Landform and Ocean Concepts

(Mountains, Plains and Plateaus are important landforms but the world below the ocean contain a large variety of landforms as well)

Mountains or hills

Himalayan Mountains

Mountains or hills are highly elevated lands, which rise above the surrounding region. When such high hills form a long chain, they are called a mountain range. They may stretch continuously and may occur in a series of parallel ranges, for example, The Great Himalayas. Mountains may be called young or old according to their age. The Alps or the Himalayas are young mountains while the Aravallis or the Appalachian mountains are old mountains. Due to erosion over the years these old mountains have become rounded and low in height. Now they are mere stumps. From the Pamir knot in Central Asia, mountain ranges spread out in different directions. Human habitation is not very dense in high hills because it is difficult to construct communication lines in such mountainous areas.

Plateaus

Tibetan Plateaus

Plateaus are moderately elevated flat-topped lands. They often resemble a table and are also called Tableland. Plateaus rise abruptly from the surrounding areas. They may stretch over thousands of square km. In other words, Plateaus are flat uplands with steep slopes. Laddakh and Deccan are two examples of plateaus. Large parts of Africa, Asia, Australia and North America consist of plateaus. Plateau of Tibet is the highest plateau in the world. Therefore it is called ‘the roof of the world’. Plateaus are moderately populated regions. Transport is not so difficult here as it is in the mountains.

Plains 

North Indian plain

Plains are low-lying and relatively flat stretches of land. They are not very high above the sea level. Rivers, emerging from the mountains carry huge amount of silt, mud, and sand. Rivers carry these materials down to the sea and deposit them along the seacoast or along their banks. The largest plains are found along the river valleys in all the continents, except in Antarctica. Deposits made by rivers consist of sand, silt, and clay. Thus the plains become very fertile. Agriculture all over the world flourishes on such plains, for example the north Indian plain. Construction of transport network is easy. Thus, these plains are very thickly populated regions of the world.

Continents

Extensive masses of land rising above the sea level are called continents. They are generally separated from one another by huge bodies of water. Most of the land area lies in the northern hemisphere. There are seven continents on the earth. They are:

Continents

• Asia
• Africa
• North America
• South America
• Europe
• Australia
• Antarctica

Asia

Asia is the largest continent lies in the northern hemisphere. Our country India situated in this continent. The cold Gobi desert and the Himalayan Mountains are situated in this continent.

Africa 

The second largest continent is Africa. The equator runs almost half way through this continent but does not divide it into equal halves. River Nile, the longest river in the world and the Sahara, the largest desert in the world are situated in this continent. This continent is filled with dense forests and rich in mineral resources.

North America

North America lies entirely in the Northern Hemisphere. It is surrounded by the Atlantic ocean, Arctic ocean and Pacific Ocean. On its west-coast, Rocky mountain, a long chain of mountains is situated.

South America

The major part of South America lies in the southern hemisphere, as little part lies in the northern hemisphere also. River Amazon, world’s largest river and the Andes, longest mountain range of the world situated in this continent. The continent of North America and South America are linked by narrow stretch of land across Panama. 

Europe 

Europe lies to the west of Asia. The Alps mountain range lies in this continent. Europe and Asia are continuous single landmasses. These two continents are separated by the Ural Mountains and the Ural River. These two continents are referred to as Eurasia.

Australia 

Australia is a continent situated in the southern hemisphere. Being the small continent it is often referred to as the ‘Island continent’ as it is surrounded by oceans from all sides. It consists of many islands like New Zealand and Fiji. Fiji Islands and Papua and New Guinea are called Oceanic islands. The Great Barrier Reef is situated the off the east coast of Australia. 

Antarctica

Antarctica is the southernmost continent. The South Pole lies at the Centre of this continent. In size, Antarctica is bigger than Australia and Europe taken together. But, because it is situated in the Antarctic Circle, it remains covered by deep snow throughout the year. Penguins, Seals and other living creatures live in Antarctica. Our country India has set up scientific research stations at Antarctica in the name of Dakshin Gangotri and Maitri. A new research station named Bharati has recently been established.

