Modern Concept Science and Technology 9

6. Why are decomposers important in the ecosystem?

Ans: Decomposers recycle the dead and decomposed substances and convert them into simpler forms.
These simpler forms of nutrients can again be used up by the plants. They are at work every time on
something dead lying in nature. Since they recycle waste matter and convert them into plant nutrients,
new food for plants is always available. This sustains the ecosystem. Therefore, they are very important
to the ecosystem.

7. Explain the role of producers in sustaining the ecosystem.

Ans: Producers produce food in the ecosystem which acts as energy for the entire biotic components of the
ecosystem. All organisms on the earth get their nutrients from food obtained directly or indirectly from
the plants. Without them, the entire ecosystem will collapse.

8. What kind of services does the ecosystem offer us in our daily life? Explain.

Ans: The services that we receive from the ecosystem in our daily life are:

a) Provisioning services: The material products obtained from ecosystem are known as the
provisioning services. For example, fresh water for drinking, bathing, cooking; foods like
vegetables, fruits, cereals; fibre for materials and clothing, e.g. cotton, wool, hemp, flax.

b) Regulating services: These are the benefits obtained from the regulation of the ecosystem
processes. For example, climate regulation and maintenance, disease control by reduction in
mosquito population, cholera etc.

c) Cultural services: These are the benefits that we obtain from non-material and abstract values of
the ecosystem. For example, spiritual, aesthetic and religious values, recreation and eco-tourism
that adds national income, etc.

d) Supporting services: These are the services that we obtain while receiving all other ecosystem

services. For example, soil formation by weathering of rocks, nutrient recycling through various

bio-geochemical cycles.

9. Identify and write the food chains involved in making this food web. Hawk

Ans: The food chains involved in making this food web are: Snake

Green plants Mouse Snake Hawk Mouse Frog

Green plants Frog Snake Hawk Green plants

10. A forest has a grasshopper being eaten by a frog. The frog is then eaten by a snake. After a while, an eagle
comes and feeds on the snake. Discuss the trophic levels of these organisms.

Ans: The question is illustrated with the following food chain:

Green plants Grasshopper Frog Snake Eagle

The green plants in this food chain are producers. They provide food and energy to the whole food
chain. The green plants are eaten by grasshopper. Grasshopper is a primary consumer. Similarly, the
grasshopper is eaten by frog. Frog is a secondary consumer. Likewise, the frog is eaten by snake which
is a tertiary consumer. The top consumer or the quaternary consumer in this food chain is eagle which
feeds on snake.

Modern Concept Science - 9 393

STEPS EXERCISE

STEP 1

1. Define an ecosystem
2. What is ecology?
3. Write the full form of CBD and MA.
4. What do you mean by abiotic and biotic factors?
5. What are producers?
6. Write the meaning of consumers.
7. Define decomposers.
8. State the meaning of holozoic nutrition.
9. Mention food chain.
10. What is food web?
11. What kind of service is provisioning service?

STEP 2

12. Write the definition of ecosystem according to the convention on Biological diversity.
13. Mention any two examples of climatic and edaphic factors.
14. Describe briefly the photosynthesis process.
15. How is ecosystem dynamic?
16. Why are lions and tigers called top carnivores?
17. Give an example of a food chain that doesn’t start with green plants.
18. Write the types of holozoic animals.
19. How is a food chain different from a food web? Write any two points.
20. What are phytoplanktons and zooplanktons?
21. Name the four ecosystem services

STEP 3

22. What are abiotic components? How do they help the biotic components to survive?
23. Draw the structure of a pyramid showing producers and different levels of consumers.

394 Nature and Environment

24. Explain the exchange of food in the following food web.

25. Draw a well labeled diagram of the following:
a) A food chain
b) A food web

STEP 4

26. What is an ecosystem? What are the
components inside an ecosystem? How
do these components interact with each
other?

27. What are biotic factors of an ecosystem?
Write the names of their different levels.
How does the food reach the decomposers
in this level?

28. What happens if all the vulture, fox and
tiger in the food web shown alongside
die? Explain.

PROJECT WORK

1. Visit a pond and note down the biotic and abiotic components. Prepare a report showing
the relationship between these components.

2. Prepare a chart of different types of ecosystems that you see around your surroundings
along with the biotic and abiotic components.

Modern Concept Science - 9 395

UNIT Estimated teaching periods Theory Practical
4 1
22
Natural Hazards

Syllabus issued by CDC Cyclone

 Introduction to natural hazard
 Introduction to man-made hazard
 Management of natural hazard
 Management of man-made hazard
 Causes, effects and protective measures of natural hazards, viz. glacier flood, glacier lake

outburst and cyclone.

LEARNING OBJECTIVES

At the end of this unit, students will be able to:
 introduce natural hazards and man-made hazards and describe the management of hazards.
 explain the causes, effects and protective measures of natural hazards, viz. glacier flood, glacier

lake outburst and cyclone.

Key terms and terminologies of the unit

1. Hazard : A hazard is an event that puts life, property and environment at risk of harm.

2. Natural hazards : The hazards that are caused by natural phenomena or incidents are known as
natural hazards.

3. Man-made hazards : The hazards that are caused by human activities are known as man-made
hazards or anthropogenic hazards.

4. Disaster management cycle : The disaster management cycle is the series of pre-designated and planned
steps that are taken to limit the effects of the disaster.

5. Avalanche : An avalanche is a collection of rocks, frozen ice and snow that rolls down a
mountain at a very high speed.

6. Glacier : A glacier is a large mass of dense ice (at least covering an area of 0.15 km2 with
thickness of 50m) moving or falling slowly over an area.

7. Glacial lake : The lake formed by the melting of a glacier is known as a glacial lake.

8. Hurricane : A hurricane is a whirlwind or a vortex that draws warm evaporated air and can
flood the coastal areas.

9. Vulnerability : Vulnerability is the state of being open to hazard or injury.

10. Disaster : If any hazard causes destruction of life, property or environment, it is called a
disaster.

11. Preparedness : The activities that are conducted prior to a disaster occurrence are known as
preparedness.

396 Natural Hazards

12. Disaster management: It is an act of controlling, risk-reducing, mitigating and even preventing the
disasters and rehabilitating and recovering the loss is termed as disaster
13. GLOF : management.
14. Storm-surge :
A type of flood that occurs due to failure of dams containing the glacial lakes is
called GLOF (Glacial Lake Outburst Flood).

Hurricanes cause the sea or ocean level to rise and this results in coastal
flooding. This is known as a sea-surge or storm-surge.

Introduction Memory Tips

A hazard is an event that puts life, property and environment More than 50% of hazards are related to
at risk of harm. The word hazard is derived from old water.
French ‘hasard’ meaning ‘to put something at stake’. The

causes of hazards can be biological, climatic, hydrological (related to water, water cycle, etc.),

environmental, meteorological etc. On the basis of nature, hazards can either be natural or

anthropogenic (man-made/artificial).

Natural Hazards

The hazards that are caused by natural phenomena or incidents are known as natural hazards.
They are caused due to weather, climate, meteorological, geological, biological and other
factors. When they occur, they generally cause destruction to life, property and environment.
For example, earthquake, volcanic eruption, tsunami, cyclones, flood etc.

Earthquake Glacier flood Landslide Volcanic eruption

FACT WITH REASON

Hazards may or may not be disasters. Why?

Hazards may or may not be disasters because hazards are the probable events that might cause harm and
they are only called disasters if they actually cause destruction to life, property or environment.

Anthropogenic Hazards (Man-made hazards)

The hazards that are caused by human activities are known as anthropogenic hazards. They
are also called man-made or artificial hazards. They also cause destruction to life, property,
health and environment but generally are less disastrous than the natural ones. Examples of
anthropogenic hazards are: fire, accidents, explosions, oil spills, nuclear disasters, building
collapse, communal riots, wars, pollution, etc.

Hazards may have single or several causes. They might be seasonal or sudden. For example,
flood can be caused by a variety of factors. In Nepal, during monsoon rainfall, floods are

Modern Concept Science - 9 397

caused by the overflow of rivers due to excessive rainfall. It is a natural cause. But floods can
also be caused by human activities like exploitation of rocks, stones, etc. in the embankments.
Sometimes a single hazard can have various causes.

Anthropogenic hazards

When hazards cause destruction to life, property or environment, it is known as a disaster. Natural
and man-made hazards turn into disasters after their occurrence.

FACT WITH REASON

Why are the number of natural disasters increasing day by day?
The number of natural disasters are increasing day by day because concentration of greenhouse gases is
increasing day by day causing global warming. It results various natural disasters.

