WBBSE Class 9 Geography Solutions Chapter 1 Earth as a Planet

Detailed explanations in West Bengal Board Class 9 Geography Book Solutions Chapter 1 Earth as a Planet offer valuable context and analysis.

WBBSE Class 9 Geography Chapter 1 Question Answer – Earth as a Planet

Very Short Questions and Answers : (1 mark for each question)

Question 1.
How many true planets are there in the solar system?
Answer:
8.

Question 2.
What is the area of the earth’s surface?
Answer:
Almost 510.72 million sq km.

Question 3.
Name two planets that do not have satellites.
Answer:
Mercury and Venus.

Question 4.
Which planet has the most number of satellites?
Answer:
Jupiter.

Question 5.
What is the estimated mass of the earth?
Answer:
5.97219 × 10²⁴ kg.

Question 6.
What is the meaning of ‘geoid’?
Answer:
Like the earth.

Question 7.
How much time does it take for the light of the sun to reach the earth?
Answer:
8.2 minutes.

Question 8.
What was the circumference of the earth according to Eratosthenes?
Answer:
46,250 km.

Question 9.
Name one Jovian planet.
Answer:
Mars.

Question 10.
What is the only source of light in the solar system?
Answer:
Sun

Question 11.
What is the full form of GPS?
Answer:
Global Positioning System.

Question 12.
What is the minimum number of satellites required to operate GPS?
Answer:
3.

Question 13.
Which planet was designated as a dwarf planet in 2006?
Answer:
Pluto.

Question 14.
Which heavenly body is at the centre of the solar system?
Answer:
The sun.

Question 15.
Which planet is known as the ‘Red Planet’?
Answer:
Mars.

Question 16.
Which heavenly body is the source of light to the solar system?
Answer:
Sun.

Question 17.
What is the greatest distance between the sun and the earth?
Answer:
152.6 million km.

Question 18.
Which is the deepest point on the earth?
Answer:
Challenger Deep in Mariana Trench, in the Pacific Ocean.

Question 19.
At which point will the angle of elevation of the North Star be 90° ?
Answer:
North Pole.

Question 20.
In which direction does the earth rotate?
Answer:
West to east.

Question 21.
What is the full form of IAU?
Answer:
International Astronomical Union.

Question 22.
Which cities of Egypt had Eratosthenes chosen in order to calculate the circumference of the earth?
Answer:
Synene and Alexandria.

Question 23.
Where was the General Assembly of the IAU held on August 24, 2006?
Answer:
Prague, capital of Czech Republic.

Question 24.
In which region is the gravitational force of the earth felt the least?
Answer:
Equatorial region.

Question 25.
Where is the earth’s diameter the greatest?
Answer:
Equatorial region (about 12,757 km).

Question 26.
Which is the largest planet in the solar system?
Answer:
arrow Jupiter.

Question 27.
Where on the earth’s surface is the main control system of GPS located?
Answer:
Colorado, USA.

Question 28.
Till what depth in oceans are organisms found?
Answer:
9 km.

Question 29.
Where was the Bedford Level Experiment conducted?
Answer:
Old Bedford River in England.

Question 30.
What is the shape of the earth really like?
Answer:
Geoid.

Question 31.
From when did GPS become operational?
Answer:
From the 1990 s.

Question 32.
From where do living organisms get the oxygen required for their survival?
Answer:
The atmosphere.

Question 33.
Which motion of the earth has caused the middle portion to bulge out?
Answer:
Rotation.

Question 34.
What does the science of ‘geodesy’ study?
Answer:
The earth’s shape.

Question 35.
Who was the first person to use the word ‘geoid’?
Answer:
Johann Benedict Listing.

Question 36.
What is the estimated age of the earth?
Answer:
About 4600 million years.

Question 37.
What is the average temperature of the earth?
Answer:
15° C.

Question 38.
What is the Kuiper Belt?
Answer:
A region of icy objects beyond the orbit of Neptune.