Oceans

Oceans are vast stretches of water covering a huge area. Unlike the continents oceans are all inter-connected. Therefore, the level of sea water remains same everywhere. This level of water is called sea level. There are four major oceans on the earth. They are pacific ocean, Atlantic Ocean, Arctic Ocean and Indian ocean. The oceans account for about 71 % of the earth’s surface. The average height of the continents is only about 700 meters above sea level whereas the average depth of the oceans is about 4000 meters below sea level. For the matter of convenience, countries divided the vast oceans into seas. For example the sea lies east to India is known as Bay of Bengal and the sea lies west to India is known as Arabian sea.

Oceans

Pacific Ocean

The Pacific Ocean is the largest as well as the deepest ocean. Earth’s deepest part is the Mariana Trench (11022 m deep) and it is situated in the Pacific Ocean. The Pacific Ocean is bounded by Asia and Australia on the one hand and North and South Americas on the other. The volcanic mountains which surrounded the Pacific Ocean are called the Pacific ring of Fire.

Atlantic Ocean

The Atlantic Ocean is the second largest ocean in the world and looks like the letter ‘S’. It is bordered by North and South Americas on the west and Europe and Africa on the east. Hurricanes are very common in this ocean. Its coastline is very much indented. The coastline may be straight or indented in nature. Indented coastlines facilitate the developments of harbors or ports, So Atlantic Ocean is the busiest ocean from the point of view of commerce. 

Indian Ocean 

The Indian Ocean is the third largest ocean in the world, bordered by the continents of Africa in the west, Asia in the north and Australia in the east. India receives rainfall due to the monsoon which originate within this ocean.

Arctic Ocean 

The Arctic Ocean is the smallest ocean in the world. It has North Pole at the Center of the ocean. For most part of the year the ocean remains frozen. This ocean has many icebergs. It is actually a northward extension of the Atlantic Ocean. Arctic Ocean is connected with the Pacific Ocean by a narrow stretch of shallow water known as the Bering Strait. It is bounded by the northern coasts of North America and Eurasia.

Ocean which surround the Antarctica is known as Southern ocean. Actually, this the culmination of South Pacific Ocean, South Atlantic ocean and Indian ocean. Apart from the oceans and seas, there are rivers, lakes, ponds and tanks.

Islands, Archipelago, Peninsula, Strait, Lagoon, Gulf, Bay & Isthmus

• A piece of land surrounded by water on all sides is called an Island. Sri Lanka is an Island country situated south of India. 

  

  Relief features

• A group of islands is termed as Archipelago. Andaman and Nicobar Islands belong to India is a group of islands, hence Archipelago.
• Peninsula is a piece of land that is surrounded by water on three sides. India is also a peninsular country.
• A strait is a narrow stretch of water that separates two large land masses. Palk Strait is a strait between India and Sri Lanka.
• A gulf is a hollow carved out in the sea coast which lets the water reach deep inland. For example the Persian Gulf.
• A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs. Lagoons are commonly divided into coastal lagoons and atoll lagoons.
• A Bay is an inlet of the sea with a wider opening than a gulf. For example the Bay of Bengal and the Persian Gulf.
• An Isthmus is a narrow strip of land separating two large water bodies. For example the Isthmus of Panama connects North America with South America.

Day & Night and Seasons: The Systematic Changes Occur on Earth

(It is because of the earths rotation and revolution, the systematic changes happen)

Inclination of the earth’s axis

Tilt of the Axis

We know that the axis of our earth is imaginary. The earth is tilted by an angle of 23 and half degree from the vertical line or perpendicular line to the orbital plane. Because of this tilt the earth is making an angle of 66 and half degree with its plane. This tilting of the earth’s axis is referred to as the Inclination of the earth’s axis.