Occurrence of Hazards

Hazards are situation and location specific. It means that same hazards that occur in one part
of the world may not occur in other parts or same type of hazards can occur simultaneously
in many parts of the world. For example, cyclones occur only in seas and oceanic areas.
Earthquake generally occurs around the tectonic belts or fault lines. Places near the rivers and
low basins are vulnerable to flooding. Some of the hazards are shown in the table below:

Natural hazards Anthropogenic hazards
Flood Fire
Earthquake and Tsunami Explosion
Cold Wave Nuclear hazard
Volcano Mass shooting
Tropical cyclone, hurricane, typhoon, tornado War
Avalanche, landslide, mudflow, flash flood, rock fall Terrorism
Heat wave (e.g. Loo) Infrastructure collapse
Lightning Riot

FACT WITH REASON

Why is management of natural hazard necessary?
Management of natural hazard is necessary because it helps to save lives and property.

398 Natural Hazards

Management of Natural Hazards

The natural hazards are the results of destructive phenomena of nature. We can neither escape
them nor prevent them. But we can control their effects and minimize the danger of loss of life,
property and environment. Some of the minimizing measures of Natural Hazards are given below:

i) Implementation and generating awareness about disaster risk management.
ii) Preparedness prior to potential hazards.
iii) Quick response: search, rescue and rehabilitation of the affected people.
iv) Mobilisation of easily accessible relief camps and programmes

Management of Anthropogenic or Man-made Hazards

With extreme caution, man-made hazards can be controlled, mitigated and even prevented.
The following should be done to manage the risk of man-made hazards:

i) Awareness generating programmes related to environment conservation, sanitation,
hygiene, diseases and potential risks should be conducted.

ii) Prevention, management and cure should be provided in case of health hazards like
epidemics, diseases etc.

iii) Environmental conservation programmes like afforestation, environmental cleanliness,
pollution control, etc. should be regularly conducted.

iv) Traffic rules should be followed and implemented strictly.
v) Carelessness, drink-and-drive, vehicle mishandling, throwing cigarette butts near

forests, etc. should be discouraged.
vi) Avoiding outdoors during heavy rainfall and lightning. Placing a discharge rod at the

top of the building for the electrical discharge during lightning.
vii) Soil, air, water and land preservation should be done.
viii) Proper handling of chemical and biohazards should be done.
ix) Formation of a disaster response team in every district or state should be done.
x) Proper land plans, utility plans, site plants, agricultural land preservation etc. should be

introduced for land conservation.

Disaster Risk Management and Disaster Management Cycle

The activities that are conducted before, during and
after a disaster is known as disaster risk management.
It is done to control, avoid, minimize, rehabilitate and
recover from the effects and consequences of disasters.
There are three major steps to be taken up for disaster
risk management:
Pre-disaster (Before a disaster)
Mitigation
Mitigation refers to risk reduction, vulnerability
analysis, capacity building, generating awareness,
enforcement of building codes, etc.

Modern Concept Science - 9 399

Preparedness

The activities that are conducted prior to a disaster occurrence are known as preparedness. It
falls under the category of activities performed under pre-disaster step. Awareness generating
programmes, building retrofitting, installation of monitoring systems and early warning
systems, drills of search and rescue and formulation of organized plans and policies by the
disaster response teams fall under this category.

On-disaster (During a disaster occurrence)

Response

These are the activities that are conducted during the occurrence of a disaster. This includes
quick dissemination of information, formulation of emergency evacuation plans, provision
for first-aid, rehabilitation programmes, quick responses like search and rescue, emergency
relief and mobilization of several mechanisms. This is also known as response. Early response
should occur during a disaster as well as just after the disaster.

Post-disaster (After a disaster)

Recovery

The recovery activities are carried out right after the occurrence of any disaster. These
activities include measures to protect life and prevent further damage. Temporary housing,
relief, health camps, management of clean drinking water, sanitation, toilets, food, medicines,
clothes, utensils, counseling and legal procurement, rehabilitation etc. are included in the
category of recovery.

The disaster management cycle is the series of pre-designated and planned steps that are taken
to limit the effects of the disaster. These are the totality of all the activities followed by a plan
before, during and after a disaster. Preparedness, Response, Recovery and Mitigation are the
four major steps of the disaster management cycle.

In this chapter, we will study a few natural hazards namely avalanche, glacial lake outburst
flood (GLOF), cyclones and hurricanes.

Avalanche (Glacier flood) Avalanche

A snow-covered mountain like Mt. Everest collects a lot of
water vapour, ice crystals, snow and solid ice every year. This
builds up on the surface of the mountain and forms different
layers varying in their compactness as shown in the figure.
The lowest region of snow is hard compact and it doesn’t fall
off. But the middle layer is weak and can slide with the loosely
bound upper layer. When the snow deposits or builds up in
a larger quantity, the upper slab and middle layer eventually
break off and cause an avalanche.

An avalanche is a collection of rocks, frozen ice and snow that rolls down a mountain at a very
high speed. It is also called snow-slide or glacier flood. if the falling substance is snow. The
cause of the avalanche is the falling of heavy snow over a weak layer of deposited snow. An
avalanche falling under gravity can move at a speed of up to 320 km/hr.

400 Natural Hazards

FACT WITH REASON

How do earthquake cause avalanche?
Earthquakes can cause avalanches because the vibration of the earth’s surface causes the loosely bound
rocks, frozen ice and snow to fall downwards.

Causes of Avalanche
i) Global warming due to which snow melts rapidly.
ii) Heavy snow load
iii) Heavy rainfall
iv) Human activities like dynamite blasts, skies etc.
v) Volcanic eruption
vi) Glacial lake outburst
vii) Earthquakes
However, these factors just trigger an avalanche. The sole reason of an avalanche is the gravity
of the earth.
Effects of an Avalanche
i) It takes the life of the mountaineers and skiers.
ii) It causes destruction to public and private properties.
iii) It destroys forests and trees.
iv) It can block roadways, railways, etc.
v) It can destroy power grids, power stations and telecom towers causing disruption in

communication and electricity.
Control Measures of Avalanches
i) Mapping of the avalanche prone areas should be done.
ii) We should do afforestation in the avalanche prone areas.
iii) We should decrease the emission of greenhouse gases that decreases the global warming.
iv) We should do continuous monitoring and tests of avalanches in mountains.
v) We should avoid explosion and earth drilling during public events.
vi) We should install early warning systems to provide information of avalanches.
vii) We should manage essential materials like oxygen cylinder, medicines, snow diggers etc.
viii) We should decrease the slope of mountains artificially in those areas where skiing is common.
ix) We should build snow fences at several areas within the slopes.

FACT WITH REASON

Many mountaineers are killed every year in an attempt to climb Mt Everest. Why?
Many mountaineers are killed every year in an attempt to climb Mt Everest because of the heavy snowfall
that causes avalanches. However, there are other reasons like hypothermia that cause the death of the
mountaineers and their guides.

Modern Concept Science - 9 401

Glacial Lake Outburst Flood (GLOF)

A glacier is a large mass of dense ice (at least covering
an area of 0.15 km2 with thickness of 50m) moving or
falling slowly over an area. Glaciers are formed in the
Himalayan regions where the snow fall is so heavy
that the deposited snow converts into dense ice by
pressurization. This dense ice slides over the lower
weak layer and moves the whole ice mass slowly.

Glaciers are mostly formed during the winter when Glacial Lake Outburst Flood

there is heavy snowfall. As they move, their paths might be blocked by other snow mass or

rocks. Then, in summer these glacier continuously melt and form a lake. The lake formed by

the melting of a glacier is known as a glacial lake. Glacial lakes can have millions of cubic

litres of water. In the Himalayan regions of countries like Nepal, Bhutan, India etc., there are

many glacial lakes out of which some are potentially dangerous. They are dangerous because

the barrier of snow or rocks around them may erode over time and they might get flooded.

When the wall of the glacial lakes fails to hold, the water outpours and results in a glacial lake

outburst flood (GLOF). A GLOF is a glacial flood that occurs due to the breakage or failure of

the walls of a glacial lake.

Causes of GLOF

i) Continuous increase in volume of water in the lake due to the melting of glaciers.

ii) Earthquakes

iii) Volcanic eruptions

iv) Avalanches

v) Human activities like explosions

vi) Global warming

vii) Weak moraine dams

Effects of GLOF

i) It floods lower valleys.

ii) It destroys human, wildlife and livestock.

iii) It destroys properties and infrastructures like roads, bridges, etc.

iv) It floods the fertile lands and decrease productivity.

v) It causes soil erosion.

Control and Protective Measures for GLOF

i) Glacial Lakes should be mapped using advanced technologies.

ii) Careful monitoring of the potentially dangerous glacial lakes.

iii) Decrease in greenhouse gases that contribute to global warming and excess melting of
glaciers.

iv) Early warning and detection systems should be deployed.

v) Evacuation plans and preparedness can minimise the damage of lives and properties.

