Detailed explanations in West Bengal Board Class 9 Geography Book Solutions Chapter 2 Movements of the Earth offer valuable context and analysis.
WBBSE Class 9 Geography Chapter 2 Question Answer – Movements of the Earth
Very Short Questions and Answers : (1 mark for each question)
Question 1.
Which movement of the earth is responsible for causing days and nights?
Answer:
Rotational movement.
Question 2.
What is the path of the sun’s apparent annual movement called?
Answer:
Ecliptic.
Question 3.
In which part of the earth is the speed of rotation minimum?
Answer:
The Polar region.
Question 4.
On which day does the autumnal equinox occur?
Answer:
September 23.
Question 5.
On which day does winter solstice take place?
Answer:
December 22.
Question 6.
Which region of the earth experiences six months of night?
Answer:
The Polar region.
Question 7.
Who proved that “the earth rotates round the sun.”
Answer:
Galileo.
Question 8.
On which date does the Northern Hemisphere experience its longest day?
Answer:
June 21.
Question 9.
Where can we see Aurora Australis?
Answer:
South Pole.
Question 10.
During aphelion, how does the sun look in the Southern Hemisphere in winter compared to that in summer?
Answer:
Smaller.
Question 11.
What is the boundary line where the illuminated and the non-illuminated portions of the earth meet?
Answer:
The Circle of Illumination or Shadow Circle.
Question 12.
What is the direction of the earth’s rotation?
Answer:
West to East.
Question 13.
Who discovered gravitation?
Answer:
Sir Isaac Newton.
Question 14.
On which day is the earth closest to the sun?
Answer:
January 3.
Question 15.
Which place experiences 24 hours daytime on June 21 ?
Answer:
North Pole.
Question 16.
In which year did Sir Isaac Newton prove the movement of earth?
Answer:
1687.
Question 17.
What is the time taken by Jupiter to complete 1 rotation and 1 revolution?
Answer:
Jupiter completes one rotation in 9 hours and 50 minutes, while it completes one revolution in 12 years.
Question 18.
What shape has the earth assumed due to rotation?
Answer:
Geoid shape.
Question 19.
What would have been the span of time between tides if the earth would not have been rotating?
Answer:
An interval of 271 / 3 days.
Question 20.
What is the name of the wind which moves in a fixed direction, and during a fixed time of the year?
Answer:
Planetary winds.
Question 21.
Which phenomena is responsible for sunrise and sunset?
Answer:
Rotation of the earth.
Question 22.
Which stream of science deals with the study of the universe?
Answer:
Astronomy.
Question 23.
What is the meaning of Equinox?
Answer:
Equal.
Short Questions and Answers : (2 marks for each question)
Question 1.
What is latitude?
Answer:
The imaginary circular lines which run from west to east, on either side of the equator, on the earth’s surface are known as latitudes, or parallels of latitude. The equator is also one of the latitudes.
Question 2.
What is the earth’s annual movement known as?
Answer:
The earth moves in its elliptical orbit round the sun in 365 days. This movement is known as the annual movement of the earth or earth’s revolution. The revolution of the earth is responsible for change of seasons, and variation in the span of days and nights.
Question 3.
What is the rotation of the earth?
Answer:
The earth rotates on its axis, from west to east within a span of 24 hours. This is known as the rotation of the earth.
Question 4.
What is Solar day?
Answer:
The time difference between two middays (12pm) on a single longitude is known as Solar day. The span of one Solar day on earth is 24 hours.
Question 5.
What is a sidereal day?
Answer:
The time taken by earth to complete one rotation on its own axis, so that a distant star appears at the same position on a single longitude twice is known as sidereal day. One sidereal day takes 23 hours 56 minutes and 4 seconds.
Question 6.
What is Ferrel’s Law?
Answer:
In 1855, American scientist William Ferrel stated that sea waves and air gets deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This phenomena happens due to the rotation of the earth. This is known as Ferrel’s law.
Question 7.
What is Coriolis force?
Answer:
In 1835, French mathematician, Gaspard de Coriolis, proved that the rotation of the earth is responsible for the creation of a Centrifugal force. This force was later named as Coriolis force.
Question 8.
Where is the rotational speed of the earth slowest and where is it fastest?
Answer:
The speed of rotation of the earth is fastest in the equatorial region, whereas, it is the slowest at the poles.
Question 9.
Which place on earth has 6 months of daytime and 6 months of night?
Answer:
From March 21 to September 23 the North Polar regions experience 6 months of daytime, and the South Polar regions experience 6 months of night during this period. From September 23 to March 21, the opposite is noticed in both these areas.
Question 10.
Which place on earth experiences no change of seasons?
Answer:
The equatorial region sees no change of season all the year round. This region receives straight rays of the sun all the year round, and hence has a hot climate.
Question 11.
What is meant by the sun’s apparent northward movement?
Answer:
From December 22 to June 21 , the sun apparently moves northward, from Tropic of Capricorn (23 \(\frac{1}{2}\)° S) towards Tropic of Cancer (23 \(\frac{1}{2}\)° N). This is known as sun’s apparent Northward movement.
