Learning-science-parti.pdf 6m3j35

LEARNING SCIENCE Part 1 Universe, Solar System, Earth

Indumati Rao C. N. R. Rao JAWAHARLAL NEHRU CENTRE FOR ADVANCED SCIENTIFIC RESEARCH, BANGALORE

About the Authors Mrs. Indumati Rao has received degrees from the Universities of Mysore, Kanpur and Purdue (U.S.A.) and certificate in Education from Oxford University (U.K.). She has worked for over four decades in the field of education. She has contributed to developing new methods of teaching and teaching aids. Mrs. Rao has worked as a geography expert in many institutions. She has participated in several teacher orientation programs conducted by the Karnataka State. She has authored books in Geography. At present, she is working as the Honorary coordinator of the multimedia group at Jawaharlal Nehru Centre for Advanced Scientific Research. C. N. R. Rao is Linus Pauling Research Professor at the Jawaharlal Nehru Centre for Advanced Scientific Research and honorary professor at the Indian Institute of Science. He was at the Indian Institute of Technology, Kanpur, and has been a visiting professor at University of California, Santa Barbara. He is a member of many science academies including the Indian National Science Academy, the Royal Society, London, U.S. National Academy of Sciences, Russian Academy, French Academy, Pontifical Academy and Japan Academy. He was President of the International Union of Pure and Applied Chemistry, and is now President of the Third World Academy of Sciences. He has received several medals and prizes which include the Marlow Medal of the Faraday Society, Centenary Fellowship of the American Chemical Society, Einstein Gold Medal of UNESCO, the Centenary Medal of the Royal Society of Chemistry, London and the Hughes medal of the Royal Society. He has published more than 1300 papers in the areas of Chemical Spectroscopy, Molecular Spectra and Chemistry of Advanced materials. He has authored 38 books and has been active in science education. He was awarded the Karnataka Ratna in 2001 by the Karnataka Government. He is the first recipient of the India Science Prize, the highest scientific award recently instituted by the Government of India and the Dan David International Prize for Science (2005). He was recently awarded the “Chevalier de la Legion d’Honneur”, the highest civilian award of (2005). He was also named the `Chemical Pioneer of 2005’ by the American Institute of Chemists, USA.

LEARNING SCIENCE Part 1

Universe, Solar System, Earth Indumati Rao C. N. R. Rao With the assistance of Jatinder Kaur Sanjay S. R. Rao

JAWAHARLAL NEHRU CENTRE FOR ADVANCED SCIENTIFIC RESEARCH, BANGALORE - 560 064.

Preface Science has become a part of our lives. Applications of science have provided us many benefits, and a better quality of life. The world today uses a language which has a lot of science in it. Without knowing, we use many words and phrases derived from science. We are also becoming conscious of our environment as well as our economy. Science has much to do with both these aspects. It is, therefore, important to learn the language of science. Children and adults alike have to know the rudiments of science and must be able to use the language of science where necessary. They must be able to apply the lessons learnt from science in daily life. It is for this purpose that we have produced a book entitled “Learning Science” in four parts. The book has the following four parts: Part1:

Universe, Solar System, Earth

Part2:

The world of physics and energy - Learning physical principles

Part3:

The world of chemistry: Of molecules and materials, Air around us, All about Water.

Part4:

Biology and life

It describes various aspects of science in simple language. It is hoped that this will be useful to school children as supplementary reading material and to all others who want to learn science and partake in the excitement of this experience. Bangalore 2005

Indumati Rao C. N. R. Rao

CONTENTS Preface

(iii)

1. Understanding the Universe

1

Objectives What is the universe? Light-Year Stars

1 2 6 9

Constellations

14

2. Exploring the Solar System

18

Objectives How did the solar family begin? Getting to know the sun Planets - chips of the old block

18 19 26 31

Special features of planets

38

3. Earth - the unique planet

49

Objectives Our unique planet The earth - our home planet

49 50 57

Motions of the earth and seasons

78

Learning Science

1

1 Understanding the Universe

Objectives In this module, we get a brief introduction to the Universe. The iron in our blood, the carbon in sugar and the oxygen in water are all from the dust created in the explosion of stars millions of years ago. In a sense, we are all children of the heavens.

2

Learning Science

What is the universe? We will try to understand the meaning of universe by taking an imaginary journey through space. Your spaceship is now blasting off into space. You are 1.6 million (1.6 x 106) kilometres into space. The earth looks big and bright in the darkness of space. The moon looks small.

You also see some familiar stars and constellations. The stars, however, remain unchanged.

Second stage of the journey, ~1.6 x 109 km into space. Now you can see Mercury, Venus, Earth and Mars close to the sun. They look bigger than they actually are. The stars do not show any change in size or position.

Learning Science

3

Deeper into space 1.6 x 1011 km into space. You are 25 times farther from the sun than Pluto, the planet farthest from the sun. You can see all the planets. The sun looks prominent and the earth looks small. You can see the Milky Way clearly.

Now you are very far into space. The distance from the earth can no longer be measured in kilometres. It has to be measured in light-years. One light-year = 9.41 x 10 10 kilometres. You are 16 lightyears into space. The Milky Way dominates your view. The sun and the planets look tiny. You can see some stars which are close to the sun shining brightly.

1.6 million light-years into intergalactic space. Our sun is just a speck. It is almost lost in the group of millions of other stars. You can see many new clusters of stars that you cannot see from the earth.

4

Learning Science

As you continue on your imaginary journey through inter galactic space, the view from your spacecraft may look like this. All things from microscopic particles to the biggest galaxies are part of the universe.

You are a citizen of the universe Give your address in full; Name House number, Street, Block number, City - pin code, State, Country, Continent, Planet, Universe.

How was the universe formed? In the beginning, there was just a point of infinite density and energy. Some 15 billion years ago, a fiery explosion now called the “Big Bang” occurred. Immediately after the explosion, the universe expanded rapidly from the size of a pin to about 2,000 times the size of the sun. When the universe was less than a second old, it was dense as iron and it was also opaque. After 500,000 years, the universe gradually cooled to ~3,0000K. The elementary particles fused to form hydrogen and helium gas - the life blood of the universe.

Learning Science

5

Hydrogen and helium formed gas pockets at random. In these pockets protogalaxies began to form. Between one and two billion years after the Big Bang, the proto-galaxies gave birth to galaxies, stars and other of the universe.

Is the universe expanding? The ever expanding universe Till the early part of the twentieth century, astronomers believed that the universe, was infinitely large, but did not change its size or shape.

However, towards the end of the 1920’s, Edwin P. Hubble showed that, the galaxies were moving away from each other and moving in all directions. That is, the universe is expanding all the time in all directions.

Edwin Powell Hubble (1889-1953) Hubble is considered to be the founder of extragalactic astronomy - the branch of astronomy that studies the existence of other galaxies. He was the first to observe the evidence of the expansion of the universe. He discovered that all nebulae in the sky are not part of the Milky Way galaxy. Until his observation in 1924, astronomers believed that all the nebulae were part of our galaxy.

6

Learning Science

Hubble made his second major discovery - In 1927 - that the galaxies he observed were moving away from our galaxy. In 1929 - the universe was expanding. Hubble received many honours for his outstanding contributions to astronomy. The most advanced telescope to study intergalactic world is named after him.

After the Big Bang, Big crunch? Will the universe expand forever? Or will it collapse at some point of time to signify the end? The density of the hydrogen atoms/m3 and the fate of the universe Some, modern astronomers believe that: if the density of the universe is less than 3 hydrogen atoms/m3, its gravity is not enough to hold the matter together; i.e. the universe will expand forever. If its density is more than this, the gravity will cause the collapse of the universe and the big crunch will occur.

Light-Year Measuring distances between stars When we look at the sky, the stars look close enough to each other to form patterns in the sky. Actually, some of them are millions of kilometres apart. How can we measure the distance between them, or for that matter, their distance from the earth?

Measuring distances A

B

We can measure the distance between A and B in centimetres (or inches). We can measure the distance in metres (or feet) as in swimming or athletics.

Learning Science

7

The distance between two cities or continents is measured in kilometres (or in miles). Each unit of measurement has a specific use. For example, we do not measure the distance between A and B in kilometres or the distance between New Delhi and London in centimetres.

The problem of measuring the distance between stars It is not possible to measure the distance of a star or any other object in the sky from the earth by the usual methods because, of the enormous distance of the stars from the earth. it is not possible to use a physical scale.

To measure this distance, therefore, scientists have found a method of using light rays that come from the stars or any celestial object in the sky.

8

Learning Science

The sun - our nearest star The distance between the sun and the earth is ~ 150,000,000 km. So, the light rays from the sun have to travel 150,000,000 km to reach the earth. Astronomers have found that a ray of light from the sun covers this distance in 8.3 minutes.

~ 150,000,000 km

What is the distance covered by the light rays (from the sun) in a second?

What is a light-year? A ray of light from any celestial object covers 9.41 x 1010 km in a year. This distance is called the light-year. The lightyear is the unit used to measure the distance between celestial objects in the sky and the earth. Proxima Centauri is the next nearest star from the earth. It is 4.2 light-years from us.

Learning Science

How old are the light rays from the stars? The light from the sun that reaches our eye at any instant will have left the sun 8.3 minutes earlier. If you see the sunlight at 7 a.m., you are actually seeing the sunlight of 6:52 a.m. When we see the light from the star Proxima Centauri, they are already 4.2 light-years old. A light ray from the Andromeda galaxy that you see, is 2.2 million years old.