402 Natural Hazards

FACT WITH REASON

GLOF is a potential hazard in Nepal. Why?
GLOF is a potential hazard in Nepal because there are hundreds of glacier lakes that have loosely bounded
dams and embankments. They might fall during earthquake, landslide, avalanche or during other human
activities.

Hurricanes, Typhoons and Tropical Cyclones – Extreme Forms of
Oceanic Winds

Name of wind Places of origination
1. Hurricane Atlantic and North-Pacific ocean
2. Typhoon Northwest Pacific
3. Tropical South-West Indian Ocean and
South-West Pacific
cyclone

Hurricanes, typhoons and tropical cyclones are all
different names for extreme winds that occur above
oceans. The names of these strong winds are given
depending on the places they originate. The table
above shows the names of winds according to the
places of their origins.

Hurricanes, typhoons and tropical cyclones are

formed by similar processes. Warm evaporated water Hurricane

and thunderstorms form them. Hurricanes usually have a season i.e. from June to November

during which they occur. The countries affected by hurricanes are USA, Australia, Taiwan,

Vietnam, Japan, Mexico, China, India etc. In this chapter, we will study hurricanes in short.

How does a hurricane form?

A hurricane is a whirlwind or a vortex that draws warm evaporated air and can flood the
coastal areas. The birth of the hurricane starts with the formation of thunderstorms in the
clouds of upper atmosphere that drift towards the areas of warm ocean water (at least 26.50C
on the ocean’s surface). Then, the warm evaporated ocean water vapour rises up towards the
storms, forming a low pressure centre at the surface of the ocean. This further sucks a large
amount of warm water vapour and thus the storm grows larger and larger. Finally, the winds
blowing in the opposite direction of these storms and the earth’s rotation cause the storm to
rotate. The more the warm air is sucked, the faster the rotation occurs and the stronger the
hurricane becomes. Ultimately, the cool air from above is drawn towards downwards rapidly
to fill the gap of lower pressure of air and thus a hurricane consistently sustains.

Modern Concept Science - 9 403

FACT WITH REASON

Why does hurricane occur frequently around equatorial region?
Hurricanes occur around the equatorial region because of the blow of hot trade winds that arise due to the
direct heating of atmospheric air around equator. These trade winds which move towards the ocean start
storms and thus cause hurricanes.

In summary, the factors that help a hurricane to originate are as follows:

a) Winds and thunderstorms : Most hurricanes start with strong winds or thunderstorms
mostly originating from warm air of the African regions. These winds blow towards
oceans, start forming a vortex and become stronger.

b) Warm evaporated water vapour : The warm water that evaporates from the oceans is
a fuel for hurricanes. The more warm water the hurricanes draw in, the stronger they
become.

c) Low wind blow : Hurricanes do not form when there are significantly different strengths
of winds blowing in the upper and the lower atmospheres. Therefore, the winds have to
be low; otherwise they disperse the hurricane clouds apart.

A hurricane sustains till it gets the opportunity to draw Hurricane at costal area
warm water vapour from the oceanic surfaces. If a hurricane
reaches the land surface, it no longer gets warm water
vapour due to which its energy constantly decreases and
thus it dies out. But when it strikes the coastal areas, it floods
coastal areas, raises ocean levels and causes a lot of damage
to life and property. Hurricanes cause the sea or ocean level
to rise and this results in coastal flooding. This is known as
a sea-surge or storm-surge.

The strength and type of damage done by a hurricane is measured by Saffir-Simpson Scale. It
divides the hurricane into five categories: category One, Two, Three, Four and Five. Category
One is less destructive and category Five is the most destructive.

FACT WITH REASON

Hurricanes lose power once they reach the land. Why?

Hurricanes lose power once they reach the land because they are unable to get a constant supply of warm
evaporated water vapour to create a low pressure. Creation of low pressure above the water is necessary
for the hurricanes to form.

Effects of Hurricanes
i) Hurricanes flood the coastal areas and ruin the settlement.
ii) They cause severe damage to life and property.
iii) They obstruct communication, sanitation, transportation etc.

404 Natural Hazards

iv) They cause a storm or sea-surge due to which the coastal areas have to be evacuated.

v) Hurricanes bring thunderstorms and they may cause heavy rainfall.

vi) They destroy crop productivity and coastal business.
Safety and Precautions for Hurricanes

i) A 24-hour hurricane monitoring system with effective information dissipation should
be maintained.

ii) People should be careful during the stormy seasons.

iii) Warning systems should be in all streets near coastal lines.

iv) Preparedness and early evacuation plans should be adopted.

v) More artificial satellites should be launched which specifically target the study of
hurricanes.

vi) Emergency shelter, medicines, food, clothing, tents etc. should be given punctually to
the affected people.

vii) People should get quick, easy and regular updates on hurricane’s trajectory (path) on
their cellphones.

ANSWER WRITING SKILL

1. What are natural hazards?
Ans: The hazards that are caused by natural phenomena or incidents are known as natural hazards.
2. Mention any two hazards.
Ans: The two hazards are: forest fire and earthquake.
3. Write the full form and meaning of GLOF.
Ans: The full form of GLOF is Glacial Lake Outburst Flood. A GLOF is a glacial flood that occurs due to the

breakage or failure of the walls of a glacial lake.
4. What is the minimum temperature for a cyclone to form over an ocean?
Ans: The minimum temperature for a cyclone to form over an ocean is 26.5OC.
5. Mention any two methods of preparedness of natural disasters.
Ans: The two methods of preparedness of natural disasters are as follows:

i) We should conduct emergency drills regularly.
ii) We should keep emergency materials and medical kits at home.
6. Write any two differences between natural and man-made hazards.
Ans: The two differences between natural and man-made disasters are as follows:

S.N. Natural disasters S.N. Man-made disaster

i They are caused due to natural phenomena. i. They are caused due to human activities.

ii. They cannot be avoided. ii. They can be avoided.

7. Write in brief about the steps to be taken before a disaster in disaster management.

Ans: The steps to be taken before a disaster are:

a) Mitigation: Mitigation refers to risk reduction, susceptibility analysis, capacity building,
awareness generating, enforcement of building codes, etc.

Modern Concept Science - 9 405

b) Preparedness: The activities that are conducted before or prior to a disaster occurrence is known
as preparedness. It falls under the category of activities performed during the pre-disaster step.
Awareness generating programmes, building retrofitting, installation of monitoring systems
and early warning systems, drills of search and rescue and formulation of organized plans and
policies by the disaster response teams fall under this category.

STEPS EXERCISE

STEP 1

1. Define a hazard.
2. What do you mean by disaster?
3. What are anthropogenic hazards?
4. Write the meaning of hurricanes.
5. Write the full form of GLOF and ICIMOD.
6. What is an avalanche?

STEP 2

7. Name any two natural and anthropogenic hazards each.
8. What is a storm surge?
9. Write any two differences between natural and anthropogenic hazards.
10. Explain recovery and mitigation.
11. Why is Nepal prone to avalanches and GLOF?
12. Write any two differences between hazard and disaster.

STEP 3

13. Mention any three ways to manage anthropogenic hazards.
14. How does an avalanche occur?
15. What is a glacial lake? How does glacial flooding occur?
16. List out the important elements necessary for the formation of a hurricane.
17. Write a difference between hurricane, typhoon and cyclone.
18. Explain the major steps in the disaster management cycle.
19. Write any four causes of

a) Avalanche b) GLOF

20. Write down any four effects of: c) Hurricane
a) Avalanche b) GLOF

21. Mention any four safety measures of: c) Hurricane
a) Avalanche b) GLOF

406 Natural Hazards

UNIT Estimated teaching periods Theory Practical
3 0
23
Greenhouse

Syllabus issued by CDC Artificial greenhouse

 Introduction to greenhouse effect
 Causes and effects of greenhouse effects
 Artificial greenhouse and its importance
 Introduction to climate change, its causes and effects

LEARNING OBJECTIVES

At the end of this unit, students will be able to:
 introduce greenhouse in natural environment and explain its effects.
 explain the importance and utilities of artificial greenhouse.
 introduce climate change and describe causes, effects and mitigating measures of climate change.

Key terms and terminologies of the unit

1. Artificial greenhouse : An artificial greenhouse is a structure covered with polyethylene (plastic) or
glass that traps solar rays inside.

2. Greenhouse effect: The phenomenon of trapping of solar heat inside the earth or an artificial
greenhouse in such a way that it is relatively at a higher temperature is
known as the greenhouse effect.

3. Greenhouse gases : Those gases which trap the solar rays and heat up the atmosphere are
known as the greenhouse gases.

4. Climate : Climate is an average weather condition recorded over a long period of
time (usually 30 years).

5. Climate change : Climate change is a regular or irregular alteration in the weather patterns
observed over a very long period of time.

6. Examples of greenhouse gases: Ozone (O3), carbon dioxide (CO2), water vapour (H2O), methane (CH4),
Nitrous oxide (N2O), chlorofluorocarbon (CFC), etc. are greenhouse gases.