Short Questions and Answers : (2 marks for each question)

Question 1.
What is meant by the solar system?
Answer:
Solar system is the name given to the sun and the various planets, satellites, comets, asteroids and other heavenly bodies, that move in their unique and specific orbits.

Question 2.
Name the planets of the solar system.
Answer:
The planets of the solar system are as follows-

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune.

Question 3.
What is meant by a star?
Answer:
A star is a gigantic, self-luminous heavenly body made of burning gas. Other celestial bodies such as planets, satellites, comets and asteroids revolve around a star. For instance, the sun is the star of our solar system.

Question 4.
What is meant by a planet?
Answer:
A planet (word derived from Greek term that means ‘wanderer’), is a large heavenly body that revolves in a particular orbit around a star. A planet does not radiate heat or light energy from internal nuclear fusion reactions-it receives heat and light from the star that it revolves around. For instance, our Earth is a planet that revolves around its star, the sun.

Question 5.
What is meant by a satellite?
Answer:
A satellite is a small heavenly object that orbits or revolves around a larger object usually a planet. Like planets, satellites also do not radiate heat or energy from internal nuclear fusion reactions. Except for Mercury and Venus, all the planets in the solar system have satellites. For instance, the moon is a satellite that orbits around its planet, the earth.

Question 6.
How can the planets of the solar system be classified?
Answer:
The planets of the solar system can be classified into two sub-categories. They are as follows-

1. planets
2. dwarf planets.

Question 7.
What is meant by a dwarf planet?
Answer:
‘Dwarf planet’ is defined by the International Astronomical Union (IAU) as a celestial body that –
1. is in orbit around the sun,
2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
3. has not cleared the neighbourhood around its orbit, and (4) is not a satellite. For example, Pluto.

Question 8.
What is meant by geodesy?
Answer:
Geodesy is a scientific discipline that makes use of surveys and other measurements to study the shape and size of the earth.

Question 9.
In which part of the world did GPS first come into use?
Answer:
GPS was first developed and implemented in the USA in the 1970 s by the U.S. Department of Defence and was used only for military purposes.

Question 10.
What is meant by ‘geoid’?
Answer:
The word ‘geoid’ is taken from the Greek word ‘geoeides’. ‘Geo’ means earth and ‘oeides’ means similar to. Therefore, geoid means similar to, or like the earth.

Question 11.
Name the major components of GPS.
Answer:
GPS consists of three major segments. They are-(1) the space segment, (2) the control segment, (3) the user segment.

Question 12.
Why is the earth called the ‘Blue Planet’?
Answer:
From space, the earth appears as a blue and white ball. The clouds in the atmosphere appear white, whereas, the areas covered by water bodies appear blue. As almost three-fourths of the earth is covered with water, it appears mostly blue from space. This is why the earth is known as the ‘Blue Planet’.

Question 13.
Why is the earth’s equatorial diameter not equal to its polar diameter?
Answer:
The equatorial region of the earth experiences centrifugal force and so, it bulges out. On the other hand, the polar regions experience centripetal force and so, are flattened. Therefore, the equatorial diameter of the earth is not equal to that of the polar diameter.

Question 14.
What is meant by GPS?
Answer:
The Global Positioning System (GPS) is a radio-based navigation system that provides a three dimensional coverage of the earth in any weather condition. Till date, it is the only system which is able to show one’s exact position on the earth- any time and anywhere.

Question 15.
What is meant by horizon?
Answer:
The boundary where the sky seems to meet the ground or the sea is known as the horizon. The higher the observer places himself, the more distant the horizon become.

Question 16.
Which experiment of Magellan proved that the earth is spherical?
Answer:
The Portuguese explorer, Magellan, set off on an expedition to circumnavigate the earth from east to west. After three years, he came back to the same place from where he had started. This proved that the earth is spherical in shape, because, had it not been so, he would not have come back to the same place that he had started from.