Stars This is how the night sky might look on a clear moonless night. Hundreds of stars twinkling in the night sky. If you observe carefully, you can see some bright objects not twinkling at all. These are the planets. Our earth is also a part of the starlit sky. If we were to look at the night sky from the moon or Mars, the earth would not be twinkling.

Planets Ancient sky-watchers or astronomers noticed that “stars” that did not twinkle “wandered” through the sky. They called these “stars”, wanderers or planets (Greek word).

9

10

Learning Science

They named them after their gods - Venus, Jupiter, Mars. According to them, the sun and the moon were also planets! Now we know better. Planets “shine” steadily. They do not have light of their own. They merely reflect sun’s light. They do not “twinkle”, because their flat discs reflect a lot of light. We can view the surface of a planet through a telescope. On a cloudless new moon night, we can see a faint band of white light glimmering across the sky. This band consists of millions and millions of stars. The stars appear in groups or clusters. Group or clusters of stars are called galaxies. The white band of stars seen across the sky is the Milky Way galaxy. Our sun is in its outer arm.

Galaxies Galaxies are systems or families containing millions of stars. In addition to stars, galaxies also contain cosmic dust and gases. Sir William Herschel, the famous 18th century astronomer, called them “islands in the sky”. There are millions of galaxies in the universe. Galaxies are millions of lightyears apart from each other. There is vast empty space between them.

Learning Science

11

Milky Way galaxy The Milky Way galaxy contains around 200 billion stars. The stars are too far away from us to be seen individually or by the naked eye. The light from these stars forms a glowing haze in the sky as their light blends with the interstellar dust and gas. This glow obviously has been seen by people from the early times. The ancient Egyptians called it the Road of Souls and the bushmen of the Kalahari desert called it the Backbone of Night. In India, it is called Akash Ganga. Astronomers call it the Milky Way galaxy, our home in the vast universe.

There are millions of galaxies in the universe All the galaxies do not have the same shape,

size and

number of stars.

Depending on the shape, galaxies are classified as: Spiral galaxy, Elliptical galaxy. The classification is called Hubble classification, named after the famous astronomer Hubble. Have you heard of the Hubble telescope?

12

Learning Science

What are the stars made of?

You must have wondered about the twinkling stars in the sky. the nursery rhyme, Twinkle Twinkle Little Star? the story of Dhruva? It is now known that stars are actually made up of hot gases - mostly hydrogen and helium. Hydrogen is being continuously changed to helium in the stars. The stars glow due to enormous heat and light energy produced by this reaction.

Why do stars twinkle? Stars are far away from the earth. The light rays from a star have to through the earth’s atmosphere. In doing so, they undergo bending or refraction. When there is a wind, the amount of bending of the light rays changes. This makes the star “twinkle”.

Stars in the sky There are stars of different sizes, brightness and even colour. Also, the stars are not evenly distributed in the sky. They are not as close to each other as they appear to us from the earth.

Learning Science

13

Brightness of stars Strike a match and hold it close to your eye. It blinds you as if you are looking at a powerful light. It is the same with the stars. The brightness of a star depends on its shape, temperature and most importantly its distance from the earth. The stars that appear merely as faint points of light may be actually bright, but appear faint because of the distance.

Colours of stars All stars do not have the same colour. Some are reddish, some yellowish and some are whitish or bluish white. The colours indicate the temperature of the stars. When we heat a metal bar, it first turns red, then orange, then yellow and finally white. The range of colours of stars also follow the same pattern. Red stars are the coolest and bluish white stars are the hottest. Our sun is a yellow star.

How are stars born? Stars are born out of a fast rotating cloud of gases and dust particles. The cloud of gases and the cosmic dust particles constitute a nebula. A nebula is a star nursery.

14

Learning Science

Do stars die? Anything that is born has to die after completing its life cycle. Some insects die within a day of being born.

Redwood trees live for hundreds of years. An elephant’s average life span is about 70 years. Human life span is approximately 100 years. A star too has its own life cycle, but it spans billions of years. At the end of its life cycle, the beautiful twinkling star loses all its light and heat, and becomes a cold black dwarf in the sky. Our sun is a middle aged star.

Constellations Some star groups form recognisable patterns. During the course of the night, they appear to change their positions in the sky. But the stars forming the pattern remain together. The star forming patterns in the sky are called constellations. There are altogether 88 constellations. Our ancestors were great sky-watchers. They noticed patterns in the sky and gave names to them. There is a group of stars occupying a fairly large patch of the sky. This is the well known constellation Ursa Major or the Great Bear. The Big Dipper is a part of this constellation.

Learning Science

15

There are seven prominent stars forming the tail and the back of the Great Bear. these stars with imaginary lines. You can see the shape of a pan with a handle. This is called the Big dipper or the Plough. It is also called the Saptarishi or the Seven Sages. The last two stars in the pan of the Big Dipper are called the Pointers.

Constellations do not remain in the same position The earth rotates from the west to the east. Therefore, the stars and constellations appear to move across the sky from the east to the west. Also, the earth revolves around the sun. As a result, different constellations are visible during different seasons.

Ursa Minor or the little Bear There is another well known group of stars or constellation close to Ursa Major. This constellation also has a group of seven stars. The stars form a Dipper. The Little Dipper is smaller and less bright than the Big Dipper. The last star in the handle of the Little Dipper is the Polestar. This constellation is also known as the polestar constellation.

16

Learning Science

The polestar The polestar always appears in the same position in the night sky. Different stars have occupied the polestar position during different periods of time. The change of stars takes place once in ~ 2,400 years! The polestar now is Polaris. The next one will be Vega. The polestar is also known as Dhruva Nakshatra.

Orion The constellation Orion has also been known to sky-watchers since prehistoric times. Orion has some of the brightest stars. Orion is called the Hunter with the unbreakable club. Orion is easily recognised with its belt of three bright stars and the bright stars of the Sword. Orion is visible clearly during winter in the Northern Hemisphere.

The largest constellation - Hydra, the Sea Serpent

This constellation is visible more clearly in the southern sky. It occupies nearly one-fourth of the sky. It stretches from Gemini to the south of Virgo.

Learning Science

17

It is long and winding. No wonder then, it looked like a serpent to our ancestors. January to February is the best time to view this constellation.

Legend of Hydra According to Greek mythology, Hydra was a nineheaded sea monster serpent (much like Kalinga in the Hindu mythology?). It was dreaded as it had the unique power to grow back a severed head. In the end, Hercules killed Hydra with the help of Iolas. Iolas cauterised the neck of the severed head with a hot iron.

The smallest constellation- the southern cross It cannot be seen by observers in the Northern Hemisphere.

Southern Cross nestles at the feet of Centaurus. Sailors used the position of the Southern Cross to know their location.

18

Learning Science

2 Exploring the Solar System

Objectives In this module, we learn about the sun and the planets. In the entire solar system, it is only on earth we find diversity in life. The sun is responsible for the diversity and evolution of life. We depend so much on the sun, that it is important that we know something about the solar system.

Learning Science

19

The sun is only one of the millions of stars moving through space. Yet, what makes this average star special?

It is the only star known to have a family of its own. Wherever the sun goes in the celestial sky, its family follows it.

How did the solar family begin? About 5 billion years ago, there was no sun. Therefore, there was no earth and no moon. Instead, only gases and cosmic dust were drifting in the darkness of space. There are two theories about the birth of the solar system.

20

Learning Science

According to an earlier theory, astronomers believed that long ago, a star ed very close to our sun. The star and our sun nearly collided.

As a result, large quantities of various materials were thrown off into space. After a long time, these materials formed the planets. The sun held the planets in place. This theory has serious drawbacks. According to modern astronomy, the solar system was formed from a slowly spinning cloud of gas and dust. Gradually, the cloud began to spin faster and faster. This formed a large spinning disk about the size of the solar system - with a large spinning whirl at the centre and smaller whirls surrounding it at various distances.

The huge and heavy centre became hot enough to start the thermonuclear reaction. As a result, the centre began to glow and became our sun. Modern astronomers believe that everything in the solar system has the same source material.

Learning Science

21

Planets or wanderers The smaller whirls attracted more and more particles from the cloud of dust and gases and grew in size. They were like snowballs rolling down a snow-covered mountain. Finally, they formed the nine planets at varying distances from the sun.

The four planets - Mercury, Venus, Earth and Mars are closer to the sun. They are called the inner planets and the other five planets - Jupiter, Saturn, Uranus, Neptune and Pluto are called the outer planets. The planets are independent of each other and they move in their own respective paths or orbits around the sun.

Solar family or the solar system Apart from the nine planets, solar family includes tiny sand-like objects as well as gigantic rocks. moons. thousands of asteroids. millions of meteors. millions of comets. All the of the solar system revolve around the sun along their respective paths at varying speeds.

22

Learning Science

The Moons The moons are the natural satellites revolving around the planets in the solar system. Except Mercury and Venus, all the other planets have moons. While the Earth and Pluto have one moon each, Uranus has 15, Jupiter 16, and Saturn 18. The size and composition of the moons vary. Some moons are only a few kilometres in diameter, while others are larger than the planet Mercury. While the earth’s moon is rocky, Saturn’s Enceladus is ~ 50% ice. Satellites, Io, Europa, Ganymede and Callisto of Jupiter are Galilean moons. They were discovered by Galileo in 1610.

Asteroids - A ring of space rocks What are asteroids? Between the orbits of Mars (the last of the inner planets) and Jupiter (the first of the outer planets), asteroids orbit around the sun. Astronomers believe that The asteroids are pieces of a planet that came too close to Jupiter. The enormous gravitational force of Jupiter broke up this object into pieces of rock and metal. And these pieces of varying sizes continued to revolve around the sun.