Question 12.
What is known as the sun’s apparent southward movement?
Answer:
From June 21 to December 22, the sun apparently moves southward from the Tropic of Cancer (23 \(\frac{1}{2}\)° N) towards the Tropic of Capricorn (23 \(\frac{1}{2}\)° S). This is known as the sun’s apparent southward movement.
Question 13.
What is the position of earth during Aphelion?
Answer:
The earth is farthest from the sun during Aphelion. This phenomena occurs on July 4. During this period the distance between the earth and the sun is approximately 152 million km.
Question 14.
What is the position of the earth during Perihelion?
Answer:
The earth is closest to the sun during Perihelion. This phenomena occurs on January 3. During this phenomena, the distance between earth and sun is approximately 147 million km.
Question 15.
What is dawn?
Answer:
Just before sunrise, the darkened part of the earth crosses the circle of illumination and begins to enter the lightened area. This is known as dawn.
Question 16.
What is the earth’s orbital plane?
Answer:
The plane along the earth’s orbit is known as the orbital plane. If the points on the elliptical orbit of earth are connected via a straight line to its diagonally opposite point, then this plane is obtained.
Question 17.
What is dusk?
Answer:
After sunset, the lightened part of the earth enters the darkened area, after crossing the circle of illumination. This is known as dusk.
Question 18.
What is season cycle?
Answer:
The coming of the four main seasons on earth (summer, monsoon, autumn and spring) in a cycle one after the other is known as season cycle.
Question 19.
What is Summer Solstice?
Answer:
On June 21, the rays of the sun fall vertically on the Tropic of Cancer (23 \(\frac{1}{2}\)° N), thus this day is known as the Summer Solstice. On this day, the days are longer and nights shorter in the Northern Hemisphere.
Question 20.
What is Winter Solstice?
Answer:
On December 22, the rays of the sun fall vertically on the Tropic of Capricorn (231° S), and thus this day is known as Winter Solstice. On this day the day are longer and nights shorter in the Southern Hemisphere.
Question 21.
What is Autumnal Equinox?
Answer:
On September 23, the earth’s position on its orbit is such that the rays of the sun fall directly on the equator. This is responsible for making the duration of days and nights equal in both the hemispheres. Due to the prevalence of autumn season in the Northern Hemisphere during this time, this day is called Autumnal Equinox.
Question 22.
What is Vernal Equinox?
Answer:
On March 21, the earth’s position on its orbit is such a way that the rays of the sun fall directly on the equator. This makes the duration of days and nights equal in both the hemispheres. Since it is spring season in the Northern Hemisphere during this period, this day is known as Vernal Equinox.
Question 23.
What is Aurora?
Answer:
When there is 6 months of darkness in the polar regions (north or south), sometimes streaks of light resembling rainbows are observed in the sky. These are known as Aurora.
Question 24.
What is Aurora Borealis?
Answer:
The streaks of light resembling rainbows, observed in the sky during the 6 months of darkness in the North Polar region (September 23-March 21) is known as Aurora Borealis.
Question 25.
What is Aurora Australis?
Answer:
The streaks of light resembling rainbows, which are observed in the sky during the 6 months of continued darkness in the South Polar region (March 21-September 23) is known as Aurora Australis.
Question 26.
What are the effects of the earth’s rotation?
Answer:
- The occurrence of days and nights.
- Change in direction of sea waves and air.
- Creation of tides.
- The occurrence of sunrise and sunset in a cyclic order.
Question 27.
What are the effects of the revolution of the earth?
Answer:
- Change of years
- change in duration of days and nights
- change of seasons
- change in position of the stars.
Question 28.
What is ecliptic?
Answer:
The path of the apparent annual movement of the sun is known as ecliptic. During the year, the sun’s rays move from falling directly on the Tropic of Cancer to the Tropic of Capricorn, and vice-versa.
Question 29.
What are the factors responsible for change of seasons?
Answer:
- The earth’s revolution,
- the tilted placement of the earth on its Axis,
- the difference in angle of the rays of sun.
Question 30.
What is ‘Midnight’ sun? Where is it seen?
Answer:
When the sun is seen in the midnight sky it is known as ‘Midnight’ sun. Midnight sun is seen in areas of the North Polar region from March 21 to September 23.
Question 31.
What is solar year?
Answer:
While rotating on its Axis, the earth completes one revolution around the sun along in its orbit in 365 days, 5 hours, 48 minutes 46 seconds. This is known as solar year.
Question 32.
What is the ‘Midday’ sun?
Answer:
When the rays of the sun fall directly on a line of longitude, at some point on that longitude, the rays of the sun are vertical. This is known as ‘Midday’ sun of that location.
Question 33.
What is Shadow circle?
Answer:
Since the earth is round, one half or the earth receives sunlight and remains illuminated, while the other side does not get sunlight and remains dark. The imaginary line of longitude which is the dividing line between the illuminated and the darkened portions of the earth is known as Shadow circle or circle of illumination.