7. Infrared rays: Energy waves of wavelength longer than visible light and shorter than

microwaves are called infrared rays.

8. UV rays: Invisible rays emitted by the sun which cause skin cancer and eye
blindness are called UV rays.

9. CFC: Chlorofluorocarbon is a compound that contains chlorine, fluorine and
carbon. It is used in coolants and aerosols. It is responsible for ozone layer
depletion.

Modern Concept Science - 9 407

Introduction

A greenhouse can either be an artificial (greenhouse made up of glass or plastic) or a natural
greenhouse i.e. the earth. In the artificial greenhouse, summer plants can be grown even in
winter. This is possible as the glass or plastic that covers the greenhouse traps the hot solar
rays maintaining a higher temperature inside. Similarly, the earth also traps the heat of the
sun in the same way as an artificial greenhouse. Therefore, it is called a natural greenhouse. In
this chapter, we will discuss what an artificial greenhouse is and how it works. Similarly, we
will also learn how the earth acts as a natural greenhouse.

FACT WITH REASON

A greenhouse is called greenhouse, why?
A greenhouse is called greenhouse because we mostly grow green plants inside it. Also, in the past, the
glass of the greenhouse was tinted light green by the addition of metal oxide.

Artificial Greenhouse Artificial Greenhouse

An artificial greenhouse is a structure covered with
polyethylene (plastic) or glass that traps solar rays inside.
Due to the trapping of the solar rays, it is hotter inside a
greenhouse than outside. This enables us to grow summer
plants or off season plants even in winter season. A
greenhouse is called a greenhouse because we mostly grow
green plants inside it.

Phenomenon of heat trapping

When the solar rays enter the glass or plastic, most of the short wavelength having higher
energy are allowed to enter inside. These rays strike the ground and plants and are absorbed.
But when they are emitted, the rays have a long wavelengths having low energy. These emitted
rays cannot penetrate the glass or plastic and thus gets trapped inside.

FACT WITH REASON

The roofs of greenhouse can be of various shapes but mostly slanted roof is preferred, why?
The roofs of greenhouse can be of various shapes but mostly slanted roof is preferred because it allows
maximum sun rays to enter even the sun is too low in the horizon.

Use and importance of a greenhouse
1. A greenhouse is used to grow the plants that need higher temperature.
2. It helps to grow fruits, vegetables and flowers commercially in a large quantity even in

the off season.
3. It protects the plants from excess frost and dew formation.
4. It helps to protect the plants from air pollution.
5. It prevents the breakage of plant stems during heavy rainfall.
6. It also protects the smaller plants from the winds.

408 Greenhouse

The main function of an artificial greenhouse is to preserve the heat of the sun inside its glass
or plastic structure. The earth also preserves the heat inside its atmosphere in the similar
manner. But what causes the earth to trap heat even it is not covered by glass or plastic?

Earth: A Natural Greenhouse Natural greenhouse

If we compare the earth and the moon, which are nearly
at same distances from the sun, the earth has suitable
temperature, but the moon is very cold and hot during the
night and the day respectively. This is due to the reason
that moon doesn’t have atmosphere. So it is not able to
trap the solar rays inside it. But the earth is surrounded
by several layers of gases called atmosphere that traps
the heat from the sun. But how? The answer is due to the
greenhouse effect.

FACT WITH REASON

It is not the sun due to which we feel hot but mostly the atmosphere, why?
It is not the sun due to which we feel hot but mostly the atmosphere because the atmosphere absorbs a
large amount of heat from the sun and ground. Without the atmosphere, we would certainly die by being
exposed to UV rays but the temperature at night would be reduced below the freezing point.

The Greenhouse Effect

The earth receives two kinds of radiation from the sun: shortwave radiation (UV rays and
visible light mostly) and long wave radiation (Infrared rays mostly). The short wave radiation
can pass easily into the earth’s atmosphere while the long wave radiation is absorbed. When
these solar rays are absorbed by the earth’s surface, it starts to reflect infrared rays (heat
carrying rays). They are immediately re-absorbed by the atmosphere. After absorption, the
atmosphere also gives heat to all directions. This atmospheric heat is again absorbed by the
ground. In this way, there is a continuous exchange of heat between the atmosphere and
the earth’s surface due to which the temperature of the earth is maintained warmer. This
phenomenon is known as the greenhouse effect.

The phenomenon of trapping of solar heat inside the earth or an artificial greenhouse in such
a way that it is relatively at a higher temperature is known as the greenhouse effect. The
greenhouse effect is mostly advantageous to us as it maintains a constant surface temperature
on the earth and prevents it from chilling. But if there is excessive greenhouse effect, it can cause
the overall temperature of the earth to rise significantly, which can then be disadvantageous to
us. Hence, greenhouse effect is both beneficial as well as harmful to us.

FACT WITH REASON

Earth acts as a natural greenhouse, why?
Earth acts as a natural greenhouse because the atmosphere that covers the earth traps the solar heat inside.

Among several gases of the atmosphere, ozone (O3), carbon dioxide (CO2), water vapour
(H2O), methane (CH4), Nitrous oxide (N2O), chlorofluorocarbon (CFC), etc. have higher ability

Modern Concept Science - 9 409

of absorbing solar rays compared to other gases. These gases which trap the solar rays and
heat up the atmosphere are known as the greenhouse gases. Some of the greenhouse gases
and their sources are given below:

i. Carbon dioxide (CO2): burning of fossil fuels, coal, petrochemicals, wood, respiration etc.
ii. Water vapour (H2O): evaporation from seas, oceans, rivers, etc., burning of hydrocarbons etc.
iii. Methane (CH4): decomposition of organic substances like dung, faeces, waste vegetables,

dead organisms, etc.
iv. Chlorofluorocarbon (CFC): aerosols and coolants
v. Nitrous oxide (N2O): fertilizers, waste water management, burning of fossil fuels,

livestock manure, etc.

FACT WITH REASON

Refrigerators, air conditions, deodorants etc. are bad for ozone layer, why?
Refrigerators, air conditions, deodorants etc. are bad for ozone layer as they release gases called
Chlorofluorocarbons which deplete the ozone layer. But some of them contain ammonia and
hydrofluorocarbons which do not deplete the ozone layer but contribute to greenhouse effect.

Adverse impacts of greenhouse effect
Some of the adverse impacts of greenhouse effect are given below:

1. Effects on animals
i. Greenhouse effect helps to spread different types of diseases like cholera,
diarrhoea, malaria, filariasis, etc.
ii. It helps to increase the generation of pathogenic insects.

2. Effects on plants
i. It affects the cultivation and harvesting periods of crops.
ii. Tropical plants are seen at the temperate region due to the effect of global warming
iii. It helps to increase the disease causing insects.

3. Effects on climate
i. Greenhouse effect increases the temperature of the earth.
ii. It helps melting of ice in the polar region, which increases the sea level. As a
result, low land areas of the earth may submerge.
iii. It changes the pattern of rain fall and weather condition.

Ways of controlling greenhouse effect
i. Greenhous effect can be controlled by controlling the production and use of CFCs.
ii. It can be controlled by using alternative sources of energy like solar energy, wind
energy, biogas, etc.
iii. Itcanbecontrolledbyreducingtheburningoffossilfuelslikepetrol,diesel,kerosene,
coal, etc.
iv. It can be controlled by decreasing deforestation and increasing afforestation.

410 Greenhouse

FACT WITH REASON

It is always warmer inside the green house than surrounding. Why?
It is always warmer inside the green house than surrounding because solar heat that enters into green
house is trapped due to decrease in wave length of the reflected rays.

Climate Change

Climate is an average weather condition recorded
over a long period of time (usually 30 years). It
is obtained by collecting the statistical data of
temperature, precipitation, seasons, humidity etc.
for a long period of time in a place. It is different
from weather as weather is a temporary condition
e.g. rainy, windy, cloudy or sunny.

Climate of a place doesn’t change much even
during many decades or it is difficult to notice
the change. Due to this slight change, the average
temperature of the earth’s surface remains
constant. Thus it supports life on the earth. If the earth’s temperature increases even slightly
e.g. by 2 degree Celsius, then it will be disastrous. If we study the climate data since 1880 A.D.,
it has been found that the change in climate has accelerated. The surface temperature has risen
by 1.80F. It is a bad news. It is to be noted that climate continuously changes due to natural
causes like solar activity, earthquake, volcano, earth’s rotation etc. But change due to natural
cause is very slight. The main problem is anthropogenic causes (human causes). Scientists
believe that the greenhouse gases, environmental pollution, deforestation, industrial gases
and other human activities are the main reasons behind the change in climate.