Question 17.
Where is the Kuiper belt located?
Answer:
The ring of icy bodies that revolves around the sun beyond the orbit of Neptune is known as the Kuiper belt. The outer limit of the Kuiper belt is poorly defined. It contains some big heavenly bodies such as Eris, Pluto as well as millions of other smaller bodies.

Question 18.
What are the effects of centrifugal force on the earth?
Answer:
When a spherical object is continuously rotated, a centrifugal force acts on it. Therefore, the middle part bulges out and at the same time, the polar areas flatten. The earth is continuously rotating on its axis from the time of its origin. This is the reason why the earth has a bulged-out middle and flattened poles, i.e., it has developed an oblate spheroid shape.

Question 19.
What information can be gathered from a GPS receiver?
Answer:
The information that can be gathered from a GPS receiver are as follows –
1. A GPS receiver operated by an user on earth measures the time taken by radio signals to travel from four or even more satellites to its location, calculates the distance of each satellite, and from this calculation, determines the user’s longitude, latitude, and altitude, thereby triangulating his or her exact position.
2. A GPS receiver also displays time. GPS satellites have atomic clocks on board to keep accurate time. General and special relativity however, assess that variations will appear between these clocks and an identical clock on earth.

Short Questions and Answers : (3 marks for each question)

Question 1
Make a comparative study of dwarf planets and satellites.
Answer:
The similarities between dwarf planets and satellites are as follows-
1. Size: Both dwarf planets and satellites are relatively small in size.
2. Light and heat: Neither dwarf planets nor satellites emit light or heat of their own.
The dissimilarities between dwarf planets and satellites are as follow-
1. Revolution: The dwarf planets revolve around the sun, whereas the satellites revolve around their individual, specific planets of origin.
2. Number: The number of dwarf planets is much less compared to the number of satellites.

Question 2
Why is the earth considered to be a planet of the solar system?
Answer:
The earth is considered to be a planet of the solar system because of the following reasons-
1. Origin: The earth has originated from the star of the solar system, the Sun.
2. Revolution: Like the other planets of the solar system (such as Venus, Jupiter), the earth also revolves around the Sun in its own orbit.
3. Light: Like the other planets of the solar system, the earth too, does not emit any light.

Question 3
Why has Pluto been classified as a dwarf planet?
Answer:
One of the characteristics that define planets is that they have enough gravitational force to dominate their neighbourhoods by clearing up the debris near their orbits. Pluto’s orbit is still somewhat cluttered. Pluto is just 0.07 times the mass of the other objects in or near its orbit and there are still many objects similar in size to Pluto moving around in its orbit. Moreover, Pluto is the smallest planet in the solar system. It is only about one-tenth of the size of the Earth. Hence, on August 24, 2006, the International Astronomical Union classified Pluto as a dwarf planet.

Question 4
Curvature of the earth’s surface is lesser at the equator than at the poles-Explain.
Answer:
The earth bulges out at the centre and gets flattened at the poles. Therefore, the earth forms an oblate spheroid. Oblate spheroids have varying curvature from north to south. It is observed that, smaller the circle, greater is the curvature. This is why the curvature of the earth’s surface is lesser at the equator than at the poles.

Question 5
Why is the average distance between the earth and the sun not the same throughout the year?
Answer:
The earth revolves around the sun in an elliptical orbit. As the sun is located at one of the foci of the ellipse, the distance of the earth from the sun varies during its period of revolution. The distance of the earth is maximum on July 4 , when it is 152.6 million kilometre away, and minimum on January 3 , when it is 147.5 million kilometre away from the sun. Had the orbit been a perfect circle, the distance between the earth and the sun would have remained the same throughout.

Question 6.
What roles have gravity and centrifugal force played in shaping the earth as we know it today?
Answer:
At the time of its origin, the earth was a swirling ball of gases and viscous matter. Gravity caused this ball of viscous matter to contract and form a sphere as it pulls the mass of an object as close to the centre of gravity as possible. Centrifugal force comes into effect when an object spins on an axis, and it causes objects to move outward and away from the centre of gravity. The rotation of the earth caused its middle part to bulge out and simultaneously, the polar areas to flatten. Therefore, the gravity and the centrifugal force have played a significant role in shaping the earth.

