "heavy elements subsided" while "gases floated upwards."

II. Fill in the blanks

1. The Pole Star can be seen at an angle of \( 90^{\circ} \) at the North Pole.
2. The earth rotates from west to east.
3. Biosphere is the narrow zone of contact between lithosphere, hydrosphere and atmosphere.
4. The earth has an average temperature of \( 17^{\circ}\text{ C} \).
5. Atmosphere is the major reservoir of carbon on the earth.
6. In terms of size, the earth is fifth planet in the Solar System.
7. Eratosthenes worked out the circumference of the earth to be \( 46,250 \text{ km} \).
8. A typical galaxy may contain large cluster of stars.

📝 Teacher's Note: These are foundational facts. Have students practice the direction of rotation (West to East) as it explains why the Sun "rises" in the East.

🎯 Exam Tip: Be careful with the number 5—Earth is the 3rd planet from the sun, but the 5th largest in size.

III. Long Answer Questions

Question 1. Study the picture and answer the questions that follow.

(a) What does the picture show you about the earth ?
(b) State briefly the Bedford Level Experiment.
(c) The sun rises and sets at different times in different places. What does the statement show about the shape of the earth.
(d) In what way is the earth’s atmosphere beneficial to mankind ?

Answer:
(a) The picture shows the spherical shape of the earth.

(b) Three pillars of equal height were put along the Bedford Level canal area in England each at \( 5 \text{ kms.} \) interval. After observation it came to know that the middle pole was higher than the sideward poles. This proved that the earth’s surface was curved and the earth is a spherical body.

(c) The times of sunset and sunrise are different in the different part of the earth, because the earth moves from west to east and the places situated in the east get sunrise first. Japan is called the land of rising sun, as it is in the eastern-most part of the Eastern Hemisphere and the date starts from the Eastern Hemisphere. It proves that the earth is spherical.

(d) The atmosphere is indispensable for life on earth, as it provides air to inhale ; rainfall and fresh waterbodies after the hydrological cycle. Moreover, it absorbs the extremely hot ultraviolet rays of the sun and makes the temperature ideal on earth. Other planets are either very hot or very cold due to the absence of atmosphere.

In simple words: This picture explains that Earth is a ball. If you place three sticks in a straight line on a long lake, the middle one looks higher because the water is curving. Because Earth is round and spins, different countries see the Sun at different times.

📝 Teacher's Note: The Bedford Level experiment is a brilliant historical proof. Explain that if Earth were flat, the tops of all three sticks would align in a perfect straight horizontal line.

🎯 Exam Tip: For subpart (c), explicitly mention the "West to East" rotation as the reason for different sunrise times.

Question 2. Provide reliable evidence to prove that the earth is spherical in shape.
Answer: The earth can be clearly seen spherical in the photographs from the satellite. Other proofs are the sight of the mast of a ship first before the ship appears, varying heights of the poles fired at different places at an interval of \( 5 \text{ km} \), through the Bedford Level Experiment and the sight of Pole Star at \( 90^{\circ} \) from North Pole and the view of the earth’s horizon as circular from higher altitudes.
In simple words: We have lots of proof Earth is a sphere: we have photos from space, we see the top of a ship first as it comes toward us, and the North Star's position changes as we travel.

📝 Teacher's Note: Use the "ship coming over the hill" analogy. When a ship arrives, you see the chimney first, then the deck, then the whole body—just like someone walking up a hill.

🎯 Exam Tip: Include "Satellite imagery" as your strongest piece of modern evidence.

Question 3. Give the distinctive features of the earth as a unique planet. Also give three points of comparison with respect to other planets.
Answer: The earth possesses all the essential features necessary for maintaining the life system. It is provided with atmosphere and hydrosphere along with the lithosphere. Moreover the average temperature of \( 17^{\circ}\text{C} \) is ideal for survival. Three points of comparison with respect to other planets are as follows.
1. Other planets are either too hot or too cold that are unfavourable for life.
2. Other planets are devoid of atmosphere and hydrosphere and some are surrounded by poisonous gases.
3. Other planets are devoid of fresh water bodies, like rivers and lakes.
In simple words: Earth is the only planet with the "perfect package" for life: liquid water, air to breathe, and a comfortable temperature. Other planets are usually dead rocks that are too hot, too cold, or filled with poison gas.

📝 Teacher's Note: This is a great place to discuss the concept of "Sustainability." Earth is the only place in the universe we know can sustain us without a space suit!