Now, let us define climate change. Climate change is a regular or irregular alteration in the
weather patterns observed over a very long period of time. To observe climate change, we
need to analyze the statistical data of decades.
Causes of Climate change
1. Excessive carbon emission
The emission of large amount of CO2 from burning of fossil fuels, coal, wood, respiration,

etc. contributes to greenhouse effect due to which the overall temperature of the earth is
increasing resulting climate change.

2. Increasing temperature of the earth
The emission of large amount of greenhouse gases increases the overall temperature

of the earth called global warming. The global warming causes the change in rainfall,
snow fall, seasonal patterns, etc. As a result, it causes climate change.

3. Ozone layer depletion
The Chlorofluorocarbons (CFCs) emitted by coolants and aerosols reach the upper

stratosphere and break down ozone gas. This lets more UV rays enter the earth
increasing temperature of the earth. This causes change of climate over a long time.

Modern Concept Science - 9 411

FACT WITH REASON

Use of CFCs in refrigerators, AC should be replaced by ammonia. Why?
Refrigerators, ACs, deodorants, etc. with CFCs are bad for ozone layer as they release gases called
Chlorofluorocarbons which deplete the ozone layer. But some of them contain ammonia and
hydrofluorocarbons which do not deplete the ozone layer but contribute to greenhouse effect.

4. Industrialization
Industrialization increases industrial gases like the oxides of nitrogen and sulphur.

They significantly alter climate due to their ability to reflect or absorb solar radiation.
Thus, over a long period of time, this can cause climate change.
5. Agricultural wastes, treatment of waste water and chemical fertilizers
Any waste and its treatment produces a lot of methane gas. Methane gas is a greenhouse
gas. Hence it causes global warming. Similarly, the excessive use of chemical fertilizers
has also increased the levels of nitrous oxide in the atmosphere which also contributes
to global warming. After a long time, this causes climate change.
6. Deforestation
Forest maintains the amount of carbon dioxide and oxygen in the atmosphere. The
amount of carbon dioxide gas in atmosphere increases due to deforestation. It results in
increase in temperature and climate change.
7. Volcanic activities
Volcanic eruption brings out tremendous amount of carbon dioxide, sulphur dioxide,
water vapour, ashes and dust particles. These substances released in the atmosphere
produce various other compounds that surround the earth’s atmosphere. If these
substances are carried by equatorial winds, they will prevent the heating effect and
thus cause cooling of climate.

FACT WITH REASON

Volcano causes climate change. Give reason.
Volcano releases lots of hot carbon dioxide, methane and other greenhouse gases which increase
temperature of the atmosphere resulting climate change.

Effects of Climate Change

1. Climate change increases the rate of snow and ice melting due to which sea levels rise.
2. Climate change can change rainfall patterns and ultimately affects plantation and

harvesting of crops.
3. Climate change can result in extreme global warming.
4. It can push several organisms towards extinction.
5. It can cause the natural disasters like hurricanes, GLOF, floods, avalanches, tornadoes,

cyclones to occur frequently and violently.

412 Greenhouse

6. It results in the flooding of coastal areas and countries like the Maldives, India (Mumbai),
Japan, New York.

7. It affects the global weather pattern.
8. It causes displacement of human population near from the coastal areas.
9. It can lead to drought, famine and natural calamities due to which instability of

governments might occur.
10. It affects natural and cultural heritages.
11. Climate change can cause various epidemic and fatal diseases in human.

Measures of climate change control and management

1. Alternative sources of energy should be promoted due to which carbon emissions can
be cut off.

2. Afforestation should be done in every possible barren land. Forests absorb large amount
of carbon dioxide from the environment and controls global warming.

3. The solid wastes should be reduced, reused and recycled. Burning of wastes should be
the last option.

4. The use of fossil fuels like petrol, kerosene, diesel etc. should be discouraged.
5. Chemical fertilizers should be replaced by organic fertilizers.
6. The ozone layer depleting gases like CFCs should be completely banned.
7. Filters and high tech chimneys should be used in factories to prevent the toxic gases to

escape in the atmosphere.
8. Public awareness programmes should be conducted extensively. People should be

encouraged to conserve energy sources and use public transportation for travelling as
far as possible.
9. Energy efficient buildings should be constructed.
10. CFCs in the refrigerators should be banned completely.
11. Development works should be conducted for necessity, not for luxury as far as possible.

ANSWER WRITING SKILL

1. Define greenhouse effect.
Ans: The phenomenon of trapping of solar heat inside the earth or an artificial greenhouse in such a way that

it is relatively at a higher temperature is known as the greenhouse effect.
2. Mention any two sources of greenhouse gases.
Ans: The two sources of greenhouse gases are:

i. Burning of fossil fuels are the sources of carbon dioxide.
ii. Some refrigerators, air conditioners, etc. are the sources of CFCs.

Modern Concept Science - 9 413

3. Is climate change only due to artificial causes?

Ans: No, climate change is not just due to artificial causes. It is also due to natural phenomena. But the main
cause of climate change in the recent years is due to the bad human activities.

4. W hat is an artificial greenhouse?

Ans: An artificial greenhouse is a structure covered with plastic or glass that traps solar rays inside and
allows the plants to grow even in winter season.

5. Nepal is sensitive to climate change effects. Justify.
Nepal has a wide range of vegetation, altitude and wildlife with socio-economic differences fully
dependent on climate. It is also rich in various medicinal plants, herbs, rare organisms and climatic
features. Climate varies drastically just over few kilometres apart. Since the past, the temperature in
Nepal has increased by 0.060C due to various greenhouse gases and pollutants emitted globally. The
water sources like the rivers, rivulets and springs have dried up. The flow volume of rivers has also
decreased. Likewise, incidents of glacial lakes outburst floods (GLOF) have also increased. Many
organisms have gone endangered and even extinct. Moreover, the snow in the Himalayas is melting
rapidly. Due to these various causes, we can conclude that Nepal is sensitive to climate change effects.

6. Write and describe any two causes of climate change due to human activities.

Ans: The two causes of climate change are:
a. Excessive carbon emission: The emission of a large amount of CO2 from burning of fossil fuels,
coal, wood, respiration, etc. contributes to greenhouse effect due to which the overall temperature
of the earth is increasing. The global warming is causing change in rainfall, snowfall, seasonal
patterns, etc. As a result, it causes climate change.
b. Industrial gases: The industrial gases like the oxides of nitrogen and sulphur can significantly
alter climate due to their ability to reflect or absorb solar radiation. These suspended particles
can cool the air by reflecting and absorbing solar rays. Thus, over a long period of time, this can
cause climate change.

7. Mention any four effects of climate change.

Ans: The four effects of climate change are:
a. It increases the rate of snow and ice melting due to which sea levels rise.
b. Climate change can change rainfall patterns and ultimately affects plantation and harvesting of
crops.
c. Climate change can result in extreme global warming.
d. It can push several organisms towards extinction.

8. If you were the prime minister of Nepal, how would you minimize the effects of climate change in the
country? Write in four points.

Ans: I would do the following things:
1. I will promote alternative sources of energy in place of fossil fuels.
2. I will encourage afforestation in every possible barren land.
3. I will encourage the farmers to use organic fertilizers in place of chemical fertilizers.
4. I will launch public awareness programmes extensively about the climate change and its effects.

414 Greenhouse

STEPS EXERCISE

STEP 1

1. What is an artificial greenhouse?
2. Define natural greenhouse.
3. What is greenhouse effect?
4. Define the term climate change.
5. Write the full form of CFC.
6. What is meant by ozone layer depletion?

STEP 2

7. Write any two uses of an artificial greenhouse.
8. Name any four greenhouse gases.
9. List out any four effects of climate change.
10. How does the earth act as a greenhouse?
11. Write a difference between greenhouse effect and global warming.

STEP 3

12. Explain the phenomenon of heat trapping in an artificial greenhouse.
13. Describe how the earth maintains greenhouse effect.
14. Write the measures to control or manage climate change.
15. What is global warming? What effect does global warming cause in Antarctica? What

effect does it cause in Nepal?
16. Write down any five causes of greenhouse effect.
17. Mention any five adverse impacts of greenhouse effect.
18. Name any four effects of climate change on environment.
19. Mention the causes of climate change and explain any two each.
20. List out any five points to reduce climate change.
PROJECT WORK
1. Construct an artificial greenhouse using a wooden base and glass or plastic. Use transparent

plastic as it is less hazardous compared to glass. Fill the base first with some mud. Grow small
plants if possible. Leave a door as an entrance. Use your creativity to place an overhead sun.

Modern Concept Science - 9 415

UNIT Estimated teaching periods Theory Practical
4 1
24
The Earth in the Universe

Syllabus issued by CDC Yuri Gagarin

 Introduction to universe
 Units related to astronomical distance (light year, AU and parsec)
 Solar system
 The rotation and revolution of the earth
 Sidereal and synodic month and phases of the moon
 Position of sun, earth and moon
 Eclipse (solar eclipse and lunar eclipse)

LEARNING OBJECTIVES

At the end of this unit, students will be able to:
 explain rotation and revolution of the earth.
 describe the phases of the moon.
 explain the relationship among the sun, earth and moon in terms of their position.
 define umbra and penumbra with figures.
 explain solar eclipse and lunar eclipse with labelled figures.