Days and Nights 

We know that, our earth is always in motion. It has two types of movements I.e. rotates on its own axis and revolves around the sun. The earth takes approximately 23 hours and 56 minutes for one rotation on its axis and this rotation is the cause of day night occur on earth. During the rotation, all parts of the earth do not experience day and night at the same time. It is because of all parts do not get sunlight at the same time. The part of the earth’s surface which faces the sun experience day and the other part which is away from the sun experiences night. So Day and night are the systematic changes that takes place everyday.

Days and Nights

In ancient times people thought that day and nights occurred due to the revolution of the sun around the earth. But it is Aryabhatta, the great ancient Indian astronomer only clarified this misconception that the earth has its own rotation. In ancient times, sunrise is marked as the beginning of the day, but now-a-days we consider 12 midnight as the beginning of the day. 

What will happen, if the earth’s axis is flat i.e. not tilted? The answer is, there would be six months of day and six months of night all over the year on the earth. This situation would change only after when earth completes one revolution. Do you know, this kind of situation existed on which planet? This situation prevails in the planet Uranus. 

Leap Year

Leap Year

Like rotation, direction of the earth’s revolution is also anti-clockwise i.e. from west to east. The earth revolves round the sun at a speed of about 100,000 km. per hour. The earth takes 365 days and about 6 hours to complete one revolution. For the sake of convenience we consider a year is of 365 days. The additional 6 hours, after 4 years, become 24 hours or one additional day and we add this to the month of February every 4 years. Thus, the month of February has 29 days instead of 28 days once in every 4 years. That particular year is called a leap-year. Leap-year is always divisible by four.

Seasonal Changes

Seasons occurred due to earth’s revolution and it is not uniform everywhere. Like the day and nights, there are seasonal changes every year. Generally there are four seasons. They are Spring, Summer, Autumn and Winter. The path which earth takes to revolve around the sun is elliptic. Hence at one position, earth is close to the Sun and other position it is away from the sun. Generally during July, the earth is very far from the sun and during the January, the earth is close to the sun. If seasons are caused due to this, then there would be summer in January and winter in July. So it is clear that earth’s elliptic orbit is not responsible for the seasons. So how seasonal changes occur? It is because of the inclination of the earth’s axis

Seasonal Changes

Because of the tilt of the earth’s axis, one hemisphere come to face the sun first and then come the other hemisphere. Therefore, variation in the weather phenomena is observed which we call seasons. When the northern hemisphere faces the sun it receives direct and vertical rays from the sun. Sunlight received in those parts during the day is longer. This is summer in northern hemisphere. But the same hemisphere, when remain away from the sun’s rays, direct sunlight does not reach there. Those regions experience lesser hours of sunlight, it is winter condition in those regions. While revolving around the sun, the earth comes in four critical positions. These changing positions are responsible for the cycle of season.

Summer Solstice: On June 21, the northern hemisphere is tilted towards the sun. The sun is directly over the 23 ½0 N latitude or the tropic of cancer. On this date maximum hours of sunlight is available in this hemisphere. This situation is called the summer solstice. During this period, the sun ray is vertical on the Tropic of Cancer and gives more heat. This is the summer condition in the northern hemisphere and winter condition in the southern hemisphere. 

Changing Positions of the Earth

Autumnal Equinox: On September 23, as the earth’s revolution continues, equator faces the sun directly. On this date day and nights are equal all over the world. Therefore it is called Equinox. As autumn season prevails during this time, it is called Autumnal Equinox in the northern hemisphere.

Winter Solstice: On December 22, the sun is directly over the 23 ½0 South latitude or the Tropic of Capricorn. This is summer solstice in the southern hemisphere. On this day the northern hemisphere experiences shortest day and it is called winter solstice in the northern hemisphere.