🎯 Exam Tip: Use the comparison points to show why life *cannot* exist elsewhere (no water, bad air, extreme heat).

Question 4. Describe the role of each of the following in making earth a habitable planet.
(a) Atmosphere
(b) Water
(c) Temperature
Answer:
(a) Atmosphere — Atmosphere play very important role in making earth a habitant planet. Atmosphere plays following role:
1. The atmosphere receives heat from the sun by solar radiation and loses heat by earth’s radiation. In this way a balance is maintained.
2. The earth’s atmosphere is made up of life supporting gases like nitrogen, oxygen and carbon dioxide. Other gases include Helium and Argon.
3. Ozone present in the earth’s atmosphere absorbs the harmful ultraviolet rays of the sun.
4. The atmosphere also prevents loss of heat from the earth’s surface and helps to keep the earth warm.

(b) Water — \( 70\% \) of the earth’s surface is covered by water. Water plays following role :
1. It is responsible for moderating the climate and surface condition of the earth.
2. Water from seas, rivers and lakes evaporates into the atmosphere where it condenses and falls back as precipitation. Most of the water on land flows back to the oceans. Thus, water moves in a continuous cycle — the hydrological cycle. There is proper balance between evaporation, condensation and precipitation without which life would not be possible.
3. Water can absorb enormous amounts of heat without causing much change in its temperature. During the day, waterbodies rapidly absorb enormous amount of heat : thus, the earth remains fairly cool. At night the waterbodies release vast amounts of heat that they absorbed during the day, which along with other atmospheric effects, keep most of the surface from freezing at night.

(c) Temperature — The earth is the third planet from the sun. It has an average temperature of \( 17^{\circ}\text{C} \) which is suitable for life to exist. If the average temperature on the earth’s surface changes by only a few degrees, many species would perish due to extreme heat or cold.

In simple words: Air gives us oxygen and blocks deadly rays from the Sun. Water moves in a big cycle to give us rain and keeps the planet from getting too hot or too cold. The temperature is just right, like a warm room.

📝 Teacher's Note: Highlight the importance of the Ozone layer specifically in the atmosphere section. For water, focus on the "heat sponge" effect—how oceans keep us warm at night.

🎯 Exam Tip: For the atmosphere, remember the term "Ozone" and "heat balance." For water, use the term "hydrological cycle."

Question 5. Name the different realms of the earth.
Answer: The different realms of the earth are lithosphere, hydrosphere and atmosphere.
In simple words: The three main parts of Earth are the hard ground (land), the liquid oceans (water), and the air above us.

📝 Teacher's Note: Ask students to name one living thing from each realm (e.g., earthworms in lithosphere, fish in hydrosphere, birds in atmosphere).

🎯 Exam Tip: Just list the three terms clearly. Make sure you spell them correctly!

Question 6. Study the picture and answer the questions that follow.

(a) What is meant by the Solar System ?
(b) Give two differences between a planet and a star.
(c) Name the planets known as terrestrial planets.
(d) Give two advantages the earth has over other planets.
(e) What are satellites ?

Answer:
(a) The sun along with eight planets and asteroids forms the Solar System.

(b)
1. The stars like the sun radiate heat and light. Planets have no light of their own. They reflect the light of their stars.
2. The stars are large burning masses very far from us while the planets are solid bodies nearer to us.

(c) The planets within the asteroids belt i.e. Mercury, Venus, Earth and Mars, are called the Terrestrial planets.

(d)
1. The earth enjoys an ideal average temperature of \( 17^{\circ}\text{C} \)
2. The earth consists of the life-system.

(e) The satellites are the celestial bodies that revolve around the planets.

In simple words: The Solar System is the Sun and its family. Stars are massive fireballs that make light, while planets are smaller rocks that just reflect it. Satellites are smaller objects, like the Moon, that circle around planets.

📝 Teacher's Note: Use the diagram to point out the distinction between "Inner" and "Outer" planets. Inner are rocky, Outer are gas giants.

🎯 Exam Tip: For the planet vs star difference, always use "light" as the primary distinction (Stars create light, Planets reflect it).

Question 7. Write a short note on the planets of the Solar System. Name the planets in order of their distance from the sun.
Answer: There are total eight planets included in the solar system which revolve around the sun in their particular elliptical tracks. According to their distance from the sun from near to far, these are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
In simple words: There are eight planets that circle the Sun in oval-shaped paths. From closest to farthest, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

📝 Teacher's Note: Note that Pluto is no longer classified as a major planet (it is a dwarf planet), which is why the list ends with Neptune. However, older textbooks might still list nine.