Key terms and terminologies of the unit

1. Universe : The vast region of space which contains all the matter and energy, seen or unseen is
known as the universe.

2. Milky Way galaxy : The galaxy where we live in is known as the Milky Way galaxy.

3. One AU : The average distance between the earth and the sun is considered as 1 AU. 1 AU = 1.5 × 108
km = 1.5 × 1011 m

4. Light year : The distance travelled by light in one year passing through vacuum is called light year.
One light year is equal to 9.46 × 1015 m.

5. Parsec : Parsec or parallax of one second is the unit to measure the distance of stars outside the
solar system. 1 parsec= 3.26 light years

6. Dwarf planet : A dwarf planet is a heavenly body that revolves around the sun It is nearly spherical in
shape but doesn’t have its own orbit.

7. An axial tilt : An axial tilt or obliquity is the angle made by the rotational axis of the earth with the
perpendicular line to orbital plane.

8. Diurnal motion : The motion of the earth around its own axis is called the diurnal motion of earth.

9. Annual motion : The motion of the earth around the sun is known as revolution or annual motion of the
earth.

416 The Earth in the Universe

10. Leap year : The number of days in every fourth year is 366. This year is called a leap year.

11. Solstice : A solastice is an astronomical event that occurs twice each year when the tilt of the
earth's axis is most oriented towards or away from the sun.

12. Equinox : Equinox is the time when both the earth’s hemispheres are at equal distances from the sun.

13. Waxing period : Over a period of 15 days, the moon continuously increases its area of brightness from
invisible to full bright moon. This period is known as the waxing period.

14. Waning period : The 15 days of the month, where area of brightness of the moon continuously decreases
from a fully bright moon to an invisible moon. This period is known as the waning period.

15. Phases of the moon : The different shapes of the moon seen in the sky over a period of the month are known
as the phases of the moon.

16. New moon : If the moon is not visible in the sky, it is known as the new moon.

17. Crescent moon : If ¼ part of the moon is illuminated, the moon is known as crescent moon.

18. Half moon : If the half part of the moon is illuminated, it is called the half moon.

19. Full moon : If the moon is fully illuminated in the sky giving a circular appearance, it is known as
the full moon.

20. Shadow : A shadow is a two-dimensional dark region formed on a surface when an opaque
object blocks the path of light on that surface.

21. Umbra : The innermost darkest part of the shadow is called umbra.

22. Solar eclipse : When the moon comes exactly in between the straight line joining the sun and the
earth, casting a shadow on the earth’s surface, the eclipse formed is known as the

solar eclipse.

23. Lunar eclipse : When the earth comes in between the sun and the moon, the shadow of the earth is
casted on the moon. This event is known as the lunar eclipse.

Introduction

The vast region of space which contains all the matter and energy, seen or unseen is known as
the universe. It contains billions of stars grouped together in a system called galaxy. A galaxy
contains many stars around which planets revolve. These systems are called planetary system.
We live in the galaxy called Milky Way and inside a planetary system called the solar system.

Milky Way galaxy

The galaxy where we live in is known as the Milky Way galaxy. It is one of the 200 billion galaxies
that human has observed through telescope. It has an average diameter of 120,000 light years.
It has width of 1,000 light years and contains over 200 billion stars. The shape of the Milky
Way galaxy is spiral. It has a centre from which two arm-like structures made up of stars
emerge. The solar system lies at a distance of 25,000 light years from the galactic centre.

Measurement of distance in the universe

The universe is very huge. If anyone travels with a maximum possible speed i.e. the speed
of light (3 x 108 m/s), one may take billions of years and still would not reach the end of it. To
measure the distance of heavenly bodies, the following units are used:

Kilometer : To measure short distance

Modern Concept Science - 9 417

Astronomical unit (AU) : To measure the distance inside the solar system.
What is 1 AU distance?
The average distance between the earth and the sun is considered as 1 AU.
1 AU = 1.5 x 108 km = 1.5 x 1011 m

Light year (ly)
The distance travelled by light in one year passing through vacuum is called light year. One
light year is equal to 9.46 x 1015 m. It is generally used to measure the distance between galaxies.
In one second, light travels about 3 x 108 m
Hence, in a year, it travels 9.46 x 1015 m

Parsec (pc)
Parsec or parallax of one second is the unit to measure the distance of stars outside the solar
system. The relation between parsec with different units can be written as:

1 parsec= 3.26 light years = 31 trillion km

The Solar System

The solar system is a well-known planetary system that exists inside one of the arms of the
Milky Way at a distance of 27,000 light years from the galactic centre. The system consists of
the sun with its planets, satellites, comets, asteroids etc. revolving around it is called solar system.
The family of solar system consists of:

a) The Sun: the central star
b) The planets: heavenly bodies that revolve round the sun
c) Asteroids: the masses of rocks between Mars and Jupiter
d) Satellites: the heavenly bodies that revolve round the planets
e) Comets: the icy and rocky bodies which have a highly elliptical orbit around the sun
f) Meteoroids: freely moving rocks inside the solar system
g) Interplanetary dust and gases

Planets (from Greek ‘Planetes’ meaning ‘wanderers’)

As understood earlier, planets were known as the heavenly bodies which revolve round the
sun and do not have their own light. This definition gave rise to many planets which made
them difficult to study. So, this concept was dropped. In 2006 A.D., a convention was held
among the astronomers of International Astronomical Union (IAU) in Czech Republic. Its’
conclusions divided the planets into two major groups:

i) Major Planet and
ii) Minor Planet
To be a major planet, the following three conditions should be met:

i) It should nearly be spherical in shape.

418 The Earth in the Universe

ii) It should revolve around the sun.

iii) It should have cleared its neighbourhood i.e. it should not have small bodies surrounding
its orbit.

There are altogether eight major planets which have met these requirements. They are: Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto, which was considered earlier
as a main planet, is now designated as a minor or dwarf planet. It is because Pluto has several
heavenly bodies near its orbit i.e. it has not cleared its neighbourhood.

So, in contrast to a major planet, a dwarf planet is a heavenly body that revolves around the sun,
is nearly spherical but doesn’t have its own orbit. Some of the well-known dwarf planets are:
Ceres, Pluto, Haumea Makemake and Eris.

Motion of the earth- along its own axis and around the sun

The earth is tilted at an angle of 23.5 degrees from North
the vertical position. It is called axial tilt or obliquity.
An axial tilt or obliquity is the angle made by the Perpendicular celestial
rotational axis of the earth with the perpendicular line to orbit pole
to orbital plane. It is shown in the figure. Also, the
Axial
tilt or
obliquity Rotation

axis

Celestial equator

angle between the rotational axis and the orbital
plane is 66.5 degrees. If this angle had been 90 Orbit direction

degrees, the earth would not have been tilted, and

the day and night time would have been same in

all places and throughout the year. This doesn’t

happen in reality as the earth is tilted. South
celestial
The earth rotates from west to east along its
rotational axis. The time taken by the earth to rotate pole

Motion of earth

once around its own axis is known as time of rotation. The motion of the earth around its own

axis is called the diurnal motion of earth. It is equal to approximately 24 hours (23 hr 56 min 4.09

sec). The diurnal motion causes the day and night. The earth rotates from west to east due to

which the sun rises in the east and sets in the west on the earth. One diurnal motion marks

one day and one night.

The earth also revolves around the sun along an elliptical orbit. It marks the completion of
a year in a yearly calendar i.e. from January to December and from Baishakh to Chaitra. The
motion of the earth around the sun is known as revolution or annual motion of the earth. The
earth takes approximately one year i.e. 365.25 days (365 days 5 hr 48 min 46 sec) to complete
one revolution around the sun. The annual motion of the earth is responsible for the change of
seasons: from spring, summer, autumn (British English) or fall (US English) to winter.

In a calendar there are only 365 days in a year covering 0.25 day every year. If we lose 0.25 day
in a year, we lose one day in every four years. This is why to compensate the loss; the number
of days in every fourth year is 366. This year is called a leap year. This extra day is added in the
month of February (28+1=29 days). A leap year is exactly divisible by 4. For example, 2016
A.D., 2012 A.D. etc.

Modern Concept Science - 9 419

FACT WITH REASON

The sun rises in the east and sets in the west, why?
The sun rises in the east and sets in the west because the rotation of the earth is from west to east due to
which the eastern part of the earth faces the sun first.