Question 7
Give a brief description of biosphere.
Answer:
The sum of all ecosystems on earth is known as the biosphere. This includes all the parts of the atmosphere, lithosphere and hydrosphere that are suitable for the sustenance of life. Water, sunrays and the proper structure of the atmosphere has made life pussible on earth, and has also supported the wide range of flora and fauna. The range of the biosphere generally extends from a height of 15 km above the earth’s surface to about 9 km below the sea level in the hydrosphere. Right from the creation of life, various evolutions taking place in the biosphere has supported its expansion. Till date the earth is the only planet in the solar system where the existence of life can be found.

Question 8
How can a pendulum clock be used to confirm the oblate spheroid shape of the earth?
Answer:
In 1671, the French astronomer, Jean Richer, observed the speed of pendulum in a few places of the earth.He noticed that his pendulum clock was running 21 \(1 / 2^{\circ}\) minutes slower when he was in Cayenne (5° N), in South America, the capital of French Guiana. However, the same pendulum clock would show the correct time when at Paris (49° N). This is because, the time period of the pendulum changes due to the difference in the gravitational pull that occurs with change in latitude. If the time period of the pendulum clock at Cayenne is greater than at Paris, it implies that the gravitational force of the earth is lower at Cayenne, indicating that Cayenne is at a greater distance from the centre of the earth than Paris. This confirms that the earth’s equatorial radius is greater than its polar radius – or that the earth’s shape is that of an oblate spheroid.

Question 9
How is the earth unique among all the other planets in the solar system?
Answer:
The earth is unique among all the other planets in the solar system because of the following reasons-

1. It is the largest among all the inner planets.
2. The distance of the earth from the sun is exactly suitable for sustenance of life.
3. The earth is the only planet to have water and so is rightly named the ‘Blue Planet’.
4. The earth’s axis is tilted at an angle of 23 \(1 / 2^{\circ}\).
5. The average temperature on the surface of the earth is 15° C, which is suitable for the growth and sustenance of life.

Write the differences between the following –

Question 1
Star and Planet
Answer:
The differences between a star and a planet are as follows-

Points of difference	Star	Planet
1. Light and heat	Stars emit light and heat of their own. E.g., the sun.	Planets do not emit light and heat of their own. E.g., the earth.
2. Nature	Stars are huge, swirling mass of fiery gases.	Planets are relatively smaller, cool and solid masses.
3. Origin	The stars were formed before the planets.	Planets were formed after the formation of stars.

Question 2
Planet and Satellite
Answer:
The differences between a planet and a satellite are as follows-

Points of difference	Planet	Sotellife
1. Revolution	Planets revolve around a star. E.g., the earth revolves around the sun.	Satellites revolve around a planet. E.g., the moon revolves around the earth.
2. Size	Planets are larger than satellites.	Satellites are smaller than planets.
3. Dependence	Planets are not dependent on satellites.	Satellites are dependent on planets.

Question 3.
Planet and Dwarf planet
Answer:
The differences between a planet and a dwarf planet are as follows-

Points of difference	Planet	Dwarf planer
1. Size	Planets are relatively larger in size.	Dwarf planets are relatively smaller in size. Dwarf planets have not been able to remove other celestial objects near them.
2. Features	Planets have been able to remove other celestial objects near them.	There are 5 dwarf planets in the solar system.
3. Number	There are 8 planets in the solar system.	Dwarf planer

Give reasons for the following –

Question 1.
The Pole Star’s angle of elevation helps to determine that the earth is spherical in shape.
Answer:
The angle of elevation of the Pole Star helps to show that the earth is round. From different places on the earth’s surface in the Northern Hemisphere, the Pole Star is visible at different angles of elevation. At the equator, it is visible at 0°, from the Tropic of Cancer at 23 \(1 / 2^{\circ}\) N, and from the North Pole at 90° N.