Question 34.
Even though the earth is rotating, why are we not falling down as a result of that rotation?
Answer:
The earth is constantly rotating from West to East. However, the gravitational pull of the earth attracts every object towards its centre. Hence we do not fall down due to the rotation of the earth.
Question 35.
Which planets rotate in a clockwise direction?
Answer:
Venus and Uranus rotate in a clockwise direction.
Short Questions and Answers : (3 marks for each question)
Question 1.
What do you mean by Coriolis force or Coriolis effect?
Answer:
Due to the rotation of the earth, winds and ocean currents on the surface of the earth do not flow in a straight line, but get slightly deflected. The force due to which winds or ocean currents are deflected is known as Coriolis force or Coriolis effect. This phenomenon has been named after the French mathematician Gaspard de Coriolis, who discovered it in 1835.
Question 2.
Why do winds and ocean currents deflect due to the the Coriolis force?
Answer:
The effect of the Coriolis force is less in the Polar regions as compared to the Equatorial region. The speed of earth’s rotation is also more in the Equatorial region and less at the Polar regions. Due to this difference in the rotational speed, winds in the equatorial region are faster than winds in the polar regions. However, the wind blowing from either side try to retain their original speed and thus they get deflected. Due to this, winds blowing from the equator towards the poles get deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Ocean currents are also deflected in the same way.
Question 3.
Discuss the features of the earth’s orbit.
Answer:
The fixed path in which the earth moves around the sun is known as the earth’s orbit.
Features of the earth’s orbit:
- It is elliptical in shape.
- Its circumference is about 960 million kilometre.
- The centres of the earth and the sun lie on the same plane in the orbit.
- The sun is located in one of the loci of the earth’s orbit.
Question 4.
What do you mean by the apparent daily motion of the sun?
Answer:
Since its origin, the earth rotates on its axis while revolving around the sun.Since we live on the earth, it seems to us that the earth is fixed, and the sun is moving east to west around it. In reality, as the earth rotates from west to east, the sun seems to move from east to west. This apparent movement of the sun around the earth is called its apparent daily motion.
Question 5.
Why does February have 29 days after every four years?
Answer:
The time taken by the earth to complete one revolution around the sun is 365 days 5 hours 48 minutes and 46 seconds. This is known as one solar year. However, for easier calculation, the time taken for one revolution is taken to be 365 days. The remaining 6 hours (5 hours 48 minutes and 46 seconds) is taken together to form one day every 4 years (6 × 4= 24 hours), and is added to the month of February. Therefore, after every four years, February has 29 days and the year has 366 days. These are known as leap years.
Question 6.
Discuss the importance of the inclination of the earth’s axis.
Answer:
The earth’s axis is inclined at an angle of 661° from the earth’s orbital plane. Due to this-
- duration of days and nights changes
- difference in temperature causes change in seasons
- the angle at which the sun’s rays fall on the earth are different at different places
- the Northern and Southern Hemispheres have opposite season pattern
- the sun appears bigger or smaller at times.
Question 7
What is the speed of the earth’s rotation on the different important lines of latitude?
Answer:
Speed of earth’s rotation on different important lines of latitude-
Question 8.
Why is the duration of winter less in the Northern Hemisphere?
Answer:
On January 3, the distance between the earth and the sun is minimum (about 147 million km ). This is known as Perihelion. During this phase, the speed of the earth’s revolution increases slightly, and thus the earth moves faster along its orbit. At this time, the Northern Hemisphere experiences winter season, while the Southern Hemisphere experiences summer. Thus, the duration of winter is less in the Northern Hemisphere.
Question 9.
What is Ferrel’s law?
Answer:
The Law: The speed of the earth’s rotation decreases as we move from the the equator towards the poles. This gives rise to Coriolis force, which causes all objects on the earth’s surface (wind, ocean currents) moving from the Northern Hemisphere to Southern Hemisphere and vice versa to deflect from their original path of motion.
- Effect: Winds and Ocean Currents deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
- Name: American scientist William Ferrel established this law, 1855, thus it is known as Ferrel’s law.
Question 10.
Why is the earth’s rotational speed different in different places?
Answer:
The earth completes one rotation on its axis in 23 hours 56 minutes and 4 seconds. As the circumference of the earth is different at different lines of latitude, the rotational speed of the earth is different at different places. As the earth’s circumference is greatest at the equatorial region, the rotational speed is greatest at the equator (about 1670 km / hr ). From the equator to the poles, the circumference of the earth gradually decreases, and thus, the rotational speed of the earth decreases from the equator to the poles.
Question 11.
The shape of the earth is like an oblate spheroid – How does this testify the existence of the earth’s rotation?
Answer:
The rotation of the earth on its axis gives rise to centrifugal force. Due to this force, liquid and gaseous matter from the polar regions moved to the equatorial region, causing this region to bulge out. On the other hand, the rise of centripetal force in the polar ragions have caused them to flatten. Therefore, it can be concluded that the earth’s rotation is responsible for its oblate spheroid shape. As the rotational speed of planets like Jupiter and Saturn is much more than that of the earth, the poles of these planets are much more flattened as compared to the earth.