Change of Seasons

Due to the axial tilt, elliptical orbit and the March 21
annual motion of the earth, the seasons
continuously change throughout the year Northern spring / Southern autum
on the earth. There are four seasons: spring,
summer, autumn and the winter. The seasons Northern summer Northern winter
change differently for northern and southern June 21 Southern winter Southern summer
hemispheres as they receive different amounts
of sunlight. Let us see this diagram and learn December 22
a few terminologies before understanding the
change of seasons: September 23

a) Equinox and solstice Northern autum / Southern spring

Change of seasons

A solastice is an astronomical event that occurs twice each year when the tilt of the earth's axis is
most oriented towards or away from the sun. If the hemisphere is nearest to the sun, it is called
summer solstice and if it is farthest from the sun, it is called winter solstice. The days and nights
in a solstice are not equal. Both the hemispheres receive unequal amount of sunlight during
a solstice.

Equinox is the time when both the earth’s hemispheres are at equal distances from the sun. In this
condition, the northern and the southern hemispheres receive equal amount of sunlight due
to which the days and nights are equal throughout the globe.

b) Tropic of Cancer and Tropic of Capricorn:

The imaginary line representing 23.5 degrees southern latitude is known as the Tropic of Capricorn.
Likewise, the imaginary line representing 23.5 degrees northern latitude is known as the
Tropic of Cancer. The Tropic of Cancer and Capricorn are the farthest north and the south
regions where the sun rays strike perpendicularly.

As the earth revolves around the sun, let us assume that it attains four main positions, as

shown in the given figure. The sun's ray Rotation Axis

i) Position 1 : Dec 22- On December 22, the southern Arctic
hemisphere of the earth faces nearest or towards circle
to the sun and the northern hemisphere faces

farther away. So, the southern hemisphere has

summer solstice and northern hemisphere has Equator
winter solstice. At this position, the overhead sun
falls perpendicularly at the Tropic of Capricorn. Antarctic
On December 22, the day is shortest and night is circle

longest in the northern hemisphere as it receives

less sunlight. Similarly, in the southern hemisphere, the day is longest and night is

shortest. Dec 22 is known as winter solstice.

420 The Earth in the Universe

b) Position 2 : March 21 – On this day, the northern and North pole
The sun's ray Equator
southern hemispheres remain at equal distances from
the sun and thus receive equal amount of sunlight. At
this position, the northern hemisphere has spring or
vernal equinox and the southern has autumnal
equinox. The length of the day and night are equal in
every part of the world on this day. March 21 is
known as the spring or vernal equinox. At this position,
the equator receives perpendicular rays of sun.

c) Position 3 : June 21- On this day, the northern Rotation Axis The sun's ray
hemisphere remains closest or towards the sun and Arctic

the southern hemisphere remains the farthest or away circle
from the sun. At this condition, the northern Equator

hemisphere has summer solstice and the southern

hemisphere has winter solstice. The rays of overhead Antarctic
sun fall perpendicularly on the Tropic of Cancer. On circle

this day, days are longer than night in the northern

hemisphere as it receives more sunlight. In fact, June 21 has the longest day and shortest

night. This day is also called the summer solstice.

d) Position 4 : Sep 23- On September 23, the northern

and southern hemispheres again remain at equal The sun's ray

distance from the sun and thus receive equal amount

of sunlight. At this position, the northern hemisphere

has autumnal equinox and the southern has spring

or vernal equinox. The length of the day and night South pole
are equal in every part of the world on this day.

September 23 is known as the autumnal equinox. At

this position, the equator receives perpendicular rays of sun. This day is also called the

autumnal equinox.

FACT WITH REASON

What happens due to annual motion of the earth?
Annual motion of the earth causes change in season, difference in the duration of day and night and leap year.
Why does change of seasons occur?
The change of seasons occur because of the following factors:
1. Revolution of the earth around the sun in an elliptical orbit.
2. Earth’s tilt at an angle of 23.5 degrees.
3. Spherical shape of the earth

Days and nights are equal on September 23 and March 21. Why?
Days and nights are equal on September 23 and March 21, since the southern and northern hemispheres
receive equal amount of sunlight on these days.

Modern Concept Science - 9 421

Phases of the moon Waning Crescen Last Quarter Half Waning Gibbous
Moon
If we watch the moon in the night sky continuously Moon Moon

over a month, we can see different variations in the

shape of the moon. Over a period of 15 days, the New moon Full moon

moon continuously increases its area of brightness

from invisible to full bright moon. This period is

known as the waxing period. Similarly, in the next

15 days of the month, the area of brightness of the Waxing Crescen First Quarter Waxing Gibbous
moon continuously decreases from a fully bright Moon Moon Moon
moon to an invisible moon. This period is known
as the waning period. A month is the combination Phase of the moon

of the waxing and the waning periods of the moon. The 15-day period of a month in which the

moon’s brightness increases is known as the bright half. Likewise, the remaining 15 days of the

month in which the moon’s brightness decreases is known as the dark half.

The different shapes of the moon seen in the sky over a period of the month are known as the phases
of the moon. Depending on the shape and illuminated part of the moon, it has different names:

a) New moon : If the moon is not visible in the sky, it is known as the new moon. It is
also called “Aunsi”. After the new moon, the beginning of the waxing part of the moon
occurs.

b) Crescent moon : If ¼ part of the moon is illuminated, the moon is known as crescent
moon. If the crescent moon is of the bright half or increases in brightness with time, it is
called the waxing crescent. But if it is of the dark half and decreases in brightness over
time, it is known as the waning crescent.

c) Half-moon : If the half part of the moon is illuminated, it is called the half moon. A half
moon is called the first quarter if it is waxing and last quarter if it is waning.

d) Gibbous moon : If ¾ part of the moon is illuminated, it is known as the gibbous moon.
A gibbous moon is called a waxing gibbous, if it increases its brightness over time.
Likewise, if it decreases its brightness over time, it is known as the waning gibbous.

e) Full moon : If the moon is fully illuminated in the sky giving a circular appearance, it is
known as the full moon. It is also called “Purnima”. A full moon is the beginning of the
waning part of the moon.

The phases of the moon start either with full moon or new moon. From full moon, the moon
starts to wane and decreases its area of brightness forming a waning gibbous, last quarter,
waning crescent and new moon respectively. It takes around 15 days to complete this dark
half. From the new moon too, it takes the same time to complete a bright half in which the
moon changes from new moon to waxing crescent, first quarter, waxing gibbous and full
moon. It takes around 3.75 days to jump from one phase of the moon to another. The phases
of the moon are explained in the figures.

422 The Earth in the Universe

Shadow Total Eclipse Annular eclipse

A shadow is a two-dimensional dark Sun
region formed on a surface when an Moon
opaque object blocks the path of light on
that surface. The innermost darkest part Partial eclipse
of the shadow is called umbra while
the outer less darker part is called the Process of shadow formation
penumbra. Sometimes, the shadow
seems to be contained in a disc of light
in an inverted cone forming a region
known as the antumbra. Given below
is an activity that demonstrates the
formation of umbral and penumbral
regions in the shadow:

ACTIVITY 1

Take any opaque object like book, ball, stone etc. on a sunny day and keep it on your hand in such a way
that the shadow it projects falls upon a white surface like paper or wall. This paper or wall acts like a
screen which receives the shadow of the object. Observe the formation of shadow. Note the umbral and
penumbral regions.

Eclipse

The earth moves around the sun and the moon moves around the earth. In every full moon
night, the earth comes in between the sun and the moon and in every new moon night, the
moon comes in between the sun and the earth. But they do not align themselves exactly in a
perfect straight line. The linear alignment of sun, moon and the earth in a straight line is called
sygyzy. The sygyzy doesn’t occur in every new moon and full moon nights because the moon’s
orbit subtends an angle of 5.150 with the earth’s orbit. Sometimes, the sun, moon and the earth
come in a straight line. When they come in a straight line and the moon is between the sun and
the earth, the moon casts a shadow on the earth’s surface. This is called the solar eclipse. Solar
eclipses occur only on new moon days. Likewise, when the earth is between the moon and the
sun, the shadow of the earth is casted on the moon. This is known as the lunar eclipse. Lunar
eclipse occur only on full moon nights.

In a certain year, there can be minimum of four eclipses (2 solar+ 2 lunar) and maximum of 7
eclipses (2 solar + 5 lunar or 5 solar + 2 lunar). Seven eclipses in a year is very rare. In the past,
it had occurred in 1935 A.D. and it will occur in 2132 A.D. But, all the eclipses cannot be seen
from a single place at a given time.

There are two main types of eclipses. They are solar and lunar eclipse.

FACT WITH REASON

Total eclipses are rare, why?
Total eclipses are rare at a certain place because the probability of forming the umbral shadow of the earth
or moon to be seen at the same place on earth is very less.