Figure 1.1: The angle of elevation of the Pole Star helps to determine that the earth is spherical

This happens only because the earth is spherical. Had the earth been just a plane, the Pole Star would have been visible at the same angle of elevation from all the places on earth.

Question 2.
The gravitational force is more at the polar regions than at the equatorial region.
Answer:
The earth is not a perfect sphere- it is an oblate spheroid. Its equatorial diameter is 12,757 km and its polar diameter is 12,714 km which means there is a difference of 43 km between the lengths of the diameters of these two regions. As the poles are closer to the centre of the earth than the equator, greater gravitational force is exerted on the polar regions.

Question 3.
An object weighs more at the poles than at the equator.
Answer:
The regions that are closer to the centre of the earth experience greater gravitational pull than those farther away from the centre. Since the earth is an oblate spheroid, the poles are closer to the centre of the earth and thus, experience greater gravitational force. As a result, all objects weigh more (about 0.5 %) at the poles than at the equator.

Question 4
The moon appears bigger than all other heavenly objects in the sky, though it is smaller.
Answer:
When we look up at the sky, the moon appears to be the largest object in space. In reality, the moon is much smaller in size than the other planets, satellites and stars. In fact, it is only about 1 / 50th of the earth’s size. However, being the earth’s only satellite, it is also our nearest neighbour in space. The distance between the moon and the earth is only 3,84,400 kilometre. So it appears larger than the other celestial bodies which are much larger, but are at greater distances from the earth.

Long Questions and Answers : (5 marks for each question)

Question 1
Discuss the main planets of our solar system.
Answer:
Solar system is the name given to the sun and the various planets, satellites, comets, asteroids and other heavenly bodies that move in their unique orbits around it in space. The names of the planets and their characteristics are listed in the table below.

Question 2
Describe the earth as a planet.
Answer:
The earth is an important planet in the solar system. The features of the earth as a planet are as follows-
1. Location: The earth is the third planet on the basis of its distance from the sun, after Mercury and Venus.
2. Distance from the sun: The average distance of the earth from the sun is 149.6 million kilometre.
3. Shape: The shape of the earth is like that of an oblate spheroid. The North and South Poles are somewhat flattened and the equatorial region bulges out a little.
4. Mass and area: The mass of the earth is about 5.97219 × 10²⁴ kg and surface area is about 510,072,000 square km.
5. Diameter: The earth’s equatorial diameter is 12,757 km and the polar diameter is 12,714 km (approx.).
6. Speed: The earth revolves around the sun while rotating on its own axis. It takes 23 hours 56 minutes and 4 seconds to complete one rotation on its axis and 365 days 5 hours 48 minutes and 46 seconds to complete one revolution around the sun.
7. Orbit: The earth’s orbit is elliptical, with the sun at one of its loci. The length of the orbit is about 960 million km.
8. Satellite: The earth has only one satellitethe moon. The moon is at a distance of about 3,84,400 km from the earth. It has no light of its own-it is illuminated by the light of the sun.
9. Structure: The exterior of the earth is made up of a hard rock strata and its interior is made up of hot, viscous magma. About 71 % of the earth’s surface is made up of water and 29 % is land. The earth’s atmosphere is primarily made up of nitrogen and oxygen.
10. Sustenance of life: The earth is the only planet in the solar system that sustains life as we know it.

Question 3.
“The earth is not flat, it is instead almost spherical.” Discuss with evidence.
Answer:
Though earlier the earth was believed to be a flat plane or a disc, it is actually almost spherical in shape- in fact, an oblate spheroid to be precise. A practical demonstration of the earth’s spherical shape was achieved by Magellan’s circumnavigation of the earth (1519-1522). Recent photographs taken from outer space have also shown that the earth is spherical in shape. Some other points in support of this view are-
1. The study of other planets: When other planets of our solar system have been studied through powerful telescopes, all of them have been observed to be spherical. Since the earth is also a part of the solar system, it stands to reason that the earth would also be spherical like the other planets.