Vernal Equinox: As the earth moves forward, on March 21, the equator again comes to face the sun, which is directly over it. Day and night again become equal on this day. This is called Vernal Equinox in the northern hemisphere.

Position of the rising Sun – Uttarayan and Dakshinayan

Uttarayan & Dakshinayan

East is the direction in which the sun rises. This statement is partially true because the position of the rising sun changes every day. From summer solstice (around 21 June), the point of sunrise gradually shift towards the south. The sun is then said to be in dakshinayan (moving south). It keeps doing so till Winter Solstice (around 22 December). Thereafter, the point of sunrise changes direction and starts moving towards north. The sun is said to be in uttarayan (moving north). From the equator, only on two days, on the days of the equinoxes (around 21 March and 23 September) the sun rises in the east. On all other days, it rises either north of east or south of east.

Day and Night on the Poles

Nights as well as days on the poles extend for six months, because the sun is never high above the horizon in the Polar Regions. It is only on the equator that the day and the night are always of equal length. As we move away from the equator, either northward or southward, the difference between the length of day and that of night generally goes on increasing.

Locating Places on the Surface of the Earth

(Any location on Earth is described by two numbers i.e. its latitude and its longitude)

We know earlier that our earth spins continuously on an imaginary axis which passes through the center of the earth. Astronomers and geographers considered the Northern tip of the axis as North Pole and the Southern tip of the axis as South Pole. But another thing, cartographers considered the largest possible circle that can be drawn on a globe or our earth is Equator. It divides the globe into two equal halves. The northern half is known as Northern hemisphere and the southern half is Southern hemisphere.

Latitudes

Parallels of Latitude

Since the distance from the equator to either of the poles is 1/4th of a circle round the earth, it will measure ¼ of 360 degree i.e.90 degree. Thus 90 degree North latitude marks the North Pole and the 90 degree South latitude marks the South Pole. Parallel to the equator one set of imaginary lines are drawn which encircle the earth and run in east west directions. These are called latitudes. Since all these lines are parallel to the equator as well as one another, they are called parallels of latitudes.

Latitude is the angular distance of a place from the equator, lying either in the north or in the south directions. It is measured in degrees from the equator toward either pole. One degree (0) is divide into sixty equal parts and each unit is called a minute (’). A minute is further divided into sixty equal parts and each unit is called a second (”). From the equator to the poles, parallels of latitudes become smaller. It becomes a point at the pole. Generally, there are 90 latitudes taken at an interval of 1 degree in the northern and southern direction of the equator. The value of the equator is zero. All points north of the equator are called ‘north latitudes’ and all points south of the equator are called south latitudes. Therefore, the value of each latitude is, followed by letters ‘N’ or ‘S’.

Important latitudes

Heat Zones & Imp. Latitudes

Besides the equator (0 degree), the North Pole (90 degree N), the South Pole (90 degree S), geographers have imagined four important parallels of latitude. They are: (i) the tropic of Cancer (23 degree 30 minute N), (ii) the tropic of Capricorn (23 degree 30 minute S), (iii) the Arctic Circle (66 degree 30 minute N), (iv) the Antarctic Circle (66 degree 30 minute S).

Heat Zones of the Earth

Torrid Zone: The mid-day sun is exactly overhead at least once a year on all latitudes in between the Tropic of Cancer and the Tropic of Capricorn. Therefore, this area gets maximum heat and is called the Torrid Zone. On 21st June the Sun is directly over the Tropic of cancer. On 22nd Dec the Sun is directly over the Tropic of Capricorn. These two latitudes form the outer limit of the Torrid Zone. It is the hottest part of the world. Most of the deserts are located here.

Temperate Zone: The midday sun never shines overhead on any latitude beyond the Tropic of Cancer and the Tropic of Capricorn. The angle of the sun’s rays goes on decreasing towards the poles. The areas bounded by the tropic of cancer and the Arctic Circle in the Northern hemisphere and the Tropic of Capricorn and the Antarctic Circle in the southern hemisphere, have moderate temperature i.e. neither very hot nor very cold. So this is Temperate Zone.