Question 12.
‘Life would not have been possible on the earth without rotation’. State three reasons in support of this statement.
Answer:
Three reasons in support of the above statement-
1. Without earth’s rotation, one side of the earth would have been burnt by the sun’s heat, and the other side would have frozen up, destroying any forms of life.
2. Only regions in the shadow circle would have little chances of sustaining life.
3. Without rotation, a balance in temperature would not have been possible, making survival of lifeforms impossible.
Therefore, one of the main reasons for the survival of life on earth is its rotation.
Write the differences between the following –
Question 1.
Vernal Equinox and Autumnal Equinox
Answer:
The differences between vernal equinox and autumnal equinox are as follows-
| Points of difference | Vernal Equinox | Autumnal Equinox |
| 1. Concept | Divides all latitudes equally, and duration of day and night is equal everywhere. This is known as the vernal equinox in Northern Hemisphere. | Divides all latitudes equally, and duration of day and night is equal everywhere. This is known as the autumnal equinox in the Southern Hemisphere. It occurs on September 23. |
| 2. Date | It occurs on March 21. | It is spring in Southern Hemisphere, and autumn in Northern Hemisphere during autumnal equinox. |
| 3. Season | It is spring in Northern Hemisphere, and autumn in Southern Hemisphere during vernal equinox. | Autumnal Equinox |
Question 2.
Solar year and Sidereal year
Answer:
The differences between solar year and sidereal year are as follows-
| Points of difference | Solar year | Sidereal year |
| 1. Concept | It is calculated on the basis of average solar days. | It is calculated on the basis of average sidereal days. |
| 2. Duration | Its duration is 365 days 5 hours 48 minutes 46 seconds. | Its duration is 365 days 6 hours 9 minutes 10 seconds. |
| 3. Change in number of days | 1 day is added to every fourth year in this case. | No change in number of days occurs in this case. |
Question 3.
Aphelion and Perihelion
Answer:
The differences between aphelion and perihelion are as follows-
| Points of difference | Aphelion | Perihelion |
| 1. Definition | Average distance between sun and earth is the maximum ( 152 million km) during aphelion. | Average distance between sun and earth is the minimum (147 million km) during perihelion. |
| 2. Date | It occurs on July 4. | It occurs on January 3. |
| 3. Speed of Revolution | Speed of revolution decreases in this case. | Speed of revolution increases in this case. |
Question 4.
Summer Solstice and Winter Solstice
Answer:
The differences between summer solstice and winter solstice are as follows-
| Subject | Summer Solstice | Winter Solstice |
| 1. Date | It occurs on June 21. | It occurs on December 22. |
| 2. Duration of day and night | Days are longer and nights are shorter in Northern Hemisphere and opposite conditions prevail in the Southern Hemisphere during summer solstice. | Days are longer and nights are shorter in Southern Hemisphere and opposite conditions prevail in the Northern Hemisphere during winter solstice. |
| 3. Season | Northern Hemisphere experiences summer, while Southern Hemisphere experiences winter during this time. | Northern Hemisphere experiences winter, while Southern Hemisphere experiences summer during this time. |
Question 5.
Earth’s Axis and Orbit
Answer:
The differences between the earth’s axis and its orbit are as follows-
| Points of difference | Axis | Orbit |
| 1. Concept | It is the imaginary line joining the two poles of the earth. | It is the earth’s path of movement around the sun. |
| 2. Length | The axis is about 12,714 km long. | The orbit is about 960 million km long. |
| 3. Importance | It is related to the rotation of the earth. | It is related to the revolution of the earth. |
Question 6.
Aurora Borealis and Aurora Australis
Answer:
The differences between Aurora Borealis and Aurora Australis are as follows-
| Points of difference | Northern Lights | Southern Lights |
| 1. Position | It is visible in the Arctic region in the Northern Hemisphere. | It is visible in the Antarctic region in the Southern Hemisphere. |
| 2. Duration | It is seen from September 23 to March 21. | It is seen from March 21 to September 23. |
| 3. Nomenclature | It is also known as Aurora Borealis. | It is also known as Aurora Australis. |
Give reasons for the following –
Question 1.
Scientists carry out expeditions to Antarctica in December.
Answer:
Three reasons why scientists carry out expeditions in Antarctica are-
1. Longer days: The apparent southward movement of the sun in December makes days longer and nights shorter. Longer days make it easier to carry out experiments and research.
2. Increase in temperature: The Southern Hemisphere experiences summer in December. The temperature in the coastal regions of Antarctica rise above the freezing point, making it easier for scientists to cope with the weather.
3. Easier passage: Large chunks of ice in Antarctica melts in summer, thus allowing scientific explorers easier passage into the continent using ships.
Question 2.
We cannot feel the earth’s rotation.
Answer:
We cannot feel the rotation of the earth because of the following reasons-
1. We are moving at a speed equal to the speed of the rotation of the earth. Thus it is not possible for us to feel the earth’s rotation.