2. The study of the earth’s shadow: When one studies the shadow of the earth that falls on the moon during a lunar eclipse, it is seen to be circular in shape. If an object casts a shadow that is circular in shape, it stands to reason that the object itself is also circular in shape.

Figure 1.3: The circular shadow of the earth on the moon during a lunar eclipse

3. Observing a ship sailing out to sea: If we observe a ship that has just set sail from the shore, we will see that the ship is not visible

Figure 1.4: A sailing ship slowly disappears from our eyesight as the earth is spherical in shape after some time even with the help of binoculars or telescopes. This happens because of the curve of the earth’s surfaceif the earth were a flat plane, the ship would never disappear from our line of sight.

4. Observing the horizon: The boundary where the sky seems to meet the ground or

Figure 1.5: As the observer moves higher, the circumference of the line of horizon increases the sea is known as the horizon. The visible horizon marks the end of our line of sight. As we go higher up in an aeroplane or up a mountain, the visible horizon becomes more distant and its circumference seems to increase – as a result of which the line of horizon appears circular. If the earth were a flat plane, the circumference of the line of horizon would not increase with increase in altitude of the position of the observer; instead, it would have remained the same.

5. Bedford Canal Experiment: In 1870 , renowned naturalist and surveyor Alfred Russel Wallace (A.R. Wallace) put in 3 sticks of equal height and diameter in a straight line at 1 kilometre interval in the slowmoving Old Bedford River in the United

Kingdom. Through a telescope, he then observed that the second stake seemed to be higher than the first and third stakes. This proved that the earth is round. If the earth were flat, the tops would be in a straight line.

6. Sunrise and sunset: The spherical shape of the earth causes the countries in the Eastern Hemisphere to experience sunrise and sunset earlier than those in the Western Hemisphere. If the earth were flat then sunrise and sunset would occur at the same time at all places on the earth.

Question 4.
Why is the earth called an oblate spheroid? Give reason for your answer. [2 + 3]
Answer:
A sphere that has a bulged-out centre and flattened poles is defined as an oblate spheroid. The equatorial region of the earth is bulged out and the polar regions are flattened due to the rotation of the earth, and this is why the earth is considered an oblate spheroid.
Evidence that the earth is an oblate spheroid:
1. Polar diameter is less than equatorial diameter: The equatorial diameter of the earth is 12,757 km whereas the polar

Figure 1.8: The difference between the earth’s equatorial diameter and its polar diameter
diameter is 12,714 km. If the earth would have been a perfect sphere, then both the diameters would have been the same.

2. Difference in time given by the same pendulum clock: In 1671 the French astronomer, Jean Richer, observed that his pendulum clock was running 2 \(\frac{1}{2}\) minutes slower when he was in Cayenne (5° N), the capital of French Guiana, in South America, However, the same pendulum clock would show the correct time when at Paris (49° N). This is because the beat of the pendulum is different with the difference in the gravitational pull with change of latitude.

Sir Isaac Newton used this example later to show that places nearer to the centre of the earth experience stronger gravitational pull. This confirms that earth’s equatorial radius is greater than its polar radius – or that the earth is oblate spheroid in shape.

Figure 1.9: The time period of oscillation is greater at the equatorial region than at the poles

3. Rotational movement of the earth: When a spherical object rotates continuously a centrifugal force acts on it. Therefore, the middle part bulges out and at the same time, the polar areas flatten. The earth is continuously rotating on its axis from the time of its origin. This is the why the earth has become an oblate spheroid in shape.

4. Variation in weight: The regions that are closer to the centre of the earth experience greater gravitational pull than those farther away from the centre. Since the earth is an oblate spheroid, the poles are closer to the centre of the earth and thus, experience greater gravitational force. As a result, all objects weigh more (about 0.5 % ) at the poles than at the equator.