Frigid Zone: Areas lying between the Arctic Circle and the North Pole in the Northern Hemisphere and the Antarctic Circle and the South Pole in the Southern Hemisphere, are very cold. It is because the sun does not rise much above the Horizon. Therefore its rays are always slanting. So this is Frigid Zone. 

Longitudes

Meridians of longtudes

A set of imaginary lines which divide the earth into eastern hemisphere and western hemisphere run over the earth in north south directions are called “meridians of longitudes”. These lines are not parallel to one another. They all converge at poles. The distances between them are measured in ‘degrees of longitude’. They form semi-circles. They are also drawn at an interval of 1 degree. These meridians of longitude and parallels of latitude form a network which is called grid. If a pilot or a ship's captain wants to specify a position on a map, these are the "coordinates" they would use. Even people who are in the defense sector or intelligence use these meridians of longitude and parallels of latitude as "coordinates". Unlike the parallels of latitudes, the meridians of longitudes are equal in length. As one go from the equator to the poles the distance between two meridians decreases.

Prime Meridian

In order to avoid confusion, there was an agreement world over that the longitude passing through Greenwich Observatory near the city of London will be considered as the Prime Meridian. Located at the eastern edge of London, the British capital, the observatory is now a public museum and a brass band stretching across its yard marks the "prime meridian." Tourists and local people often get photographed as they straddle it i.e. one foot in the eastern hemisphere of the Earth, the other in the western hemisphere. On the opposite side of the Earth at 180 degree longitude lies an imaginary line called the International Date Line. It is considered to be 0 degree Longitude and from it we count 180 degree eastward as well as 180 degree westward. It is interesting that 180 degree east and 180 degree west meridians are the same line in the middle of the Pacific Ocean. To avoid the confusion letters ‘E’ and ‘W’ are written with the values of meridians for the eastern hemisphere and western hemisphere respectively.

Time Zones

Time Zones of the World

The earth takes about 24 hours to complete a rotation. This period of rotation is known as the earth-day. This means in 24 hours the earth completes 360 degree. Therefore, it takes about one hour to complete each 15 degree or 4 minutes for each degree. Accordingly, the earth has been divided into 24 time zones of one hour each. As the earth rotates from west to east, day starts at different times in different places. When we are marking time world-wide, not tied to our locality it is Coordinated Universal Time (UTC), which can be defined as the local time in Greenwich, England, at the zero meridian. Places east of the Greenwich meridian experience sunrise earlier than the place lying west of the prime meridian. All the places situated along the same meridian will have the same sunrise hence same local time.

An interesting story of America

In the middle of the 19th century, each community across the US defined its own local time, by which the Sun, on the average, reached the farthest point from the horizon (for that day) at 12’ O clock. However, travelers crossing the US by train had to re-adjust their watches at every city, and long distance telegraph operators had to coordinate their times. This confusion led railroad companies to adopt time zones, broad strips (about 15° wide) which observed the same local time, differing by 1 hour from neighboring zones. Then the US adopted the system. Now, the continental US has 4 main time zones. They are eastern, central, mountain and western, plus several more for Alaska, the Aleut islands and Hawaii. Canadian provinces east of Maine observe Atlantic time. One may find those zones outlined on the map giving area codes. Other countries of the world have their own time zones; for example Russia has 11 standard times. Only Saudi Arabia uses local times, because of religious considerations.