Asteroids Asteroids are too small and too far from the earth to be seen without a telescope, Even through a telescope, they look like faint stars. Guiseppe Piazzi, an Italian astronomer, discovered the first asteroid. He called it Ceres after the Roman goddess of agriculture. It is the biggest asteroid and yet it takes 5000 Cereses to balance the earth in a gigantic balance!

Learning Science

23

What are meteors? Meteors are the stony or metallic pieces travelling around the sun. There are billions of meteors in the solar system. Meteors can be as small as sand grains or as big as boulders. They travel alone around the sun or travel together in swarms of billions of particles. Meteors travel at great speeds around the sun. Sometimes when a meteor comes close to the earth, the meteor is pulled towards the earth by the gravitational force of the earth. When a meteor enters the earth’s atmosphere, it streaks across the sky as a shooting star. While most of the meteors burn out in the earth’s atmosphere, some big ones land on earth. Those that land on earth are called meteorites.

Comet - the mysterious member of the solar family A comet streaking across the sky has inspired awe and fear in people from ancient times. The orbits of comets are very large with the sun off centre. Comets come closer to the sun at one end and go almost to the edge of the solar system at the other end.

24

Learning Science

A comet has no light of its own. Its light is entirely reflected sunlight. Comets do not streak across the sky. Even though they travel at great speed, they stay visible in the sky for weeks as they are far away from us. Comets return to view at regular intervals. Edmund Halley, of England was the first to observe this phenomenon. Halley’s comet is the most famous comet. It visits the earth once in 76 years. Its most recent visit was in 1986. Halley’s comet was first observed in 240 BC by Chinese astronomers. Hale-Bopp is the brightest comet that was seen in the 20th century. It was discovered in 1997 by Alan Hale and Thomas Bopp of USA.

The shape and structure of a comet A comet is shaped like a thin torpedo. It has two distinct parts; a nucleus or head and a tail. The head of a comet is made up of frozen gases, cosmic dust and ice. It tail looks like a dirty snow ball. When a comet comes close to the sun, the outer layers of the head begin to evaporate. The winds from the sun nucleus or head blow the gas and dust particles away from the sun. This looks like a bright tail. The comet develops a long tail extending millions of kilometres into space.

Learning Science

25

The revolving planets do not fly into space. Why? The sun is a very powerful master. He keeps all the in place. How does the sun do it? We will try to understand it by a simple activity. Take a string about 3 metres. Tie a small round object to one end. Stand in an open place and rotate the string horizontally as fast as you can.

Let go of the string. What do you notice? The stone flies off. As long as you were holding the string, the stone was moving in a circle.

This is how the sun holds his family together Nearly 99% of the total mass of the solar system is in the sun. This gives the sun an enormous force of attraction. The force produced by the revolution of the planets is balanced

by the sun’s gravitational force. This balance of forces keeps the of the solar family in place. Sitting in the centre, the sun perhaps keeps an eye on them!

26

Learning Science

Getting to know the sun The sun is literally the star of the family. Like all stars, the sun is a huge ball of hot gases. Its temperature is so high that all the elements in the sun are in the gaseous state. The sun consists mostly of hydrogen. The other element present in large quantities is helium.

How old and how many more years to go? The sun is 4.6 billion years old and has ~ 10 billion years to go. The stages in the life of our sun (or any other star) are:

Main sequence

Red giant stage

White dwarf stage

Black dwarf stage

Main sequence of a star The main sequence stage is the longest and perhaps the most important stage in the life cycle of a star. The contracting nebula has a central bulging core in which thermonuclear reactions cause the nebula to shrink and finally end itself. Then the hot centre begins to glow with its own light and changes into a star that is stable. At this stage, the star gives out its own heat and light. Our sun is in its main sequence now.

The sun is in the main sequence stage now.

Learning Science

27

Based on the surface temperature of the sun, it is classified as a yellow star which is in the middle of its main sequence.

The red giant stage In this stage, the sun expands upto 100 times of its original size. Its surface resembles a red hot burning charcoal due to the high temperature. Then the sun emits enormous solar energy, as a result of which the planets close to the sun could melt and turn into a liquid state.

On earth, all the water in the oceans could evaporate and form giant clouds of vapour.

The white dwarf stage The red giant stage cannot last forever. When the fuel in the red giant, i.e. hydrogen, is exhausted the red giant becomes unstable. Its outer layers begin to separate and disappear in space. The centre of the sun gradually cools and the light becomes dim. Then the sun becomes a white dwarf.

The black dwarf stage After a billion years, the white dwarf cools and gradually the light fades. Lastly, the sun which gave heat and light to all the of the solar family and was responsible for the evolution and sustenance of life on earth,

28

Learning Science

itself loses all the heat and light to become a black dwarf. This is the last stage in the life cycle of the sun.

Layers of the sun The sun we see day in and day out is only one layer of the sun. Let us find out about the different layers of the sun.

Core, the engine room All the energy of the sun is produced here. It is deep inside the sun. The energy released in the core travels to the outer layers. The temperature in the core is ~15,000,0000C

Photosphere - the sphere of light Photosphere is what we see. Its surface temperature is around 60000C. It is a thin layer. Its thickness is ~544 km. Sunspots were first discovered by Galileo in the 16th century. Photosphere has dark patches. They are called sunspots.

Galileo Galileo discovered that the surface of the moon was not smooth. that the Milky Way consisted of millions of stars. that Venus also had phases. the rotation of the sun. the sunspots covering the face of the sun. the moons of Jupiter. Galileo was the first to actually see the moons of Jupiter when he focussed his telescope on Jupiter in 1610. He also saw the rings around Saturn. He discovered all these by looking through his telescope night after night!!

Learning Science

Sunspots Sunspots are the dark patches of different sizes and shapes that are seen on the photosphere of the sun. Galileo was one of the first to observe the sunspots through his telescope in the 17th century. The sunspots have two distinct parts the umbra or the dark core and the surrounding penumbra.

Chromosphere - the sphere of colour Chromosphere is a transparent layer. It is much hotter than the photosphere. Chromosphere can be seen only during total solar eclipse. Chromosphere is seen as a thin red ring around the dark disk of the sun.

Corona - the crown Corona is the outermost layer of the sun. It stretches millions of kilometres into space. The temperature of the corona is around 2,200,0000C. Corona can be seen only during total solar eclipse. Corona presents a spectacular sight as white streamers or white petals around a darkened sun.

29

30

Learning Science

Sun’s motions Ask your friend, `Does the sun have motion?’ Your friend is likely to laugh and reply `Have you not seen the sun move across the sky from sunrise to sunset?’ The sun has two motions. They are: Rotation on its axis and Circular movement among its neighbouring stars and constellations. Its rotation period at its equator is 25 earth days and at its poles, it is 35 earth days. (1 earth day = 24 hours) The sun seems to be in a hurry to visit its friends, for, it travels at 250 km per sec. The sun does not move across the sky from sunrise to sunset.

Vital statistics of the sun Age

~4.6 billion years

Mass

330,000 (earth = 1)

Density

1.41(water = 1)

Waist (diameter at the equator)

1,400 million km.

Average distance from earth

~150 million km.

Surface temperature

60000C

Core Temperature

~15 million 0C

Rotation period at the equator

25 Earth days

at the poles

35 Earth days

Learning Science

31

Planets - chips of the old block Planets are made of the same elements as the sun. Planets have solid rocks / gases. varying amounts of water as water / water vapour / frozen ice (depending on their distance from the sun), and an atmosphere comprising of various gases. It is only on earth that water is found in all the three states i.e. ice (solid state), water (liquid state), and water vapour (gaseous state).

Same, yet different The planets vary in size, their distance from the sun and in their structure. Jupiter is the largest planet and Pluto is the smallest. The sun is ~600 times bigger than Pluto, but only ~10 times bigger than Jupiter.

Sun

Jupiter

Pluto

32

Learning Science

Nearest and the farthest Mercury is nearest to the (57.9 million kms) sun and Pluto is farthest from the sun (5,900 million kms) Pluto is a lonely planet. Its closest neighbour Neptune is nearly 1,414 million km from it.

Mercury, Venus, Earth and Mars are relatively close to the sun. They are grouped together as terrestrial planets.

Mercury Mercury is the closest planet to the sun. It is difficult to spot mercury in the night sky as it is very close to the sun. Mercury is a small planet. Its mass is 0.055 (Earth = 1). Its surface temperature ranges from very cold to very hot (-1800C to 4300C). Mercury revolves fastest around the sun (47.9 km/s). It rotates slowly (58.65 earth days to complete one rotation). Its surface is scarred by craters. It has a huge iron core.

Terrestrial planets Terrestrial means earth-like. Mercury, Venus, Earth and Mars - the terrestrial planets, are also called inner planets. Venus and Earth are about the same size. They are also called twin planets.

Learning Science

33

Giant planets or the outer planets Jupiter, Saturn, Uranus and Neptune are called the giant planets or the gaseous planets. They are massive. rotate faster. have many moons.

They are so far away from the sun that they retain most of the volatile elements from the original cosmic cloud. Water seems to be frozen on their rocky cores.

The moon - earth’s satellite What is special about the earth’s satellite (moon) in the solar system? According to our present knowledge, in the entire solar system, man has set foot only on the moon. In 1969, Neil Armstrong of USA was the first to set foot on the moon. His foot prints will remain for thousands of years on the moon’s surface. Why? (Clue: there is no atmosphere on the moon)

34

Learning Science

A day on a planet A day is the time taken by a planet to complete one rotation Earth on its axis. Duration of a day is not the same on all planets. A day is shortest on Jupiter. It is a 10 hour day on Jupiter. It is difficult to know the duration Venus of a day on Venus because of its dense atmosphere. The smaller planets closer to the sun have longer days.