🎯 Exam Tip: Be very careful not to mix up the order. Use the "MVEMJSUN" mnemonic to keep them straight.

Question 8. Distinguish between a planet and a star.
Answer:
A Planet :
1. A planet revolves around a star
2. A planet has not got its own light.
3. A planet is a solid body.

A Star :
1. A star remains still on its position.
2. A star has got its own light.
3. A star is a burning gaseous body.

In simple words: Stars are stationary, burning balls of gas that glow. Planets are moving, solid objects that orbit stars and only shine because they are hit by starlight.

📝 Teacher's Note: While stars appear "still" from Earth, explain that they are actually moving through space, but are so far away that we don't notice it in our lifetimes.

🎯 Exam Tip: Contrast "burning gaseous body" (star) with "solid body" (planet).

Practice Questions (Solved)

Question 1. In which unit the distances of universe are measured ?
Answer: Light year.
In simple words: Space is so big that kilometers are too small to use. We use "light years," which is how far light travels in one whole year.

🎯 Exam Tip: Always remember that a light year measures distance, not time.

Question 2. How much time does a ray of sunlight take to reach the Earth ?
Answer: \( 8 \) minutes approximately.
In simple words: The light you see right now actually left the Sun about \( 8 \) minutes ago!

Question 3. How much time does a ray of moonlight take to reach the Earth ?
Answer: One second approximately.
In simple words: Because the Moon is so close to us, its light reaches Earth in just a second.

Question 4. How many planets are there in the Solar System ?
Answer: Eight
In simple words: There are eight main planets circling the Sun.

Question 5. Name the largest planet of the Solar System.
Answer: Jupiter.
In simple words: Jupiter is the giant of the family—it's the biggest planet of all.

Question 6. Name the planet closest to the Sun.
Answer: Mercury
In simple words: Mercury is the Sun's nearest neighbor.

Question 7. Name the planet farthest from the Sun.
Answer: Neptune.
In simple words: Neptune is the loneliest planet at the very edge of the system.

Question 8. What is position of the Earth from the Sun ?
Answer: Third
In simple words: Earth is the third "rock" from the Sun.

Question 9. Name the planets between the Sun and the earth.
Answer: Mercury and Venus.
In simple words: Only Mercury and Venus are closer to the Sun than we are.

Question 10. Which planet is known as ‘blue planet’ ?
Answer: Earth
In simple words: Earth is the Blue Planet because of all its water.

Question 11. Which planet is known as ‘red planet’ ?
Answer: Mars
In simple words: Mars is the Red Planet because its surface is covered in rusty dust.

Question 12. Which is the brightest planet ?
Answer: Venus
In simple words: Venus is the brightest thing in the night sky other than the Moon.

Question 13. Which planet has the largest number of satellites ?
Answer: Jupiter.
In simple words: Giant Jupiter has dozens of moons circling it.

Question 14. Which planet has only one satellite ?
Answer: Earth
In simple words: Earth has just one companion moon.

Question 15. Which planet has three rings round it ?
Answer: Saturn.
In simple words: Saturn is famous for its bright, wide rings.

Question 16. Which star is known as ‘Evening Star or “Morning Star” ?
Answer: Venus
In simple words: Venus is often the first "star" to appear at night and the last to vanish in the morning.

Question 17. How many satellites are there in Solar System ?
Answer: \( 100 \) approximately.
In simple words: There are roughly a hundred moons across all the planets.

Question 18. Which is the centre of the Solar System-Sun or Earth?
Answer: Sun
In simple words: The Sun is the boss; everything revolves around it.

Question 19. What is the period of rotation of Moon ?
Answer: \( 27 \) days \( 7 \) hours \( 43 \) minutes.
In simple words: It takes the Moon about a month to spin once on its axis.

Question 20. ‘Although the Moon has no light of its own, yet it shines’. Why ?
Answer: Sun’s light is reflected from the surface of the moon.
In simple words: The Moon is like a mirror; it doesn't make its own light, it just bounces the Sun's light back to us.

Question 21. Mention two important features of Earth that make it a planet suitable for life.
Answer: (a) Presence of atmosphere (b) Presence of water
In simple words: Air and water are the two ingredients that let us live on Earth.