2. The gravitational force of the earth pulls all objects on the earth’s surface towards its centre. Due to this force, we are not flown off the surface of the earth in spite of its rotation.
3. We are so small compared to the size of the earth, that it is not possible for us to sense the earth’s rotation.
Question 3.
Season changes are not felt in the equatorial region.
Answer:
Season changes are not felt in the equatorial region, i.e., the region around the equator (0°. latitude), due to the following reasons-
1. The circle of illumination intersects the equator at right angles; thus, length of days and nights are equal throughout the year in this region.
2. The rays of the sun fall vertically on the equator almost throughout the year. Hence it remains warm all the year round.
3. The equatorial region experiences convectional rainfall for most of the year.
Question 4.
New Zealand celebrates Christmas in summer.
Answer:
As the earth revolves around the sun, during November to January, the Southern Hemisphere remains tilted towards the sun, while the Northern Hemisphere remains away form the sun. Due to this, during these three months, the Southern Hemisphere experiences summer. Thus, December 25 , i.e., Christmas, is celebrated during summer in New Zealand, which lies in the Southern Hemisphere.
Question 5.
Temperature decreases from the equator towards the poles.
Answer:
The rays of the sun do not fall equally on all parts of the earth due to various reasons – the shape of the earth, its rotation, etc.
Thus, the sun’s rays fall vertically on the equatorial region, spreading over a smaller area, and are slanted at the poles, spreading over a larger region. Besides, as the rays have to travel over a longer distance in order to reach the polar regions, they lose much of their heat in the process. Hence, temperature gradually decrease as we move from the equator to the poles.
Long Questions and Answers : (5 marks for each question)
Question 1
How can we prove that the earth has diurnal motion?
Answer:
The movement of the earth on its axis from west to east in an anti-clockwise motion within a duration of 24 hours is known as rotation of the earth or earth’s diurnal motion. The fact that the earth has diurnal motion can be proven by the following points –
1. Earth is shaped like an oblate sphere: When a circular object rotates constantly on its axis, its centre bulges outwards, while the top and the bottom ends get flattened. The same is the case of earth. The earth is bulging at the equator and is flattened at the poles.
2. Occurrence of day and night in a cyclic pattern: Due to the fact that earth rotates on its axis within a span of 24 hours, places on earth experience days and nights in a cyclic pattern. If the earth had not been rotating, the side facing the sun would have eternal day while the opposite side would have eternal night.
3. Observing a stone falling from a certain height: When a stone is dropped from a certain height, it does not fall vertically in a straight line. Instead it shifts somewhat to the east. This proves that the earth rotates from the west to the east.
Figure 2.4: A stone dropped from above deflects slightly to the east due to the rotation of the earth
4. Pictures taken from artificial satellites: Several man made satellites sent to outer space have captured pictures of the earth, which are proof of the fact that the earth is rotating on its axis.
5. Rotation of the other planets on their axes: The other planets of the solar system like Venus, Mars, Jupiter and others rotate on their respective axes. Since earth is also a planet of the same solar system, it is therefore deduced that the earth too rotates on its axis.
6. Change in direction of air and sea waves: The direction of sea waves and air does not move straight in either hemisphere from the high pressure belt to low pressure belts. Instead it shifts to the left in the Southern Hemisphere and to the right in the Northern Hemisphere. This change in direction occurs due to the rotation of the earth.
7. Regular occurrence of tides: Tides occur on earth due to the gravitational pull of the moon. If the earth would not have been rotating any place on the earth would have experienced near tidal bulge once in every 27 days. It is only because of earth’s rotation that near tidal bulge occurs on a daily basis.
8. Foucault’s experiment: French scientist Foucault (1851) did an experiment with a pendulum and proved that the earth is rotating.
Question 2
What are the results of the diurnal motion of the earth?
Answer:
The rotation of the earth on its axis in 23 hours 56 minutes 4 seconds is called the diurnal motion of the earth or rotation of the earth. This movement is an anti-clockwise movement. The results of this diurnal motion are-
1. Cyclic occurrence of days and nights: The earth is round in shape and has no light of its own. The rays of the sun are the only source of heat and light for the earth. Therefore during the rotation of the earth, one half of the earth receives the rays of the sun and it is daytime in that area, while the region which remains in darkness has night.
2. Sunrise and sunset: Due to the fact that the earth rotates from west to east, the sun rises in the east and sets in the west.
3. Calculation of time: One complete rotation of the earth takes almost 24 hours or 1 complete day. Each hour is divided into 60 minutes and each second into 60 seconds for easier calculation of time.
4. Change in direction of sea waves and air movement: From the equator to the poles, the area of the earth gradually decreases, hence its speed of rotation also decreases as we proceed towards the poles. This causes air to get deflected near the poles. The same happens in case of sea waves.
5. Creation of tides: As a result of the gravity, both the sun and the moon exert gravitational force over the earth. However, since the moon is closer to the earth than the sun, it exerts more gravitational force on the earth than the sun. Due to the rotation of the earth, the side facing the moon experiences near tidal bulge, while the side opposite to the moon experiences far tidal bulge. The sides in the middle on the other hand experience low tide.