Jupiter

Each planet has its own day i.e. the time taken to complete one rotation. Still `a day’ of any planet is expressed in of the earth day (24 hour period). A day in a planet Planet

Duration of a day

Mercury Venus Mars Jupiter Saturn Uranus Neptune Pluto

88 earth days around 30 earth days 24 hours 15 m less than 10 hrs 10 hours 15 m 11 hours 16 hours 6 days and 9 hours

Learning Science

35

Is life possible on other planets? The sun provides light and heat energy to all other planets. Why is development of life not possible on other planets? Some planets are too hot (Mercury) while some are too cold (Pluto).

Some lack an atmosphere while others are covered by an atmosphere that has little life-ing gases. Life ing soil and water (in liquid state) are not available in other planets.

What are the criteria for evolution of life on a planet? The two important criteria for evolution of life on a planet are: (i) the distance of a planet from the sun and (ii) the size of the planet.

36

Learning Science

The distance of a planet from the sun: If the planet is too close to the sun, chemical compounds cannot remain stable. If it is too far from the sun, water will be frozen and chemical reactions cannot take place there. The size of a planet: If the planet is too small, its gravitational pull will not be enough to hold the atmosphere.

If the planet is too big, its gravitational pull holds a very thick layer of atmosphere. This dense atmosphere will not let solar energy reach the planet. Only the earth is at the ideal distance from the sun and has the ideal size to life.

Is there life on Mars? Why do scientists believe that Mars is the only other planet which can life? Detailed studies of Mars have revealed that the length of a day on Mars is almost the same as an earth day. the martian year and its seasons are twice as long as ours. it is green during its spring and brown during its autumn. its polar caps grow larger during its winter and shrink during its summer. But its atmosphere is thinner, seems to have very little oxygen and water vapour.

Learning Science

37

Is there life elsewhere in the Universe? There are billions of stars in the universe. Astronomers strongly believe that there are many stars similar to our sun in the universe and these stars are likely to have planetary systems. if even one planet is at the ideal distance from its star, and has the right size, there can be living organisms on the planet.

How can we find out if there is anyone else in the universe? In principle, every television programme or radio broadcast can be heard - many light-years later of course - on the other stars. Carl Sagan the famous radio astronomer was convinced that we were not alone in the universe. His group sent an image signal in 1974 towards the Great Cluster of stars 25,000 light-years away. The reply will arrive 50,000 lightyears later if our friends reply as soon as they receive the greetings.

The search for extraterrestrial intelligence Radio signals or clapping hands? How do you catch your friend’s attention in a crowded room?

38

Learning Science

You can either shout or clap your hands. As these sounds do not blend with the other sounds in the crowded room, you can catch your friend’s attention. Likewise, a signal from an alien from another world may be a sound that is different from any other earth sound or may be, he/she will send a photo image! digital of course.

Special features of planets

Distance of the planets from the sun The nine planets are arranged at varying distances from the sun. Earth is ~150,000,000 km from the sun. This is taken as the astronomical unit (AU). Bode’s law: In 1770, Bode of , put forward a strange law to calculate the distance of a planet from the sun. He allotted numbers at random to the planets. Mercury = 0, Venus = 3, Earth = 6, Mars = 12, .......... He added 4 to these numbers and divided the sum by 10. The resulting value gave the distance in astronomical unit. Planet Distance of the planet Actual distance (According to Bode) of the planet Mercury 0.4 0.39 Venus 0.7 0.72 Earth 1.0 1.0 Mars 1.6 1.52 Jupiter 5.2 5.22 Saturn 10.0 9.59 Uranus 19.6 19.36 Neptune 38.8 30.10 Pluto 77.2 39.78

Learning Science

39

But according to knowledge available then, there was no planet in the position of 24 and 192. Later, astronomers found asteroids in the position of 24 and in 1781, they discovered Uranus in the position of 192. Only Neptune and Pluto’s distances did not conform to Bode’s law. Even though modern astronomers may consider Bode’s law as a coincidence, this law is one of the profound statements about the laws of nature.

Inclination of the axes of planets The planets (except Pluto) revolve around the sun in approximately the same plane. Their axes point to the celestial north. However, their axes are tilted in varying degrees. Only the axis of Mercury is almost perpendicular to the plane of its orbit or its inclination is 00. Planets and the inclination of their axes Jupiter 30 07’ Mercury 00 Saturn 26044’ Venus 2042’ Uranus 82008’ Earth 23027’ Neptune 29034’ Mars 25021’

Orbits of all the planets are not the same Orbits of the planets are not circular. They are elliptical or eccentric in varying degrees. As the eccentricity increases, the orbit becomes more elliptical. Eccentricity of a circle is zero. The eccentricity of the orbit of Venus is the least. (Its orbit is almost circular.)

40

Learning Science

The eccentricity of the orbit of Pluto is the greatest. (It is elliptical.) In the solar system, the orbits of comets are most eccentric.

Density of planets The main difference between the gaseous giant planets and the inner planets is in their densities. Density of a substance tells us how tightly the matter in a body is packed.

mass of the substance volume of the substance Density of water is taken as the standard to measure the density of a substance. Density of a substance =

Mercury Venus Earth Mars

5.4 5.2 5.5 3.9

Density of planets Jupiter 1.3 (mass = 318e) Saturn 0.7 (mass = 95e) Uranus 1.3 (mass = 14.5e) Neptune 1.8 (mass = 17.2e) Density of Pluto = 2.1(?)

The individual special characteristic of planets: a brief look Mercury - the planet that is hurtling around the sun. Mercury is closest to the sun. Naturally, it is the hottest planet. Why does it hurtle at ~170,000 km around the sun?

Learning Science

41

If it did not move at this great speed, the tremendous gravitational attraction of the sun would have pulled the planet towards its centre. What would have happened?

Venus, the earth’s twin Earlier, astronomers believed that there was an abundance of life on Venus. But, the reality is different. Venus is a violent planet with violent storms and acid rain. Venus is the brightest planet. It shines as the morning star in the constellation Taurus and as the evening star in Libra. The highest mountain in the solar system, Maxel mounts, is on this planet. This planet rotates from east to west. The Mayans based their calendar on the revolution of the planet Venus.

Venus, the greenhouse planet The atmosphere here consists mainly of carbon dioxide and clouds of sulfuric acid. As a result, incoming solar insolation is trapped. The dense layer of CO 2 and clouds of sulfuric acid in the atmosphere of Venus absorb and trap enough incoming solar radiation to keep Venus hot even through its 4 month long night. As a result, this planet has the greatest greenhouse effect.

42

Learning Science

It gives this warning to us: If we do not mend our irresponsible ways, our planet also will hurtle towards the same fate.

Earth, the home of lifeforms In the entire solar system, life as we know it, exists only on earth. This is due, among other factors, to the water cycle. the rock cycle. the carbon dioxide cycle. the oxygen cycle.

Oxygen

CO2

The solar rays and earth’s atmosphere The sun provides heat and light to all the of the solar system. But the sun’s radiation includes harmful gamma rays, x-rays and ultraviolet rays. The earth’s atmosphere protects the earth from the harmful gamma rays radiation.

Learning Science

43

Gift of the earth’s atmosphere At a height of approximately 50 km, the ozone layer protects all life on earth from the harmful ultra-violet rays. Clouds and dust particles absorb the infrared radiation to a large extent and reflect it. A small portion of the white light coming through the atmosphere is scattered by clouds and dust particles. This scattering of white light makes the sky appear blue during the day and orange at sunrise and sunset. In 1930, Professor C.V. Raman received the Nobel Prize in physics for his work on the scattering of light. This is also known as the Raman effect.

Jupiter - the planet with the red spot Astronomers had observed the red spot on Jupiter’s surface around 300 years ago. But, its nature and cause have been understood only recently. In the lower atmosphere of Jupiter, there are alternate belts of rising and falling gases. They circulate in opposite directions.

44

Learning Science

The red spot is actually a huge storm between two rising belts. It gets its red colour from a chemical in the lower atmosphere. This chemical rises with the rising air currents and turns red in sunlight. The red spot rotates in an anticlockwise direction. Its period of rotation is six earth days. How many hours does it take to complete one rotation?

Could Jupiter have become a star? The mass of the sun is 99% of the total mass of the solar system. The mass of Jupiter is 99% of the remaining 1%. This is the only reason why the sun became a star and Jupiter remained a planet. Astronomers believe that if the mass of Jupiter had been 100 times more than its actual mass, the temperature in its core would have been high enough to start nuclear fusion. About 4.6 billion years ago, when the solar system began to form, Jupiter was a huge ball of cosmic cloud and dust. But, its mass was not sufficient to exert great pressure on its core. As a result, the heat generated there was not enough to start nuclear fusion. Gradually, Jupiter cooled and became a planet. Jupiter is cooling down even today. It is the only planet that radiates more heat than the isolation it receives from the sun.

Learning Science

Comparison between Jupiter and the Sun Sun Mass

Jupiter

3,30,000

318

1,304,000

1,316

1,50,00,0000K 200 billion earth atmospheres

30,0000K 100 million earth atmospheres

(Earth as standard)

Volume (Earth as standard)

Core temperature Pressure

Miniature models of solar system within the solar system The gas g i a n t s Jupiter and Saturn have 16 and 18 satellites. These two planets have held their satellites in place by their gravitational force just as the sun holds them in their place in the solar system.