5. Variation in curvature: Oblate spheroids have varying curvature from north to south. It is observed that, smaller the circle, greater is the curvature. In case of the earth, the curvature is observed to be lesser at the equator than at the poles. This means that the earth is bulged out at the centre and flattened at the poles. Therefore, from the variation in curvature, we can deduce that the earth is an oblate spheroid.

Figure 1.10: Bulging of the equatorial region of the earth due to rotation on its own axis

Question 5.
How does the earth look from space?
Describe the actual shape of the earth. [2 + 3]
OR,
‘The shape of the earth is geoid.’Explain.
Answer:
From space, the earth appears to be a sphere, half of which is lit by the rays of the sun and the other half is in darkness. The areas covered by water bodies appear blue, the mountain and plateau regions appear brown, the forest areas appear green and the areas under snow or cloud cover appear white. As almost 71 % of the earth is covered by water bodies, it appears mostly blue from space and is also called the ‘Blue Planet’.

The shape of the earth is geoid (like the earth itself): As a spherical object is continuously rotated, a centrifugal force acts on it. Therefore, the middle part bulges out and at the same time, the polar areas flatten. As the earth is continuously rotating on its axis from the time of its origin, the oblate spheroid shape was formed. Later on, further studies revealed that the shape of the earth is not a perfect oblate spheroid either. This is because of the following reasons-

1. Various types of landforms such as mountains, hills, plateaus, valleys and rifts are present on the earth’s surface. Apart from this, the water bodies with their waves, high and low tides are also present. The highest point on the earth’s surface is Mount Everest (8,848 m), and the lowest point is Challenger Deep (10,994 m) in Mariana Trench in the Pacific Ocean. Therefore, the surface of the earth is undulating and uneven.

Figure 1.11: The actual shape of the earth (geoid)

2. Recent data from artificial satellites show that –

1. The South Pole of the earth is more flattened than the North Pole.
2. The North Pole is about 20 m higher and the South Pole is 20 m lower than perfect oblate spheroid shape. If it were a perfect oblate spheroid, both would have been the same.
3. The latitudinal circumference is 8 m more in the Southern Hemisphere than in the Northern Hemisphere.

Therefore, the shape of the earth cannot be compared with the shape of any other object on earth. So, it can be said that the shape of the earth is like the earth itself, or that the earth is ‘geoid’.

Question 6.
Give a brief note on GPS. How does the GPS help in understanding the shape of the earth?
[2 \(\frac{1}{2}\) + 2 \(\frac{1}{2}\)]
Answer:
The Global Positioning System (GPS) is a radio-based navigation system that is able to locate the exact position of someone or something on the earth – at any time. A further elaboration of the system is given below:
1. Location: Several artificial satellites have been launched in space to orbit the earth at an altitude of about 20,200 km. The functioning of GPS requires a minimum of three artificial satellites.

2. Components: GPS consists of three major parts. They are-

• the space segment
• the control segment
• the user segment

3. Information: GPS is mainly capable of providing four types of information about a place. They are-

• ‘X’ = Latitude
• ‘Y’ = Longitude
• ‘Z’ = Altitude
• ‘T’ = Time.

4. Usage: Though GPS was first developed in the USA in the 1970 s by the U.S. Department of Defence for military purposes, but the use of GPS for everyday use became common from the 1990 s. This system helps us to-

• find out the exact location of a place on the earth’s surface,
• find out the average elevation of a place,
• determine the direction for flying aircrafts and sailing ships
• make maps for various development projects.

Application of GPS in understanding the shape of the earth: As the receiver of the GPS on the surface of the earth receives the signals sent from the artificial satellites positioned in space, the exact position on the earth can be determined. The GPS signals are so accurate and powerful that even a small variation in height can be easily read. The following example will further explain this –
For instance, say three points (A, B and C) are marked at a distance of 10 m on the same latitude. The GPS information from all these places are noted, where, ‘ X ‘ = Latitude, ‘ Y ‘ = Longitude, ‘Z’ = Altitude and ‘ T ‘ = Time.