6. Existence of flora and fauna: If the earth would not have been rotating, one part of the earth would have always been away from the sun. This side would not receive any light or heat. Therefore, nothing would grow on this side of the earth. On the other hand the side facing the sun would get an abundance of heat and light, again making it impossible for life to sustain. The rotation of the earth is responsible for creation of day and night and equal distribution of heat and light, thus making earth a planet fit for habitation.
Question 3
What proof do we have about the revolution of the earth?
Answer:
The movement of the earth on an elliptical orbit around the sun, while rotating on its own axis is known as the revolution of the earth. The proof of this revolution of earth are-
1. Change in position of star: There is a slight shift in the position of the stars everyday. After a few days, the older stars disappear from the sky and a new set of stars appear. The older set of stars gradually show up in the sky after a year. This proves that the earth revolves around the sun, and gradually comes back to the position it started from.
2. Change in the position of sunrise and sunset: Due to the earth’s revolution, an apparent annual movement of the sun is noticed. The sun rises exactly in the east and sets in the west from March 21 and September 23. On the other days of the year, the sun rises and sets with a slightly northward and southward shift. If the earth had not been revolving, the sun would have been rising and setting exactly in the east and the west for the entire year.
3. Change of seasons and the duration of days and nights: Due to the revolution of the earth, there is change of seasons on earth. The duration of days and nights also varies round the year due to the same reason.
4. Observing the revolution of the other planets: With the help of powerful telescopes scientists have observed that the other planets like Venus, Mars, Jupiter and others revolve around the sun. As the earth is a planet of the same solar system as the rest, therefore the earth is also revolving round the sun.
5. Pictures taken from satellites: The pictures taken by satellites prove that the earth is revolves round the sun.
Question 4
What are the results of the earth’s annual movement or revolution?
Answer:
Revolution of the earth is the movement of the earth around the sun, while rotating on its axis, from west to east in an anti-clockwise pattern within a span of 365 days and 6 hours. The results of the revolution of the earth are-
1. Change in the duration of days and nights: The earth is inclined on its axis at an angle of 661 / 2°. As a result of this inclination, the rays of the sun are sometimes vertical on the Tropic of Capricorn in the Northern Hemisphere, and sometimes on the Tropic of Cancer in the Southern Hemisphere. This causes changes in the duration of days and nights in both hemispheres.
2. Change of Seasons: The distance of the earth from the sun changes during the revolution of the earth. Since the earth is inclined at an angle of 661° and is somewhat circular in shape, the rays of the sun do not fall in the same, manner on all the parts of the earth. Some parts receive direct rays of the sun, while the rest may receive inclined rays of the sun. This is responsible for the variation in heat received by the places. This difference in heat distribution causes difference of seasons in the regions. For example-
| Timespan of revolution | Northern Hemisphere | Southern Hemisphere |
| 1 and 1 / 2 months preceding and following June 21 | Summer | Winter |
| 1 and 1 / 2 months preceding and following September 23 | Autumn | Spring |
| 1 and 1 / 2 months preceding and following December 22 | Winter | Summer |
| 1 and 1 / 2 months preceding and following March 21 | Spring | Autumn |
3. Apparent annual movement of the sun: The sun rises exactly in the east and sets exactly in the west on March 21 and September 23. On the other days the sun rises shifted to the north or the south. This northward or southward movement of the sun is known as the apparent annual movement of the sun.
4. Change in position of stars: If noticed closely, the star in the night sky shift their position from east to west. After this shift for a few days, the stars gradually disappear from the sky and a new set of stars appear. After a year, the same stars are again seen in the sky.
Question 5
What are reasons for season change on earth?
Answer:
The various regions of the earth do not receive equal amount of sunlight throughout the year. Thus when one place experiences summer another might experience winter, a year is divided into seasons on the basis of this variation. Seasonal changes mark the passing of a year. The reasons that lead to seasonal changes are-
1. Revolution of the Earth: The earth rotates upon its own axis in particular path and in a particular direction (anti-clockwise). The earth also revolves around the sun in a particular path, taking a particular amount of time. Variations in the amount of sunlight received amongst the various regions of the earth is due to this reason.
2. Position of the earth at 661° angle on its Axis: The earth is inclined at an angle of 661° on its axis. This is one of the causes behind the change in duration of days and nights. This creates a difference in temperature. For example, if the days are longer and nights shorter, then the total heat absorbed by the earth during the day cannot be radiated out. Therefore, increasing the warmth in the respective area.
3. Rotation of the earth: The earth rotates on its axis and thereby brings days and nights. The rotation of the earth is also responsible for causing changes in the duration of days and nights. Thus rotation is responsible for both temperature variations as well as change of seasons.
4. Geoid shape of the earth: The earth is not perfectly round in shape, it is geoid in shape. This is the reason for the rays of the sun falling directly on some areas and tangentially on others. The direct rays of the sun heat up the earth faster and more than the tangent rays of the sun thereby creating a variation in temperature.