45

46

Learning Science

However, unlike the sun, Jupiter and Saturn do not give heat and light to their satellites. The sun is the source of heat and light of these satellites also.

Why does Uranus rotate almost on its side?

Astronomers give many explanations to the strange inclination of Uranus’s axis. According to one of the explanations, when Uranus was being formed, a round object, an intruder from space, hit the young planet near one of its poles. The force of this collision turned Uranus on its side and the intruding object broke into pieces of rock and ice. The pieces of rocks, clouds of steam and gases began to orbit around the plane of Uranus’s equator. Gradually, these became the planets, rings and satellites.

Neptune - the stormiest planet in the solar system For a long time, astronomers believed that Neptune was a calm blue planet. But Neptune is the windiest planet, with winds blowing upto 2,400 km per hour. The width of some of the storms is wider than the earth.

Learning Science

47

How were the moons formed? The mental picture we get of the moon is the beautiful milky white object in the night sky. But moon actually is the satellite of a planet.

Astronomers believe that after the main planets were formed, the remaining small bits came together and became moons. They became the satellite of the planets which held them by their gravitational attraction. All moons are not the same. Let us look at some unusual moons.

Io, the moon with active volcanoes

Io

Europa

Callisto

Gyanmede

The Galilean moons of Jupiter are Io, Europa, Callisto and Gyanmede. There are at least 10 volcanoes on Io. The volcanoes on Io spew out liquid sulfur and silicate rocks. What is the difference between volcanic eruption on the earth and on Io?

48

Learning Science

Triton - the satellite with cold volcanoes Cold volcano may sound like a contradictory term. We are used to hot lava pouring out of the volcanoes here on earth but the volcano on Triton spews out liquid nitrogen. As its temperature is very low, liquid nitrogen gets frozen.

Titan - the sole satellite with atmosphere

Titan is the biggest satellite of Saturn. There is atmosphere only in this satellite in the entire solar system. Its atmosphere consists mostly of nitrogen and traces of methane. As the satellite is far away from the sun, methane is in liquid form.

Learning Science

49

3 Earth - the unique planet

Objectives “What is unique about our planet? ” forms the theme of this module. The rock and minerals, the motions of the earth and the seasons are explored in this module.

50

Learning Science

Our unique planet Our earth is just a minor planet revolving around an average star. Looking from the vastness of space, our planet may be no more than a speck.

Yet, this is the only member of the vast universe to have life.

Seeing is not believing - shape of the earth For a long, long time, people believed in only what they could see, and they did not see the round shape of the earth.

Learning Science

51

The ancient Babylonians, the Egyptians, the Greeks, the Indians and the Chinese had their own ideas about the shape of the earth.

The turtle, the elephants and the serpent: The Indian idea of the earth According to one idea, the earth was held up by elephants. The elephants stood on a turtle. The turtle was one of the ten incarnations of Vishnu, the supreme God. The turtle rested on a cobra with its tail in its mouth. As the cobra symbolised water, the earth was surrounded by water. Whenever the elephants shifted their legs, earthquakes took place.

The circular disc, the mountain and the travelling sun - the Chinese idea The earth was a circular disc with the ocean surrounding it. There was a great mountain in the centre of the circular disc (the earth). Once a day, the sun went round the mountain going behind the western side of the mountain in the evening. After travelling behind the mountain throughout the night, the sun came out on the eastern side in the morning.

52

Learning Science

Changing shape of the earth If you look at the earth from the top of a mountain, it will look conical. If you go down a wide valley and look at the earth, it will look like the inside of a bowl. Ordinarily, if you look at it from where you are the earth will look flat.

The round earth - early Greek idea Pythagoras, the renowned Greek mathematician suggested that the earth was round. Around 350 BC, Aristotle taught his pupils that the earth was round. Around 250 BC, Eratosthenes calculated the circumference of the earth for the first time by using the idea of a round earth. Astronomers of ancient and medieval India were also aware of the true shape of the earth.

Pythagoras 580 BC - 500 BC Pythagoras was perhaps one of the greatest mathematicians of all time. He contributed to the development of mathematics and western philosophy. He started a school or brotherhood. He believed that reality was mathematical in nature, and that it was possible for the soul to achieve union with the divine. Why did Pythagoras conclude that the earth was round? According to mathematicians, a sphere is the most perfect shape. Like all Greek philosophers and mathematicians, Pythagoras believed that the earth was a perfect body. He concluded, therefore, that the earth was round. He was nearly correct in his conclusion about the shape of the earth.

Learning Science

53

How did Eratosthenes calculate the circumference of the earth? Eratosthenes was the first to actually calculate the circumference of the earth. Eratosthenes lived in Alexandria, an Egyptian town. There was a deep and dry well in Syene, a town ~5000 stadia ( 800 km) to the south of Alexandria. Eratosthenes came to know that on summer solstice day (June 21), one could see the bottom of the well as there was no shadow at noon on that day. On the same day, he noticed that the sun at noon time was not vertical but cast a shadow at Alexandria. Using Pythagoras’ idea of a round earth he calculated the circumference of the earth.

Actual shape of the earth The earth is not a perfect sphere. It bulges at the equator and is slightly flattened at the poles. This shape is called an oblate spheroid. This is the result of the earth spinning or rotating very fast on its axis, and rotating faster at its equator than at its poles. Isaac Newton was the first to suggest this. The earth’s shape is also called geoid.

Picture of the exact shape of the earth The model suggested by Newton was not completely correct. Information from space gave the exact shape of the earth.

54

Learning Science

Vanguard I in 1958 confirmed that the earth’s bulge is: not uniform. not exactly at the equator. slightly more at some places in the southern hemisphere and at the north pole Nearly 4.6 billion years after the earth was formed, we finally know the shape of the earth.

If the earth is round, why do we see it as a flat earth? We will find the answer to this question through a simple activity. Take a sheet of foolscap paper, a pencil and a piece of string at least 120 cm long. Tie the string to the sharpened end of the pencil. Fix the paper to the top of a drawing board or a flat table. Draw arcs of circles of

radius 5 cm, 35 cm and 110 cm on the paper. Compare the curvatures of the arcs of the three circles. What can you conclude? The biggest arc is curved least. It is for the same reason that the round earth appears flat. If you want to draw the arc of the earth’s surface, you will need a string ~6400 km long!

Learning Science

Why is the earth round? Even the smallest particle of dust in the vast universe has the power to attract other things. At the time the earth was being formed, particles of dust etc., attracted each other to form the tightest possible shape - a sphere. This force of attraction is called gravity or gravitational force.

Gravitational force Do you know the story of Newton and falling apple? London was hit by the plague epidemic. So, Newton went to a quiet village. While sitting under a apple tree, an apple fell on his head. He wondered why the apple fell down? Working on this problem, Newton gave the Law of gravitation. Gravitational force is the force of attraction between objects. All things big and small, including the tiniest dust particle or atom in the universe, experience this force. The strength of this force depends upon the weight of the object- the heavier the object, the stronger is the force.

Centre of the earth and gravity As in a sphere, the particles of matter of the earth are crowded at its centre. Therefore, the force of attraction or gravity is strongest at the centre of the earth.

55

56

Learning Science

Why don’t we fall off the sides and bottom of a round earth? The reasons for us not falling off the round earth into space are: there is no top or bottom or sides of the earth. Everything that we see on earth is on its outside, or the surface. all objects on the surface are attracted towards the inside or the centre of the earth. this force, or the earth’s gravity, holds all things on the outside or on the surface of the earth, and prevents them from falling off.

Why do things weigh a little more at the poles than at the equator? Gravitational force decreases as the distance from the centre of gravity increases. ! Weight of a body = mass x acceleration due to gravity; W=mxg The earth is not a perfect sphere. It is slightly flattened at the poles. Therefore, the poles are slightly closer to the centre of the earth. As a result, a person weighs slightly more at the poles. Armstrong, the first man to land on the moon, weighed on the moon 1/6th his weight on earth. Why? Can we escape from the grip of earth’s gravity? We can do this, provided we are prepared to live in a spaceship beyond the reach of earth’s gravity. Why are the astronauts attached to the spaceship by a long rope when they carry out repairs outside the spaceship?

Learning Science

57

Escape from earth’s gravity To escape the pull of the earth’s gravity, we must reach the limits of the atmosphere. Then, we have to enter into space. To do this, the spacecraft with you in it has to travel at a speed of ~40,000 km per hour or 11.2 km per second! This speed is called the escape velocity.

The earth - our home planet We have explored space successfully. We have gathered evidence of what lies beneath the waves.

It is strange but true that we know little about what lies beneath our feet.

Getting to know our planet It is now known that the radius of the earth is ~6400 km. The deepest mine, the Champion Reef mine in Kolar Gold Fields, (Karnataka) is only ~2 km deep. One of the deepest oil wells in Pecos Oil Field in Texas (USA) is only 8 km deep. They are merely like scratches on the earth’s surface.

Kolar

How can we get evidence about the earth’s interior? We cannot take a pickaxe and dig through the layers of the earth. We do not have any instruments to look through the layers. We cannot drill through the

58

Learning Science

layers of the earth to its centre and get samples. It is, therefore, not possible to get direct evidence. We can only have indirect evidence.

Earthquake waves and the indirect evidence of the earth’s interior An earthquake is always associated with destruction. But the earthquake waves have unlocked some secrets of the interior of the earth. By the manner in which they travel through the different depths, they provide evidence of the nature of the earth’s interior.