Short Answer Questions

Question 1. State three unique features of the Earth.
Answer: There unique features of earth are as follows :
1. It is neither too hot nor too cold.
2. It had atmosphere surrounding it.
3. There is plenty of water on it.
In simple words: Earth is special because it has perfect weather, air to breathe, and lots of water to drink.

Question 2. State three factors which have made life possible on the Earth and on no other planet.
Answer: The factors which have made life possible on earth and on no other planet are as follows :
1. Earth is the only planet in the Solar System which is neither too hot nor too cold.
2. The atmospheric blanket around the earth protects it from ultra-violet and other deadly sun rays.
3. The mixture of gases in the atmosphere is suited to growth and survival of life.
4. Plants, through the process of photosynthesis take away carbon dioxide from atmosphere and make oxygen available for animals to breathe.
5. Oceans play a great supportive role. They cover three-fourths of earths surface and are responsible for operation of hydrological cycle which makes water on land surface available.
6. Circulation of many life cycles not make energy and food available but also process the wastes which otherwise would harm life.
In simple words: Earth protects us with its air "blanket," keeps us at a good temperature, and has plants that create oxygen for us.

🎯 Exam Tip: Mentioning "Photosynthesis" and the "Hydrological Cycle" shows advanced knowledge of how Earth supports life.

Question 3. Give reasons for the following :
(a) Earth is called a “Watery Planet”.
(b) Earth has the most ideal temperature conditions.
(c) Earth and its Moon are called a double planet.
Answer:
(a) Because the Earth is the only planet which has water and \( 70\% \) if its surface is covered by water.
(b) Earth has the most ideal temperature conditions because it is neither too close nor too distant from the sun. This position in the Solar System makes it possible to receive neither excessive nor totally deficient heat from the Sun. Its average surface temperature is \( 17 \) degrees Celsius far more favourable for living conditions than about \( 500^{\circ}\text{C} \) on the nearest planets and about \( 150^{\circ} \) to \( 200^{\circ} \) Celsius below freezing point on the sunlit sides of distant planets. Earth is, thus, neither a burning furnace nor an extremely cold planet.
(c) Earth and its Moon are called a double planet because they revolve round the sun nearly in one and the same orbit.
In simple words: Earth is watery because it's covered in oceans. It has great temperature because it sits in the perfect middle spot in space. We call it a "double planet" because the Moon is so big that it and Earth travel through space together.

Question 4. Draw a fully labelled diagram of water cycle and explain its working and significance with reference to the diagram.
Answer: The water is evaporated from the surfaces of open oceans, seas, rivers, lakes and from the surface of living organisms. The evaporated water gets condensed, precipitated and form the water droplets. These water droplets are returned to the earth in the form of rain and snow or it may drop directly into the oceans. If it falls on land, it again flows back into the oceans through rivers and streams. The evaporation rainfall cycle repeats several times to get the average rainfall.

In simple words: The Sun heats the ocean, turning water into vapor (Evaporation). This vapor cools down to make clouds (Condensation). When the clouds get too heavy, they release water as rain or snow (Precipitation), which flows back to the ocean to start again.

📝 Teacher's Note: Remind students that plants also contribute to this cycle through "transpiration," which is like plants "sweating" water into the air.

🎯 Exam Tip: Use the three key terms: Evaporation, Condensation, and Precipitation. Labeling these correctly in a diagram will get you full marks.

Question 5.
(a) What makes our Sun as an ordinary star in the universe?
(b) What heavenly bodies are the members of the Solar System?
(c) Arrange the nine planet of the solar system in order of their sizes.
(d) Why do planets, being opaque bodies, shine in the sky?
Answer:
(a) There are several galaxies in the limitless universe. Each galaxy consists of millions of stars like the Sun. Thus, the Sun is an ordinary star in no way different from many of the other stars.

(b) Components of the Solar System : Our Solar System consists of the Sun and its nine planets, major and minor, \( 63 \) satellites, a large number of very small planets called asteroids, millions of other heavenly bodies called meteors or shooting stars and the scores of comets. All these components are held together by the Sun’s great gravitational pull.

(c) The largest planet is Jupiter and the smallest is Mercury. According to their size the planets are :
1. Jupiter
2. Saturn
3. Neptune
4. Uranus
5. Earth
6. Venus
7. Pluto
8. Mars
9. Mercury

(d) Planets are opaque heavenly bodies. They radiate no light of their own but shine with the light that is reflected from the Sun.

In simple words: The Sun is just one of billions of stars in space. Our Solar System is held together by the Sun's gravity. Planets shine because they bounce sunlight off their surfaces, like a ball glowing when you shine a flashlight on it.