International date Line

International date Line

As we know the opposite side of 0 degree latitude is the International Date Line, it passes through the mid-Pacific Ocean and roughly follows a 180 degrees longitude north-south line on the Earth. It is established in 1884. It is located halfway round the world from the prime meridian. It serves as a “line of demarcation” i.e. separating two consecutive calendar dates. Despite its name, the International Date Line has no legal international status and countries are free to choose the dates that they observe. It is not a perfectly straight line and has been moved slightly over the years to accommodate needs (or requests) of varied countries in the Pacific Ocean. Thus, the date line generally runs from north pole to south pole, it zigzags around political borders such as eastern Russia and Alaska’s Aleutian Islands. Immediately to the left of the International Date Line in the eastern Hemisphere, the date is always one day ahead of the date immediately to the right of the International Date Line in the Western Hemisphere. So, travel east across the International Date Line results in a day, or 24 hours, being subtracted. Travel west across the International Date Line results in a day being added.

Indian Standard Time

IST

In India the longitudinal span is from 68 degree 7 minute to 97 degree 25 minute E. Therefore, it was felt necessary to adopt the local time of central meridian of our country as the standard time for the country as a whole. In India 82 degree 30 minute E has been considered for this purpose. It is called the standard meridian. This is known as Indian Standard Time (IST). The Greenwich Mean Time is 5 hours and 30 minutes behind the Indian standard time.  India Standard Time is a half-hour time zone. Its local time differs by 30 minutes instead of the normal whole hour. This time zone is often called Indian Time. In global context Greenwich (0 degree) time is followed which is called the Greenwich Mean Time (GMT).

Our Earth and The Solar System

The solar system

(The sun has a magnetic field, the heliosphere, which envelops our entire solar system)

Our Solar System

The solar system is dominated by the sun and it accounts for almost 99.9% of the matter of the whole system. It is the source of light and heat for our earth. Scientists believe that the Sun has been formed from a moving cloud of gases, which is called Nebula. The sun and the planets were born out of this cloud. The force of gravity has created them. For over millions of years, these balls of dust and gas are moving around the sun. The Sun by virtue of its mass and weight, controls the movement of the planets. This force is called the force of gravity.

Sun is at the Centre of the solar system with eight planets (i.e. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), satellites, asteroids, meteors and comets. They all move around the Sun. Till 2006, there were nine planets in the solar system. Pluto was the farthest planet from the sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto and other celestial bodies like Ceres, 2003 UB313 does not fit this definition. It is no longer considered as a planet of the solar system.

The sun

It is believed that the sun was born about 5 billion years ago. Since that time, it is continuously emitting huge amount of heat and light, and it is expected to glow in a similar manner for the next 5 billion years or so. The sun is essentially a sphere of hot gases. The disc is called photosphere. The layers of gas above the photosphere are extremely hot, but their densities are extremely low. These layers are very faint and are not visible in the presence of strong light from the disc of the sun. At the time of the total solar eclipse, when the light of the sun’s disc is completely cut off, the outermost layer becomes visible. It appears like a crown round the sun. This layer is called Corona.

Our Sun

The radius of the sun is almost 100 times of the radius of our earth and its mass is about a million times the mass of our earth. The sun is the nearest star from us. The sun is the source of almost all energy on the earth. In fact, the sun is the main source of heat and light for all the planets. The temperature of the bright disc is about 6000 K. At the center of the Sun, the temperature is about 15 million K. The pressure is also extremely high. All these conditions are right for fusion reactions.

In the sun, four protons (nuclei of hydrogen atoms) combine to form a helium nucleus. The mass of the helium nucleus is slightly smaller than the combined mass of four protons. So there is a loss of mass in the process. This loss of mass is the source of sun’s energy. According to Einstein’s equation i.e. E=mc² , E is the energy produced when a mass, m, is lost. The quantity, c, is the velocity of light. Most stars produce their energy by the process of fusion.

The sun is about 150 million km away from the earth. As light travels at a speed of 300,000 km/s., the light of the sun takes about 8.3 light minutes to reach us. The light of the nearest star (Proxima or Alpha centuari) to the sun reach earth in about 4.3 light years.