What is an earthquake? An earthquake is the shaking of the ground under your feet. This is caused by the sudden release of energy stored in the rocks. This energy is released in the form of waves. They are called seismic waves. The place where an earthquake originates is called the focus and the point directly above the focus (on the earth’s surface) is called the epicentre. An earthquake can be felt when it occurs just a few kilometres below the earth’s surface.

Ground surface Epicentre

Focus

Earthquake waves or seismic waves There are three types of seismic or earthquake waves. They are; The P wave or the primary wave - This wave can travel through both the solid and liquid forms of the material in the earth’s interior. These waves reach the earth’s surface first.

Learning Science

S wave or the secondary wave - This wave can travel only through solids. Their speed is approximately half the speed of the P waves. L waves or the surface waves - These waves move only on the surface. They are slower than the P and S waves. The earthquake waves start from the focus of the earthquake and reach the earth’s surface.

L - waves

59

S - waves

P - waves

S - waves

What do the earthquake waves reveal? By studying the way earthquake waves travel through the earth’s interior, we now know that: the materials of the crust and the upper mantle are in the solid state. there is a soft zone in the mantle. the iron and nickel outer core is in the liquid state (This is due to the melting of the substances at high temperatures). the iron-nickel inner core is in the solid state due to the tremendous pressure there.

Tsunami Tsunami literally means “harbour wave” in Japanese language. It is also called seismic sea wave. This is one of the most catastrophic natural disasters. It is caused by an underwater earthquake occurring less than 50 kilometres below the seafloor. This takes place due to two tectonic plates either converging or one plate slipping under another plate. The magnitude of the earthquake has to be greater than 5.5 on the Richter scale for the tsunami waves to be generated. Coastal or underwater landslide or underwater volcanic eruption may also cause a tsunami.

60

Learning Science

Tsunami is a more catastrophic disaster as its destructive force can be felt in coastal regions thousands of kilometres away from the epicentre of the underwater earthquake or volcanic eruption. Immediately after the earthquake, a train of waves travel over great distances at the ocean surface in ever- widening circles. This can be compared to the waves produced by a pebble thrown into a shallow pool of water. As the tsunami waves approach the continental coasts or coasts of islands, increasingly shallow seafloor reduces the velocity of the waves. This in turn makes the coastal water rise as high as 30 m in 10 to 15 minutes and giant waves come crashing inland. The effects of the tsunami waves depends upon the bottom topography near the shore and nature of the coast. As result, their effects vary widely from place to place. Often, the first sign of the arrival of a tsunami at a coast is the water receding and exposing the shallow seafloor. Lisbon, Portugal witnessed such a phenomenon on November 1, 1755. This unusual event attracted many curious people to the bay; many were swept away by the tsunami wave that arrived only minutes later. Tsunami that struck Awa in Japan killed more than 100,000. As tsunamis are common in the Pacific Ocean, a tsunami early warning system has been put in place there. The most catastrophic tsunami occurred on the morning of 26 December 2004. This was caused by an underwater earthquake measuring 8.5 on the Richter scale off the coast of Sumatra, Indonesia. Within minutes it devastated Aceh, Sumatra, and moved with great speed and hit Thailand, Car Nicobar, Nicobar and Andaman Islands, India in the Bay of Bengal, southern part of the Tamil Nadu coast on the mainland and eastern and southern coasts of Sri Lanka. More than 220,000 people died in Indonesia alone. India and Sri Lanka also suffered casualties in tens of thousands. Tsunami was not known to strike the coasts in the Indian Ocean and The Bay of Bengal. Here

Learning Science

61

also, as in Lisbon, many paid with their lives when curiosity drew them to the beaches. The tsunami of 26 December 2004 is perhaps the greatest human tragedy as it killed nearly 300,000 people and destroyed the coastal regions of the affected countries.

The layers of the earth

The Crust The Mantle

The earth has four layers. The crust is the topmost layer. It is found both under the continents and the oceans. The mantle is found between the crust Outer core and the core. It stretches from ~40 km to Inner core ~2900 km. The outer core is in a molten state. This contains mostly iron and nickel. The inner core stretches from a depth of ~5200 km to ~6400 km.

From a molten rocky mass to a layered planet How did this happen? We have to look into the earth’s distant past to understand its present layered interior. Earth scientists have established that in the beginning, primitive earth was a mixture of molten rocky material. There were no continents or oceans. As the earth cooled, the densest materials (iron and nickel) sank to the centre of the earth. This formed the core of the earth. It begins at a depth of ~2900 km and stretches to the centre of the earth. The lighter materials floated to the top and on further cooling formed the crust. The residual material formed a layer, called the mantle, in the middle.

Comparative distribution of the elements in the earth as a whole and in the earth’s crust Percentage of minerals in the whole of the solid earth Iron Oxygen Silicon Magnesium Nickel * 13% 2.4% Others 35% 30% 15% * Sulfur 1.9% Calcium 1.1% Aluminium 1.1% Rest < 1%

62

Learning Science

Percentage of minerals in the earth’s crust Al Iron Mg * Oxygen Silicon 8% 6% 4% Others 46% 28% * Calcium 2.4% Potassium 2.3% Sodium 2.1% Rest < 1% ~ 82 % of the crust is made up of just three elements - oxygen, silicon and aluminium. Oxygen occurs only in combination with other elements in the solid part of the earth. Most of the 35% of iron is found in the core. Evolution of the earth is the result of the internal and the external heat engines simultaneously driving the earth.

Role of internal heat The internal heat melts rocks. throws out magma. supplies energy to build and move continents around. pushes up the mountains.

Role of the external heat engine In the beginning, the earth’s external heat was the result of meteorites hitting the earth with tremendous force. This heat contributed to the melting of the young earth. Once the earth cooled, the earth’s external heat engine is driven by the solar energy received by the earth’s surface. Now, external heat is responsible for: erosion by wind and rain. weathering. Wind and rain weather and climate. water cycle.

Water cycle

Weather and climate

Weathering

Learning Science

63

Earth’s atmosphere, oceans and the continents evolved as result of the effects of the external heat and the internal heat acting upon the primitive earth.

Atmosphere Most earth scientists believe that the atmosphere was also formed when the primitive earth began to cool. As the molten rocky mass of the infant earth cooled, the lightest and the most volatile substances escaped as gases from the earth’s interior. These gases formed the atmosphere. The composition of the early or primitive atmosphere was very different from the composition of the atmosphere now. Sunlight and plants were mainly responsible for the change in the composition of gases in the earth’s atmosphere.

Formation of continents Scientists believe that the formation of the continents started when the magma started coming to the earth’s surface. On reaching the earth’s surface, the molten rock material slowly cooled and solidified and formed the earth’s crust. Here also, the heavier materials sank to the bottom and lighter materials floated to the top. The lighter crust was decomposed and broken into fragments by forces of nature. These fragments were deposited in layers at various sites to form different landforms of the primitive continents.

Birth of the ocean It is hard to believe that in the beginning, there was no water on our planet. As the rock pieces from space hit the young earth with great force, many huge basins were formed on the earth’s surface.

64

Learning Science

As the earth cooled, steam escaped from the interior and entered the atmosphere. Gradually, it cooled and formed clouds. At first, the clouds brought incessant rains. The rain water filled the basins for tens of thousands of years and eventually a super ocean was formed.

Lithosphere - the outer shell of the earth Lithosphere is composed of the crust and the solid part of the upper mantle. It is not an unbroken and continuous shell. It is made up of rock “slabs” called crustal plates. The plates are in constant motion. The change in the shape of continents and oceans are the result of the motion of these plates.

Crustal plates Earth’s crust is made up of many smaller pieces called crustal plates. These are like pieces of a jigsaw puzzle. The crustal plates ride or float over the soft layer of the mantle. The crustal plates are in constant motion. They come together (convergent movement), move apart (divergent movement) and slide under (subsidence). Movement of the crustal plates (or plate tectonics) result in many natural phenomena.

How hot is the interior of the earth? We know for certain that the temperatures in the borewells and the mines increase by about 20 or 30C per 100 m depth. How can we know about the temperatures below a depth of 8 km? You can imagine the difficulty when we compare the depth of the deepest oil well (8 km) to the depth from the crust to the centre of the earth (~6400 km). To find out about the earth’s fever chart, the earth scientists have studied the temperature of the lava as it emerges from the interior of the earth. the speed changes in the seismic waves as they travel through the different layers. the melting points of various metals and minerals.

Learning Science

65

The earth’s fever chart The temperature increase is not uniform throughout the interior of the earth. The temperature increases by ~120C per km in the first 100 km and by ~20C between 100 km and 300 km. The temperature increases to ~20000C from 2900 km. This temperature is higher than the melting point of iron. (Hence the outer iron core is in a liquid state). The temperature of the inner core or the centre of the earth is ~22000C to 27500C. But the inner core is a solid because of the tremendous pressure there. Many believe that the temperature of the core is as high as 50000C. Atmospheric pressure at the core is ~ 4 million times greater than it is at sea level.

Minerals: the gift of the lithosphere What are minerals? Where are they found? Minerals are naturally occurring, non-living substances. can be made up of elements or of chemical compounds. are found in the thin uppermost layer of the crust. are sources of energy. can be metallic or non-metallic. are the source of soil. In fact, rocks are made up of minerals.

How many minerals are known to us? Mineralogists have identified more than 2000 minerals which are distinct from one another. All these minerals do not form rocks of the earth’s crust.