Question 6
Give an idea about Perihelion and Aphelion.
Answer:
Perihelion: The earth is closest to the sun on January 3. The distance between the earth and the sun is approximately 147 million km. This position of the earth from the sun, on its orbit is called Perihelion.
Result:
- The distance between the sun and the earth is reduced during this position.
- The sun appears larger in the Northern Hemisphere due to this.
- The rotation speed of the earth increases during this position.
Aphelion: The distance between the earth and the sun is maximum on July 4 . The distance is approximately 152 million km. This position of the earth from the sun is called Aphelion.
Result:
- The distance between the sun and the earth increase during this position.
- Due to Aphelion, the sun appear to be smaller in the Southern Hemisphere, during summers.
- The rotation speed of the earth decreases slightly during Aphelion.
Question 7
How do days and nights occur? Discuss the different phases of day and night on earth. [2 + 3]
Answer:
Occurrence of days and nights: The earth rotates west to east on its axis. During rotation, the half of the earth that faces the sun has day, and the half that remains away form the sun has night.
Different phases of day and night: The different phases of day and night are-1. morning and evening, 2. dawn and dusk, 3. midday and midnight.
1. Morning and evening: Due to the rotation of the earth, when a particular region crosses the circle of illumination and faces the sun, it has morning; and the region opposite to this point, which moves away from the sun, has evening.
2. Dawn and dusk: Just before sunrise, the dust particles in the atmosphere reflect the sun’s rays and a diffused light spreads over the eastern sky. This time period is known as dawn. Again, just after sunset, the dust particles in the atmosphere reflect the sun’s rays. As a result, a diffused light spreads over the western sky. This time period is known as dusk.
3. Midday and midnight: When the sun is overhead a place, this place is said to have midday; and the place that lies on the opposite side of this point is said to have midnight.
Question 8
Why does the duration of days and nights change?
Answer:
March 21 and September 23 are the two days apart from which the duration of days and nights on the earth vary. This is because-(1) The revolution of the earth and (2) the position of the earth on its axis along its orbital plane, (3) the geiod shape of the earth, (4) earth’s revolution, (5) elliptical orbit of the earth, (6) the position of the earth’s poles towards the North Star, (7) the sun’s apparent daily and annual movement etc. The earth stands in several different positions in its orbit on different periods of the year. The several reasons for the change in duration of days and nights are given below-
1. During Summer Solstice: On June 21 the position of the earth is such that the Northern Hemisphere is inclined towards the sun and the Tropic of Cancer receives its direct rays. Hence, on this day the Northern Hemisphere experiences the longest day and the shortest night. The Southern Hemisphere on the other hand experiences the complete opposite phenomena.
2. After Summer Solstice: After June 21, the position of the earth gradually changes and the Southern Hemisphere gradually starts shifting towards the sun. The rays of the sun fall directly on the Southern Hemisphere and thus the days become longer and the nights shorter there, while it is the opposite in the Northern Hemisphere.
3. During Autumnal Equinox: On September 23, the position of the earth on its orbit is such that both the hemispheres are equidistant from the sun. Hence days and nights are of equal duration in both hemispheres.
4. After Autumnal Equinox: After the Autumnal Equinox (September 23), the Southern Hemisphere comes even closer to the sun while the Northern Hemisphere shifts further away from it. The duration of day become longer and nights shorter in the Southern Hemisphere, and it is just the opposite in the Northern Hemisphere.
5. During Winter Solstice: On December 22, the sun rays fall directly on the Tropic of Capricorn. This is the longest day of the Southern Hemisphere, and it is just the opposite in the Northern Hemisphere.
6. After Winter Solstice: After December 22, the Southern Hemisphere begins shifting away from the sun, while the Northern Hemisphere begins moving toward the sun. This causes the shorter days and longer nights in the Southern Hemisphere.
7. During Vernal Equinox: On March 21 the position of the earth on its orbit is such that the both the Northern and the Southern Hemispheres are equidistant from the sun. Thus, duration of days and nights are equal.
8. After Vernal Equinox: The Northern hemisphere gradually begins shifting towards the sun, while the Southern Hemisphere shifts away from the sun. This makes nights shorter and days longer in the Northern Hemisphere, and it is just the opposite in the Southern Hemisphere.
The lengths of days and nights according to seasons and dates in the Northern and Southern Hemispheres-
Question 9
Why do winds and ocean currents get deflected?
Answer:
Planetary winds and ocean currents get deflected due to the earth’s rotation. According to William Ferrel’s (American scientist) law, moving objects on the earth’s surface, such as winds and ocean currents, get deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. A detailed explanation of this phenomenon is given below. The circumference of the earth decreases gradually from the equator to the poles.
The speed of the earth’s rotation is also the maximum at the equator, and decreases gradually towards the poles.Therefore, when winds blow southwards from any region in the Northern Hemisphere towards the equator, its velocity is less in comparison to that in the equatorial region. However, as it tries to maintain its original velocity, it deviates from its path of flow and deflects to the right. Due to this reason, winds blowing from the equator towards the north also get deflected to their right.