Modern Concept Science - 9 423

FACT WITH REASON

A partial eclipse is longer than a total eclipse, why?
A partial eclipse is longer than a total eclipse. This happens because partial eclipse is formed by the
penumbral part of the shadow and total is formed by the umbral part. The area of the penumbra is larger
than the area of umbra. Hence, to pass by the penumbral shadow it takes a longer time.

Solar eclipse

When the moon comes exactly in between the straight line joining the sun and the earth, casting
a shadow on the earth’s surface, the eclipse formed is known as the solar eclipse. A solar eclipse
occurs only in the new moon day because the moon comes in between the sun and the earth
at this time.

FACT WITH REASON

A solar eclipse occurs only in the new moon day, why?
A solar eclipse occurs only in the new moon day because the moon comes in between the sun and the earth
at this time.
Why does solar eclipse occur?
A solar eclipse occur due to the following reasons:
1. Light always moves in the straight line.
2. The sun, the moon and the earth, come to lie in a straight line.
3. The earth revolves round the sun and the moon revolves round the earth.
4. The moon casts a shadow on the on the earth.

Sun Partial eclipse

Total eclipse Earth

Moon

Umbra Moon's Earth's
Penumbra orbit orbit

Solar eclipse

When the moon comes in between the earth and the sun, two basic regions of its shadows are

formed. They are umbral region and penumbral region. A person in the umbral region of the

moon’s shadow experience an eclipse in which it goes total darkness. This is known as the

total solar eclipse. If the person is in the penumbral region of the moon’s shadow, he/she

experiences a partial darkness known as the

partial solar eclipse. In rare occasions, the moon

covers the sun in such a way that a bright ring is

seen at the edges of the sun. This is known as the

annular solar eclipse. Partial Annular Total

Types of eclipse

424 The Earth in the Universe

A total solar eclipse occurs after 360 years while an annular eclipse occurs only after thousands
of years in the same place. But around the world, solar eclipses occur in every 2.5 years in
average. A solar eclipse lasts for almost 7 minutes because the earth and the moon travel faster
in the opposite direction and also the moon casts a smaller shadow on the earth.

Annular solar eclipses are very important events for astronomers because they get an
opportunity to study about the sun’s surfaces which otherwise is very bright for the human
eyes and instruments to see.

FACT WITH REASON

A solar eclipse lasts for almost 7 minutes, why?
A solar eclipse lasts for almost 7 minutes because the earth and the moon travel faster in the opposite
direction and also the moon casts a smaller shadow on the earth.
Annular solar eclipses are very important events for astronomers, why?
Annular solar eclipses are very important events for astronomers because they get an opportunity to study
about the sun’s surfaces which otherwise is very bright for the human eyes and instruments to see.

Lunar eclipse

When the earth comes in between the sun and the moon, the shadow of the earth is casted on the
moon. This event is known as the lunar eclipse. Since the shadow of the earth casted on the moon
is much bigger, the moon takes almost 107 minutes to completely pass through the shadow.

a
Penumbra

b

c

d
Penumbra

e

Lunar eclipse

Look at the above diagram of the lunar eclipse. When the moon is in position (a), there is no
eclipse as the moon does not fall on the earth’s shadow. When the moon reaches position (b),
it falls on the penumbral region of the earth’s shadow. So, a partial eclipse is observed. As the
moon reaches the position (c), the moon gets completely darkened and total lunar eclipse is
observed as it falls on the umbral region of the earth’s shadow. Likewise, at positions (d) and
(e), the moon experiences partial lunar eclipse and skips out of the eclipse respectively.
A total lunar eclipse occurs for about 107 minutes. In comparison to a solar eclipse, a lunar
eclipse is longer in duration because the length of the earth’s shadow falling on the moon is
longer and both the moon and earth move in the same direction.

Modern Concept Science - 9 425

FACT WITH REASON

A lunar eclipse occurs only in the full moon night, why?
A lunar eclipse occurs only in the full moon night because the earth comes in between the sun and the moon
at this time.
Why does lunar eclipse occur?
A lunar eclipse occur due to the following reasons:
1. Light always moves in the straight line.
2. The sun, the earth and the moon, come to lie in a straight line.
3. The earth revolves round the sun and the moon revolves round the earth.
4. The earth casts a shadow on the on the moon.
In comparison to a solar eclipse, a lunar eclipse is longer in duration, why?
In comparison to a solar eclipse, a lunar eclipse is longer in duration because the length of the earth’s
shadow falling on the moon is longer and both the moon and earth move in the same direction.

Rahu and Ketu: nodes of the moon

The earth’s orbital plane and the moon’s orbital plane intersect at two points making an angle
of 5.15 degrees. These two points are known as nodes. They are called Rahu and Ketu. The
Rahu node is the point of intersection where the moon ascends up the earth’s orbit. So, this
node is also called the ascending node. The Ketu node is called the descending node because
it is the point of intersection where the moon descends down the earth’s orbit.

FACT WITH REASON

Eclipses don’t occur in every full moon and new moon, why?
Eclipses don’t occur in every full moon and new moon because the sun, moon and the earth rarely come in
a straight line on these days as the moon usually inclines at an angle of 5.15 degrees.

ANSWER WRITING SKILL

1. What is a light year? Write its relation with parsec.
Ans: The distance travelled by light in one-year time is called light year. One parsec is equalt ot 3.26 light years.
2. In which phase is the moon seen as half lit from the earth?
Ans: The moon is seen half lit in the first and the last quarter.
3. Which eclipse darkens a part of the earth?
Ans: The eclipse that darkens a part of the earth is solar eclipse.
4. What is an artificial satellite?
Ans: The satellites which are made by man and launched in the space to revolve round a planet are called

artificial satellites.
5. What are the purposes of launching artificial satellites?
Ans: The purposes of launching artificial satellites are:

i. To study and forecast the weather
ii. To broadcast radio and television programmes on the earth

426 The Earth in the Universe

iii. To study the space and the different regions of the earth
iv. To build sky-labs to do research on the planets and other places
6. Write any two differences between synodic month and sidereal month.
Ans: The differences are given in the table below:

S.N. Sidereal month S.N. Synodic month

1. The time taken by the moon to revolve once 1. The time taken by the moon to complete
around the earth is called sidereal month. one new moon or full moon to another new
moon or full moon is called synodic month.

2. It is equal to 27 1 days. 2. It is equal to 29 1 days.
3 2

7. Why is a lunar eclipse longer than a solar eclipse in duration?

Ans: Solar eclipse occurs due to the shadow casted by the moon on the earth. Similarly, lunar eclipse occurs
due to the shadow casted by the earth on the moon. As the size of the earth is larger than the size of the
moon, the shadow casted by the earth on the moon is much bigger than the shadow casted by the moon
on the earth. So, the moon takes more time to pass through the earth’s shadow. Hence, lunar eclipse is
longer than a solar eclipse in duration.

8. What causes the change in seasons on the earth?

Ans: The causes of the change in seasons are:

a) Due to the elliptical path of the earth around the sun

b) Because of the tilting of the earth at an angle of 23.5 degrees.

c) Due to the revolution of the earth

9. Study the given figure and answer the following questions:

a) What phenomenon is shown here?

Ans: The phenomenon shown in the figure is solar eclipse.

b) What are the position A and B? A
B
Ans: The position A is umbra and the position B is
penumbra.

c) How often does this phenomenon occur?

Ans: This phenomenon occurs after every 2.5 years in average (2 to 4 years).

d) What is the duration of darkness in position A?

Ans: The duration of darkness in position A or total solar eclipse is around 7 minutes.

STEPS EXERCISE

STEP 1

1. In which galaxy do we live in?
2. Name the natural satellite of the earth.
3. In which category of the planets does the Pluto lie?

Modern Concept Science - 9 427

4. What is meant by annual and diurnal motion of the earth?
5. Define equinox and solstice.
6. What are the phases of the moon?
7. Write the meaning of new moon and full moon.
8. Define umbra and penumbra.
9. What are Rahu and Ketu?

STEP 2

10. Why does solar eclipse occur?
11. Why does lunar eclipse occur?
12. Write any two differences between umbra and penumbra.
13. Write any two differences between sidereal and synodic months.
14. Why does lunar eclipse occur only in full moon nights?
15. Solar eclipse occurs during the new moon day. Why?
16. Total eclipses are rarely seen at a certain place. Give reason.
17. Days and nights are equal on September 23 and March 21. Write the reason.
18. Astronomers tend study every eclipse. Why?
19. Eclipses don’t occur in every full moon and new moon. Why?

STEP 3

20. What is a shadow? How are shadows formed? Write the names of different parts of the
shadows with the help of diagram.

21. Draw a well-labeled diagram of a solar eclipse and describe it.
22. Draw a well-labeled diagram of a lunar eclipse and describe it.
23. Mention the causes of change in seasons on the earth with the help of diagram.
24. Draw and describe the various phases of the moon.
25. Describe total, partial and annular solar eclipse with diagram?

428 The Earth in the Universe