Question 6. Distinguish between the following pairs.
(a) Superior and Inferior planets.
(b) Planet and Planetoids
(c) Periodical and Non-periodical comets
(d) Meteors and Meteorites
(e) Planet and Satellite.
(f) Inner and Outer planets.
Answer:
(a) Superior Planets : The six planets outside the Earth’s Orbit are known as the superior planets (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto).
Inferior Planets : The two planets inside the Earth’s orbit are known as the inferior planets (Mercury and Venus).

(b) Planets : The planets are bigger bodies and visible as nine planets. These move in their elliptical orbits around the sun. They have particular names.
Planetoids : These are smaller bodies in the form of debris. About \( 50,000 \) planetoids revolve round the Sun between Mars and Jupiter.

(c) Periodical comets : Comets which have regular orbits and occur at fixed intervals. Halley’s comet appears every \( 76 \) years.
Non-periodical comets : Such comets are seen very rarely and do not appear at regular intervals.

(d) Meteors : Often called "shooting stars," these are pieces of rock that burn up in Earth's atmosphere, creating a streak of light.
Meteorites : These are the pieces of meteors that do not burn up completely and actually fall on the Earth's surface.

(e) Planet : Opaque bodies that revolve around the Sun and originate from it.
Satellite : Small spherical bodies that revolve around a particular planet.

(f) Inner Planets : Planets similar to Earth (small, rocky), including Mercury, Venus, Earth, and Mars.
Outer Planets : Planets not similar to Earth (large, gaseous), including Jupiter, Saturn, Uranus, Neptune, and Pluto.

In simple words: Superior planets are farther from the sun than we are. Meteors burn in the sky, but meteorites actually hit the ground. Planetoids are just tiny "space crumbs" between Mars and Jupiter.

🎯 Exam Tip: For Meteors vs Meteorites, the key difference is "burning in air" vs "hitting the ground."

Question 7. (a) How many satellites are there in the Solar System?
(b) Name the planet having no satellite.
(c) Which planet has the largest number of satellites. Or Name the largest known satellite of any planet.
Answer: (a) There are \( 57 \) known satellites in our Solar System (as per source text).
(b) Mercury, Venus and Pluto.
(c) Titan, one of the \( 22 \) satellites of Saturn is the largest known satellite of any planet. Its size is larger than mercury.
In simple words: Mercury and Venus are "lonely" because they have no moons. The biggest moon in the system is Titan, which circles Saturn and is even bigger than the planet Mercury!

Question 8. Why are the following planets not inhabitable ?
(a) Mercury
(b) Venus
(c) Jupiter
(d) Neptune
(e) Saturn
Answer:
(a) Mercury is not inhabitable because one side is too hot (\( 450^{\circ}\text{C} \)), the other is too cold (\( -150^{\circ}\text{C} \)), and it has almost no air.
(b) Venus is inhabitable because its air is almost all carbon dioxide and it is the hottest planet (\( 480^{\circ}\text{C} \)).
(c) Jupiter is not inhabitable because it is too cold (\( -148^{\circ}\text{C} \)), has no water, and has poisonous air like ammonia and methane.
(d) Neptune is not inhabitable because it is freezing (\( -216^{\circ}\text{C} \)) and surrounded by methane clouds.
(e) Saturn is mostly made of light gases and is less dense than water, meaning it has no solid surface to stand on.
In simple words: Other planets are "deadly" because they are either like ovens, like freezers, or are made of poison gas instead of solid ground.

Question 9. Name the following:
(a) Two planets which are nearer to the sun than the Earth.
(b) Two planets which are farther from the sun than the Earth.
(c) The planet farthest from the Sun.
(d) The planet nearest to the Sun.
Answer: (a) Mercury and Venus (b) Jupiter and Saturn (c) Pluto (d) Mercury
In simple words: Mercury and Venus are our "inner neighbors," while Jupiter and Saturn are "outer neighbors." Mercury is the Sun's closest neighbor.

Question 10. (a) Which is the unit adopted for measuring the distance in the Universe ? Give a reason for its choice.
(b) Find out the number of years to reach the following celestial bodies from the Earth : Nearest star from the Earth (Distance 150 million km)
Answer: (a) The unit adopted for measuring the distances in the universe is light year. The reason for the choice of this unit is that a kilometer is too short a unit to measure the massive distances between stars.
(b) \( 15.8 \) light year (as per source text).
In simple words: A kilometer is too tiny for space, like trying to measure the distance between cities using a toothpick. So we use "light years."