A graphical representation of the entire data will give us a clear picture of the variation of the height of the places studied.

Question 7.
How did the Greek scholar Eratosthenes calculate the circumference of the earth? Write a note on the use of GPS. [3 + 2]
Answer:
In the third century B C, Eratosthenes was the first person who attempted to calculate the size of the earth, with an assumption that the earth is a perfect sphere. Eratosthenes observed that on June 21 (Summer Solstice), the sun in Syene 23 \(\frac{1}{2}\) ° N is exactly vertical at noon. On the same day in Alexandria 3012° N, the angle between the sun’s rays and the vertical was 7° 12′. Now, 7° 12′ is (360° 7° 12′) = 1 / 50 part of the circumference of the earth. Eratosthenes had already calculated the actual distance between Alexandria and Syene, which he found to be 5000 stadia. From these, he calculated the circumference of the earth to be 5000 stadia x 50 = 2,50,000 stadia, or 46,250 km. (Now the circumference of the earth has been measured to be 40,000 km.)

Figure 1.13: Eratosthenes’s mathematical calculation of the circumference of the earth

The Global Positioning System (GPS) is a radiobased navigation system that is able to locate the exact position of an object on the earth-at any time and any where. The applications of GPS are as follows-

1. Town planning: Nowadays, GPS is used for town planning in most urban areas.
2. Shape of the earth’s surface: GPS is used to determine the shape of the earth’s surface.
3. Distance and ruggedness: GPS is used to determine the difference in distance and ruggedness between two places on the surface of the earth.
4. Transportation system: GPS is useful for determining the direction in order to fly aircrafts and sail ships or even for determining road routes to different places.
5. Defence: Currently, GPS is used to track movement and carry out rescue operations by defence forces.
6. Geometric location: The geometric location of any object on the surface of the earth can be traced with the help of GPS.
7. Weather: Information on weather can be found through the use of GPS and dependable forecasts can be made.
8. Cartography: Cartography becomes easier, more informative and more accurate if drawn on a computer using data gathered from GPS.
9. Rescue work: GPS helps in tracking any aircraft or ship that has met with an accident or has gone missing.
10. Routing: GPS plays a significant and useful role in tracking routes on the surface of the earth.

Question 8.
‘Earth is the abode of man’ – Elucidate.
OR,
Describe why life has been found only on the earth instead of all the other planets in the solar system.
Answer:
Among all the planets in the solar system, the earth is the only planet where life exists. This is because of suitable living conditions on earth. So, earth has become the home to man and other living organisms. The reasons for this are as follows-
1. Suitable temperature: The earth receives light and heat from the sun. Some of it is retained and the rest is reflected back by the atmosphere-this helps to regulate the temperature of the earth and maintain it at an average of 15° C, which is suitable for sustenance of life forms. Both the planets that are nearer to the sun (such as Mercury and Venus) and planets far away from the sun (such as Jupiter and Saturn) cannot sustain life because either they are too hot or too cold.

2. Abundant availability of water: Life is not possible without water. Water constitutes 71 % of the earth’s surface – this is another major reason for the existence of life on earth. No traces of water have been found in other planets like Jupiter or Venus.

3. Availability of oxygen: Oxygen content in the earth’s atmosphere is about 21 %. Oxygen is used by life forms to breathe and survive.

4. Availability of food: The earth’s environment is favourable for growing food. All the necessary elements such as sunlight, water, gaseous elements (O and N), mineral components (Fe, Cu) are found in suitable proportions on earth. The green plants make their food from these elements and the food chain continues.

5. Sunlight: Sun is the source of all known energy, and life is not possible without sunlight. The first link of any food chainthe autotrophs – use solar energy to produce their food.

Figure 1.14: Factors which sustain life on earth

6. Other factors: Life has become possible on the earth also because of the following reasons-

• Right proportion of gases in the atmosphere.
• Weather elements such as formation of cloud, rain, wind flow and others.
• Occurrence of day and night.
• Occurrence of seasons and various other factors.