66

Learning Science

The main rock-forming minerals are the silicates, carbonates and sulfides. There may be only one mineral in some rocks (Limestone often consists only of calcite). There may be more than one mineral in some other rocks. (Granite contains quartz, feldspar and at least one other mineral.)

Identification of minerals Minerals are generally identified on the basis of the following properties.

Colour

Transparency

Crystal form

Lustre

Streak

Cleavage

Hardness

From salt to transistors Importance of materials from the earth. Some commonly used materials are: stones for buildings. chemical salts for fertilizers. limestone for making cement. sand (silica) for transistors and optical fibres. clays for ceramics. gemstones Can you imagine life without common salt at home?

Uses of minerals Man has used minerals in a variety of ways throughout history and in all civilizations. Minerals have been used in the following ways:

Learning Science

67

for making rock shelters. for making weapons. as mineral fuels. as gemstones.

Gems - the beautiful minerals Gems are beautiful, durable and rare. There are about 80 known gems. The most precious and sought after ones are diamond, ruby, sapphire and emerald. These beautiful gems are actually drab elements or compounds. Diamond is carbon, ruby and sapphire are both forms of aluminium oxide (corundum (Al2O3)). Ruby and sapphire are transformed to beautiful gems by certain impurities in corundum (Al2O3). Ruby is red because of chromium, sapphire is cornflower blue because of iron and titanium.

How are gems useful? Artificial diamond, ruby and sapphire have been made in the laboratory by imitating the conditions under which they are formed in nature. Gems are useful in industry as well. Ruby and sapphire are used in grinding tools in industry. Large ruby crystals are used in lasers. Sapphires are used in watches. Diamonds are used as tips in cutting tools.

Mineral wealth of India Distribution of some of the important mineral ores in India Haematite Fe2O3 Bauxite Al2O3. 2H2O

Ilmenite FeTiO3 Monazite sands (rare earth elements)

Pyrolusite MnO2

68

Learning Science

Rocks What are rocks? Rocks make up the crust of the earth we live in, are aggregates of minerals, are generally porous but all porous rocks are not permeable. They are necessary for soil formation and are the key for understanding the evolution of the physical earth through geologic time.

Rock dating or measuring geological time At first geologic time was measured using rate of deposition: It takes 4,000 to 10,000 years for a layer of ~1 foot thick sedimentary rock to be formed. rate of erosion and salt in the ocean. Modern methods of measuring geologic time are based on more reliable radioactive clocks. These radioactive clocks measure the decay of radioactive elements and the amount of radiocarbon present. Radiocarbon dating: All plants and animals have a tiny amount of radiocarbon. The radiocarbon content begins to decrease as soon as the plant or animal dies.

Same minerals but different rocks! Rocks are like cakes which may be made of the same ingredients and minerals are like nuts and dried fruits. Just as two pieces of a cake may have different amounts of nuts, raisins and other goodies, two different samples of the same rock may have different proportions of the minerals making up the rock.

Learning Science

69

Classification of rocks Just as we can prepare different things using the same ingredients but by using different processes, rocks differ when they are formed by different processes. Rocks are classified into three major groups. They are: Igneous rocks, sedimentary rocks, metamorphic rocks. Type of rock

What is it made up of?

What is the process?

Igneous rock

Melted rocks in the earth’s hot and deep crust and upper mantle.

Solidification of the rock melt and crystallization.

Sedimentary rock

Material from weathering and erosion of the earth’s surface.

Transportation, sedimentation and lithification

Metamorphic rock

Solid rocks in the earth’s deep crust and mantle.

Re-crystallization and formation of a new class of rocks.

How are the three types of rocks distributed? Rocks are not distributed with fixed and known boundaries, each in a separate enclosure as it were. They occur together without any fixed boundaries. Sedimentary rocks cover a greater percentage of land surface and ocean floors. Igneous and metamorphic rocks make up most of the crustal volume.

Surface Sedimentary rock area

Metamorphic Crustal Volume rock

Sedimentary rock Sedimentary rock

Meta morphic rock Metamorphic rock

Igneous rock

Igneous rock

Igneous rock

70

Learning Science

Distribution of the three groups of rocks is different in the horizontal and vertical planes of the earth’s lithosphere. Horizontal Crust plane of the Upper mantle earth’s crust and the upper Continental mantle. crust Oceanic crust

Vertical plane of the earth’s crust and the upper mantle.

On the horizontal plane of the crust and the upper mantle sedimentary rocks make up 75% of the rocks found. The continents and the ocean floors are made up of sedimentary rocks upto 75%.

Sedimentary rocks

Distribution of rocks in the vertical plane

Sedimentary rocks

In the vertical plane, as one goes down from the crust to the upper mantle, igneous and metamorphic rocks make up 95% of the volume of the rocks.

Decoding the information in the rocks Rocks may provide the key to unlock the geological secrets of the earth, buried deep within it and strewn all around us on the surface. In order to decode the information hidden in the rocks, we need to know when, where and how a rock is made and to that the individual characteristics of a rock is the result of its process of formation.

123456789 123456789 123456789 123456789 Igneous and 123456789 123456789 metamorphic 123456789 123456789 rocks 123456789

Learning Science

71

Earth as a rock factory In this factory, rocks are made and unmade at all times without a break. The raw materials are never in short supply here. There is never any power shut down and the work force never quits. weathering deposition uplift

burial and lithification uplift uplift

Rock cycle heat and pressure

heat & pressure cooling melting

I rock - Igneous rock S rock - Sedimentary rock M rock - Metamorphic rock

Like most natural phenomena in our physical world, the formation of different types of rocks also follows a cyclic pattern. Rock cycle is the result of certain geological processes that have been going on for millions of years on our planet. As result of these processes, the three types of rocks are continuously changing from one type of rock to either of the other two types of rocks. In the rock cycle, igneous rock is considered as the primary rock or the “starting block”.

Process of formation of igneous rock The origin of igneous rocks is as ancient as the origin of the earth itself! It is believed that during the early part of the earth’s history, hot and sticky “rock melt” called magma was the parent rock material of all the rocks that were formed in due course of time. This molten rock melt is found in the magma chambers present in the earth’s crust or the upper mantle .

> 7000C

72

Learning Science

All igneous rocks are formed by the cooling of magma or rock melt – whether the rocks are formed inside the lithosphere (intrusive igneous rocks) or on the outer surface of the earth (extrusive igneous rocks). Intrusive igneous rocks are formed when the magma is squeezed into the subsurface space, cools slowly and eventually solidifies. Intrusive rocks form typical structures called plutons. Intrusive igneous rocks have granular structures with larger crystals. Granite is a typical example of intrusive igneous rock. Extrusive igneous rocks are formed when molten magma is squeezed onto the surface of the earth through a vent and the molten magma or lava spreads and cools rapidly. Extrusive igneous rocks have smooth texture and very fine crystals. Basalt is a typical extrusive igneous rock.

Process of formation of sedimentary rocks Sedimentary rocks occupy an important stage in the rock cycle. They are placed between the rocks that are uplifted to the earth’s surface and the rocks that are formed deep inside the Weathering earth’s crust. Formation of Transportation Sedimentation sedimentary rocks goes through B definite stages; each stage is u dependent upon its previous stage. r i However, these stages occur a Weathering concurrently and continuously. l The stages are: weathering, Lithification transportation, sedimentation, burial and lithification. Different types of sedimentary rocks Sedimentary rocks are classified on the basis of their formation. The three main types of sedimentary rocks are: mechanically formed sedimentary rocks (example: sandstone), chemically formed sedimentary rocks (example: gypsum) and organically formed sedimentary rocks (example: limestone).

Learning Science

73

Environments of mechanically formed sedimentary rocks Sedimentation

Glacier Lake River Desert

B u r i a

Beach Delta Deep sea

l Lithification

These environments also provide dissolved chemical salts for the formation of chemically formed sedimentary rocks. Sorting of the raw materials of mechanically formed sedimentary rocks Pebbles, sand and silt form the raw Sorting in the ocean materials of mechanically formed sedimentary rocks. They get sorted out continental shelf and are buried in layers on land and at pebbles the bottom of water bodies. Typically, sands pebbles get deposited first. This is silt and clay followed by sand. Finally silt and clay settle down as sediments. Sedimentary rocks are the only rocks that are formed by the raw materials from the earth’s surface and the geological processes taking place there.

Change from sediment to sedimentary rock Sediment Gravel Clay Sand Lime

Sedimentary rock Conglomerate Shale Sandstone Limestone

74

Learning Science

Process of formation of metamorphic rocks When igneous and sedimentary rocks are subjected to intense pressure and extremely high temperature deep inside the earth’s crust or the upper mantle, their composition and texture undergo change. As result, rocks with entirely different composition and texture are formed. These rocks are called metamorphic rocks. (Here, meta means “form” and morphic means “change”). What are the causes of metamorphism? The two main causes of metamorphism of rocks are high temperature and extreme pressure. Extreme pressure and high temperature are the results of the earth’s internal heat. High temperature Extreme pressure the weight of layers from overlying rocks of rocks. the pressure developed when rocks are deformed. Where does metamorphism occur? Metamorphism can occur wherever conditions of extreme pressure and extremely high temperature are found in the earth’s crust or the upper mantle. Metamorphic rocks, therefore, are found at various depths in the earth’s crust and the mantle. It is also found sometimes close to the earth’s surface.