Question 11. Which unit is used for measuring distances in the universe?
Answer: The unit used for measuring distances in the Universe is light year. One light year is defined as the distance traveled by light in vacuum in one year. One light year = \( 9.5 \times 10^{12} \text{ km} \).
In simple words: A light year is the distance light can zoom across in one year—it's about \( 9.5 \) trillion kilometers!

Question 12. What makes the Sun so hot ?
Answer: The Sun is a large hot gaseous body. Such extremely high temperature is produced by the conversion of hydrogen into helium by the process of fusion. This process releases tremendous amounts of the energy.
In simple words: The Sun is a giant nuclear engine. It takes hydrogen gas and smashes it together to make helium, which releases an incredible amount of heat and light.

Question 13. Name the three planets which have rings around them.
Answer: 1. Saturn 2. Uranus 3. Neptune
In simple words: While Saturn's rings are the most famous, Uranus and Neptune have them too!

Question 14. Give reasons for the following :
1. Mercury completes its orbit in less time than the Earth.
2. Venus is considered as the Earth’s twin.
3. No life is possible on Saturn.
4. Pluto is the coldest planet.
5. Comets appear very rarely.
Answer:
1. Mercury is closer to the Sun, so its orbit is much shorter.
2. Venus is similar to Earth in size, weight and density.
3. Saturn is made mostly of light gases and is extremely cold.
4. Pluto is the farthest planet from the Sun, so it receives very little heat.
5. Comets have very large and highly eccentric (oval) orbits, taking many years to return.
In simple words: Mercury is on the "inside track," so it finishes its lap faster. Venus is our "size-twin." Comets are like space travelers that only visit us once in a lifetime.

Question 15. What do you understand by ‘Terrestrial Planet’ ?
Answer: The first four planet in order of their distance from the Sun, Mercury, Venus, Earth and Mars are called terrestrial planets, because of their similarity with the Earth.
In simple words: Terrestrial planets are the rocky worlds that have a hard surface you can actually stand on.

Question 16. Why does the moon appear big in the sky though it is smaller than other heavenly bodies ?
Answer: The moon appears big because it is much nearer to the earth than any other heavenly bodies.
In simple words: It's all about perspective. A marble right in front of your eye looks bigger than a mountain that is miles away.

Question 17. The shape of the earth is not exactly that of a sphere Why?
Answer: The shape of the earth is not exactly that of a sphere due to centrifugal force created by the rotation of the earth around its own axis.
In simple words: Because the Earth spins so fast, it "bulges" out at the middle, like a ball of dough being flattened slightly by a baker.

🎯 Exam Tip: Use the term "Centrifugal force" to explain the bulge at the Equator.

Question 18. Why is Mercury the hottest planet of the Solar System?
Answer: Mercury is the smallest planet and the closest to the Sun. Due to its nearness to the Sun, it is the hottest planet (Note: Venus is scientifically hotter, but based on distance, Mercury is most exposed).
In simple words: Being the Sun's closest neighbor makes Mercury's daytime surface incredibly hot.

Question 19. Why is Neptune the coldest planet of the Solar System?
Answer: Neptune is the farthest planet from the Sun. Due to its extreme remoteness, it is the coldest planet of the Solar System.
In simple words: Neptune is so far away that the Sun's warmth hardly reaches it, making it a frozen world.

Question 20. ‘Venus is considered Earth’s Twin’. Why ?
Answer: Venus is similar to the Earth in size, weight and density. Both the planets are very close to each other.
In simple words: If you put Earth and Venus next to each other, they would look almost like twins because they are the same size.

Question 21. (a) Describe the position of the Earth in Solar System.
(b) What is the size of the Earth ?
(c) Which is the satellite of the Earth ?
Answer: (a) The Earth is the third planet of the Solar System, lying between Venus and Mars.
(b) Its equatorial diameter is \( 12,756 \text{ km} \) and polar diameter is \( 12,713 \text{ km} \). Its circumference is about \( 40,000 \text{ km} \).
(c) The Moon is the only satellite of the Earth.
In simple words: Earth is the third planet out from the Sun. It is about \( 12,700 \text{ km} \) wide and has one moon.

📝 Teacher's Note: The difference in diameter (\( 12,756 \) vs \( 12,713 \)) proves the Earth is not a perfect sphere.