The planets

Planets are actually spinning around the Sun. They have their own elliptical path of movement known as the orbits. Movement around its own axis is called rotation and around the sun is revolution. They have their own speed also. Planets, unlike stars, have no light or heat of their own. They are lit by the light of the stars. That’s why, stars twinkle whereas planets do not. The word ‘planet’ comes from the Greek word “Planetai” which means ‘wanderers’. Planets keep changing their positions with respect to the stars. 

Mercury (Budha)

Mercury

It is nearest and the smallest planet of the solar system i.e. nearly of the same size and mass as the moon. Mercury takes 88 days for one orbit around the sun and 59 days for one spin on its axis. It has no satellite. There is no atmosphere on mercury. The surface of mercury is rocky and mountainous. One side of the surface facing the sun receives maximum heat and light. The surface of this planet does not receive sunlight or heat on its other side. One part of mercury, therefore, is very hot while the other part is very cold. Because Mercury is very close to the sun, it is difficult to observe it, as most of the time it is hidden in the glare of the sun. However, it can be observed in September and October just before sunrise in the eastern sky as a morning star. It is also seen in the western region of the sky immediately after sunset in March and April as an evening star.

Venus (Shukra)

Venus

Venus has no moon or satellite of its own. It rotates on its axis is somewhat unusual i.e. from east to west. Venus shows phases just like the moon. Its size is nearly the same as that of the earth. The mass of Venus is nearly 4/5 times that of the earth. It takes 255 days for one orbit around the sun and 243 days for one spin on its axis. Venus has an atmosphere that consists of mainly carbon dioxide. Minute quantities of water vapour and oxygen have also been found on Venus. The cloudy atmosphere of Venus sends back almost 3/4th of the sunlight that falls on it. That is why Venus could be seen as the brightest object in the sky after the sun and the moon. Sometimes Venus appears in the eastern sky before sunrise and sometimes it appears in the western sky just after sunset. Therefore it is often called a morning or an evening star. However the surface of Venus is much hotter than the earth but not as hot as that of mercury. Though mercury has no atmosphere, Venus is the hottest planet in our solar system.

The Earth (Prithvi)

Our earth

Our earth rotates from west to east. Our earth is more or less like a sphere, which is slightly flattened in the north and south. Slightly flattened or tapered at the poles, the earth is best to be described as Geoid which means earth like shape. Earth is the 3rd nearest planet to the sun. In size it ranks 5th. In size and make up the earth is almost identical to Venus. Due to presence of water and landmasses the earth appears blue-green in color from the space. Therefore, it is called blue planet. The earth is the only planet where some special environment conditions are responsible for the existence and continuation of life. These include just the right distance from the sun, so it has the right temperature range, the presence of water, soil, minerals, suitable atmosphere and a blanket of ozone. 

Earth’s Natural Satellite – The Moon (Chandra)

A small body revolving around a bigger body is called its satellite. The moon is a satellite of the earth. Planets are the satellites of the sun. They are natural satellites. Like planets, satellites also have no light or heat of their own. As the Moon revolve round the earth, and also around the sun its position changes daily in relation with the Sun. As a result, the relative positions of the moon keep on changing everyday. That is how moon appears different each night. The Moon takes almost the same time, 27 days and 7 hours to complete both the movements i.e. rotation and revolution. Thus it always presents the same side towards our earth. The moon has no atmosphere. It has no water.

Our Moon

On a full moon day, the earth lies in between the moon and the sun. So the full face of the moon is visible to us. On a new moon day, the moon lies in between the earth and the sun. So sunlight falls on that face of the moon, which is on the other side of the earth and we cannot see it. As we know the moon completes one revolution around the earth in 27 days and 7 hours. But, in the meanwhile the earth moves ahead in its orbit. Thus from the earth, the moon appears to complete one revolution between one new moon day and the next, in 29 ½ days. Nearly one-half of the Moon is always illuminated by the sunlight. But, on the day following the new moon day, the illuminated part of the moon that is visible from the earth is crescent in shape. The various shapes of the bright part of the moon as seen during a month are called phases of the moon. 