How do rocks change? Conglomerate Sandstone Shale Shale Slate

Schist Quartzite Slate Schist

Coal Limestone Granite

Anthracite Marble Gneiss

Learning Science

75

Soil What is soil? Soil is the uppermost thin layer on the earth’s surface. It takes hundreds of years to form a centimetre of soil! Most importantly, it is the layer that s plant life. Some interesting facts about soil: soil is carried by wind. takes a long, long time to form. goes with the flowing water. has many colours and textures. Also, the sun dries it, rain makes it slushy and children and some animals love to play in it.

When was soil formed on our planet? There was no soil on our planet when it was formed. In the beginning, billions of years ago, there were only rocks on the earth’s surface. The rocks were exposed to the heat from the sun, force of the wind and rain. Gradually, the rocks developed cracks and broke into smaller pieces. When the plants appeared on the earth, they grew in the small cracks in the rocks pieces. Some plants grew on the rock pieces. Gradually, the rock pieces grew finer and the tiny dead plants got mixed in the fine rock pieces and soil was formed.

76

Learning Science

Rock, climate and soil Just like we share the characteristics of our parents, soil shares the characteristics of its parent rock and the climate of the place. Climate plays an important role in the formation of soil. 12345678901234567 12345678901234567 12345678901234567 12345678901234567 12345678901234567 12345678901234567 12345678901234567 12345678901234567 12345678901234567 12345678901234567 12345678901234567 12345678901234567 12345678901234567 12345678901234567

to form

breaks up the rocks

sand

clay silt

Climate

determines the natural vegetation of the place

The dead plants on decaying form

humus

All soils are not the same The three main types of soil based on texture are: Sandy soil: This has more sand and no humus. It cannot retain water. Water melons and pumpkins grow well in it. So do cacti. Clayey soil: This soil has more clay than sand. It retains moisture for long periods. It becomes hard when it is dry. Cotton and wheat grow well in it. Loamy soil: This has humus, silt and clay. It retains moisture for a long time and does not become hard even when it is dry. It is a plantfriendly soil.

soil

Learning Science

77

Can soil become “useless”? Soil becomes useless when plants cannot grow in it. This happens when the top soil is removed by rain, wind and flowing water. This is called soil erosion. There are many types of soil erosion.

Soil is a precious natural resource. Conserve it. Soil erosion and depletion of its fertility can be prevented by growing grass and trees wherever possible. fencing the fields with trees. rotation of crops. strip cropping. growing crops that can fix nitrogen to the soil. preventing overgrazing. contour ploughing in hilly regions. not cutting down trees excessively.

Time has come to green the deserts Soil used to be called an inexhaustible natural resource. It is no longer so. By our indiscriminate use, soil is fast becoming an exhausted resource. Due to thoughtless deforestation and overgrazing, creeping deserts are consuming the once fertile soil in many countries. Time has come for us to reverse this trend and start to green our deserts. Or our planet will soon return to its original state when there was no soil on it. Do your bit by spreading this awareness.

78

Learning Science

Motions of the earth and seasons The earth and its motions Rotation of the earth on its axis and its revolution around the sun affect our daily lives. Yet, we go about our daily lives unaware of the eternal motions of our home planet.

Earth’s rotation on its axis Have you seen a top rotate? Perpendicular to the plane It rotates better if the nail is Axis of the earth exactly in the centre. The 0 earth also rotates on an Plane imaginary line ing 0 through its Equator north pole and the south pole. This imaginary line is called the axis of the earth. The earth’s axis is tilted making the earth face the sun at 23.5 degree. We often forget how important this tilt is for life on earth.

Tilt of the earth’s axis The earth’s axis is tilted at a fixed angle. The angle does not change either during rotation or during revolution. The axis is tilted at 66.50 to the plane of the earth’s orbit or 23.50 to the perpendicular to the plane of the earth’s orbit. This tilt can be shown either to the right or to the left. But the axis is always perpendicular to the earth’s equator.

Learning Science

79

Rotating earth and stationary us The earth is rotating at ~1600 km per hour at the equator and yet, we do not feel it. This is because we are rotating along with the rotating earth. ! When we are travelling in a train or a car or an aeroplane, we feel that we are stationary and the things outside are moving. In which direction do things appear to move? The earth rotates fastest at the equator. The speed of rotation goes on decreasing as the distance from the equator increases.

Rotation of the earth and sunrise and sunset In reality, there is no sunrise or sunset. As the earth rotates on its axis, different parts of the earth face the sun. This happens gradually, resulting in sunrise when it faces the sun and sunset when it moves away from the sun. The sun appears to rise in the east and set in the west. In which direction does the earth rotate?

The earth’s journey around the sun or the apparent annual migration of the sun The earth like all the others planets, does not rotate at the same place in space. It must pay its respects to the sun by going round it not once or any fixed number of times but eternally. The revolution of the earth around the sun is commonly expressed as the annual journey of Earth’s orbit the sun! The path along which the earth goes around the sun is called the orbit. The orbit or the path is like an ellipse. The sun is slightly off centre.

80

Learning Science

How long does the earth take to go around the sun? The earth revolves around the sun in the same direction as it rotates on its axis. The earth takes 365.25 days or a year to complete one revolution. This is also called the “Earth Year”. You are also revolving around the sun along with the earth. How many times have you gone around the sun? Clue: count your birthdays.

Earth Year The earth travels around the sun along a fixed track or orbit. The track is 960,000,000 km long (600,000,000 miles). It always runs from west to east. The earth completes one lap around the sun in 365.25 days or approximately 105,600 km a day (66,000 miles)! This lap of the earth is called a Year.

Revolution and its importance As the earth goes around the sun, it receives different amounts of energy from the sun. The sun sends out rays in all directions into space.

~ 150,000,000 km

The rays reach the earth as parallel rays because of the enormous distance between the sun and the earth and the small size of the earth (compared to that of the sun). Direct rays

Some rays from the sun reach the earth’s surface as direct rays and some as slanting rays. The direct rays provide more heat than the slanting rays.

Slanting rays

Learning Science

81

Seasons - the cyclic change Seasons are the direct result of the amount of energy the earth receives from the sun. When the earth is tilted towards the sun, it receives more energy from the sun. What will be the season? When the earth is tilted away from the sun, it receives less energy from the sun. What will be the season? All places on earth do not receive the same amount of energy all the time.

Why do seasons occur? The tilt of the axis, the axis always pointing to the North Star and the revolution of the earth cause a cyclic change in the amount of heat energy received by the two hemispheres of the earth. As a result, seasons occur.

Season in the two hemispheres In December, the southern hemisphere is tilted towards the sun and the northern hemisphere is tilted away from the sun. As a result, the southern hemisphere receives more heat energy from the sun. Countries in the southern hemisphere experience summer season. During the same period, what is the position of the northern hemisphere with respect to the sun? What will be the season here? Make a list of the continents which are completely in the southern hemisphere and of the continents completely in the northern hemisphere.

82

Learning Science

Spring and autumn seasons As the earth moves from its December position to its March position, there is a change in the amount of heat and light received by the two hemispheres. In which hemisphere will it be the beginning of spring season in March? Spring is more noticeable in countries in North America and Europe. Why? Summer season gives way to the autumn season in September. This is the season of mellow sun and fruits. In hot countries, these two seasons are not very noticeable. They can be experienced only in the mountains. Where can we experience spring and autumn in India?

Summer in India and winter in Australia India is in the northern hemisphere. Australia is in the southern hemisphere. When the northern hemisphere is tilted towards the sun, the southern hemisphere is tilted away from the sun. Therefore, India and Australia have opposite seasons during the same months. Why don’t they have a white Christmas in the southern hemisphere? Seasons are mirror images in the two hemispheres.

Learning Science

Duration of seasons Each of the seasons, winter, spring, summer and autumn last for three months. The calender months of the four seasons in the northern hemisphere are as shown in the picture. What are the calendar months for the four seasons in Australia?

83

March April May June July August

December January February September October November

Seasons in India In most parts of India, we have only a mild winter season, a hot summer season and the rainy season. Importance of seasons Seasons greatly determine the way we dress and perhaps what we eat. It is said seasons affect even the moods of people. Seasons and plants Even the plants respond to seasons. Some plants flower only in summer and some in the cool winter. The trees in hot countries shed their leaves in summer and the trees in cooler climates shed their leaves in winter.

Why do trees shed leaves? Trees shed leaves either to conserve moisture or heat. In the tropical regions, summers are very hot. The trees, therefore, shed their leaves to minimise loss of water through transpiration. In the cold countries, it is very cold during winter. The trees in these countries shed their leaves in autumn to conserve heat. Trees which shed their leaves according to season are called deciduous trees.

84

Learning Science

Seasons and body - clock of animals The behaviour of animals during different seasons is truly one of the wonders of life on earth. To avoid the bitter cold of the polar region, the polar bear hibernates. The whales swim from the frozen Arctic along the coast of North America to the warm waters of the California Bay during winter, and start their return journey from California Bay to the Arctic once the winter is over. Birds from Siberia visit sanctuaries in India during winter. The annual seasonal migration of birds is truly fascinating.

Hibernation Hibernate means the winter in sleep! As winter approaches, days become shorter and colder. During winter, in Sweden, for some days there is no sun for 24 hours! Animals know that winter is approaching and food will become scarce. Squirrels, bears and other animals eat a lot so that they can get layers of fat. In winter, they go into deep sleep to save energy (hibernation). Bears hide in caves, squirrels in nests. They are woken up by the arrival of spring season.

Same season yet different climate Seasons are the same in all the countries in a particular hemisphere. For example, during the months of June, July and August, in all the countries in the northern hemisphere, it is summer and in all the countries in the southern hemisphere, it is winter. However, even when the season is the same, temperature and rainfall are not the same in the entire hemisphere.