Similarly, winds blowing northwards or southwards in the Southern Hemisphere get deflected to their left. For example, Trade Winds blowing towards the Equatorial Doldrums in the Northern Hemisphere deflect to the right and are known as Northeast Trade Winds, and deflect to the left in the Southern Hemisphere and are known as Southeast Trade Winds.
Ocean currents too get deflected in the Northern and Southern Hemispheres due to the same reason.
Question 10.
What do you mean by ‘Land of Midnight Sun’? What is solar path or ecliptic? [2 + 3]
Answer:
‘Land of Midnight Sun’: When the Arctic region has day for about six months at a stretch from March 21 to September 23, the sun can be seen even at night from Hammerfest, located in the northern part of Norway (70°. North latitude). Thus Hammerfest is known as the ‘Land of Midnight Sun’.
Solar path or ecliptic: The path followed by the sun during its apparent movement around the earth throughout the year is known as the solar path or ecliptic. This apparent movement of the sun takes place between the Tropic of Cancer in the north and the Tropic of Capricorn in the south. From June 21 to December 22 , the sun gradually shifts to the south. This is known as the apparent southward movement of the sun.
On June 21 , the sun is vertically overhead the Tropic of Cancer, on September 23 and March 21 , it remains vertically overhead the Equator, and on December 22, the sun is vertically overhead the Tropic of Capricorn. From December 22 to June 21 , the sun gradually shifts to the north. This is known as the apparent northward movement of the sun. This path through which the sun moves throughout the year is known as solar path or ecliptic.
Question 11
How did scientist Leon Foucault prove that the earth rotates? What is a leap year? [2 + 3]
Answer:
Foucault’s experiment: In 1851, French scientist Foucault carried out an experiment by fixing a pin beneath a pendulum bob, and suspending it from Pantheon in Paris with a 67 m long wire, over some sand. He noticed that though the pendulum was oscillating from north to south in a fixed path, the markings on the sand were gradually moving from west to east.
This proved that the earth rotates from west to east, otherwise the markings on the sand would have been on the same spot each time.
If you take a look at the picture above, you will notice that the pin will move from A to A’ first, then from B to B’, then C to C’, then D to D’, then E to E’, and exactly after 24 hours, it will return to A-A’. If all the points are joined, you will find that they form an arc. This phenomenon occurs due to the rotation of the earth.
Leap year: The time taken by the earth to complete one revolution around the sun is 365 days 5 hours 48 minutes and 46 seconds. This is known as one solar year. However, for easier calculation, the time taken for one revolution is taken to be 365 days. The remaining 6 hours (5 hours 48 minutes and 46 seconds) is taken together to form one day every 4 years (6 × 4 = 24 hours), and is added to the month of February. Therefore, after every four years, February has 29 days and the year has 366 days. This is known as a leap year. For example, the years 2000, 2004, 2008, 2012, 2016, etc. are leap years. If the digits constituting the year can be divided by 4, it is a leap year.
Question 12
What do you mean by seasonal cycle? Explain in brief. [1 + 4]
Answer:
Seasonal cycle: The apparent northward and southward movement of the sun, and variation in the length of days and nights are caused by the revolution of the earth. This creates a difference in temperature in different places over the earth which, in turn, affects the weather and climate of that region. This is known as change in season. There are four main seasons on the earth-summer, autumn, winter and spring. This is known as seasonal cycle.
Explanation:
1. During one and a half months preceding and following June 21, the Northern Hemisphere remains tilted towards the sun, while the Southern Hemisphere remains away from the sun. The Northern Hemisphere receives almost vertical rays of the sun, while the Southern Hemisphere receives slanting rays. Hence the Northern Hemisphere experiences summer and the Southern Hemisphere experiences winter.
2. After July, as the earth moves around its orbit, the amount of sun’s rays received by the Northern Hemisphere decreases while the amount received increases in the Southern Hemisphere. Due to this, during one and a half months preceding and following September 23, the Northern Hemisphere experiences autumn, and the Southern Hemisphere experiences spring.
3. During one and a half months preceding and following December 22, the Southern Hemisphere remains tilted towards the sun, while the Northern Hemisphere remains away from the sun. The Southern Hemisphere receives almost vertical rays of the sun, while the Northern Hemisphere receives slanting rays. Hence, from November to January, the Southern Hemisphere experiences summer and the Northern Hemisphere experiences winter.
4. After January, as the earth moves around its orbit, the amount of sun’s rays received by the Southern Hemisphere decreases while the amount received increases in the Northern Hemisphere. Due to this, during one and a half months preceding and following March 21, the Northern Hemisphere experiences spring, and the Southern Hemisphere experiences autumn.
Two regions on the earth’s surface do not experience change in seasons:
1. The equatorial region receives vertical rays of the sun throughout the year, thus it always experiences summer.
2. The North and South Poles receive slanting rays of the sun throughout the year, thus these regions always experience winter.