The diameter of the moon is only 1/4th of that of the earth. The Moon is about 384,400 km away from the earth. The light reflected by the moon reaches us in just one and a quarter seconds. Neil Armstrong, Edwin Aldrin and Collins set foot on the Moon for the first time on July 21, 1969. They found that the moon’s surface is dusty and barren. There are many craters of different sizes. It also has a large number of steep and high mountains. These cast shadows on the moon’s surface.

Mars (Mangal)

Mars

Mars is almost half of the size of the earth. The diameter of Mars is slightly more than half of that of the earth but its mass is only 1/10th of that of the earth. It takes 687 days for one orbit around the sun and 1 day for one spin on its axis. The atmosphere of Mars is much thinner than that of the earth. It perhaps consist mainly nitrogen. Traces of oxygen have also been found. It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites named Phobos and Deimos.

The Mars is visible from the earth for most part of the year. However, it is best situated for observation when it is opposite the sun’s position in the sky with respect to the earth. On these days it is closer to the earth as well. Astronomers have noticed certain changes on the surface of Mars that gave them idea that water may also be present on the planet and it may have life in some form. However, so far, no evidence of water or life has been found on it although possibility of their existence is still being investigated.

Jupiter (Brihaspati)

Jupiter

Jupiter is the largest planet of the solar system. It takes 11 years and 11 months for one orbit around the sun and 9 hours, 56 minutes for one spin on its axis. It has 16 satellites. It has faint rings around it. Its most distinguishing feature is the great red spot. It is believed that it is a complex storm in the atmosphere of the planet ranging for a few hundred years. Its mass is more than the combined mass of all other planets. Because of its large mass, Jupiter exerts a strong gravitational pull on other objects which pass by it. A comet passing close to it is likely to experience this pull and stray from its original path. In 1994, the comet, Shoemaker-levy, actually broke up into several pieces when it came too near this planet. The pieces then collided with the planet.

Jupiter’s bright appearance in the sky after Venus and occasionally Mars is due to its thick atmosphere that reflects most of the sunlight falling on it. It is believed that Jupiter mainly consists of hydrogen and helium in gaseous form. Its cloud like outer regions consists of methane in gaseous form while ammonia is present in crystalline form.

Saturn (Shani)

Saturn

Beyond Jupiter is Saturn which appears yellowish in colour. What make it unique in the solar system are its three beautiful rings. It takes 29 years, 5 months for one orbit around the sun and 10 hours, 40 minutes for one spin on its axis. It has 18 satellites. Saturn is the least dense among all the planets. Its density is less than that of water. It is similar in size, mass and composition to Jupiter. It is however cooler than the Jupiter.

Uranus

Uranus

Uranus was the 1st planet to be discovered with the help of a telescope. William Herchel discovered the planet in 1781. Hydrogen and methane have been detected in the atmosphere of Uranus. Like Venus, Uranus also rotates from east to west. Except Venus and Uranus, all other planets rotate in the same direction in which they revolve. The most remarkable feature of Uranus is that it has highly tilted rotational axis. As a result, in its orbital motion it appears to roll on its side. It takes 84 years for one orbit around the sun and 17 hours, 14 minutes for one spin on its axis. It has 17 satellites. 

Neptune

Neptune

Neptune discovered by Sir William Herchel on the basis of the law of gravitation given by Sir Issac Newton which he gave almost 180 years before it. Neptune takes 164 years for one orbit around the sun and 16 hours 7 minutes for one spin on its axis. It has 8 satellites. Uranus and Neptune are quite faint and cannot be seen with the naked eye. That is why only six planets were known in ancient times. These two planets have been discovered only after telescopes came into use in astronomy. 

Asteroids (meaning starlike)

Asteroid Belt

There are numerous tiny bodies found between the orbits of Mars and Jupiter, which also revolve around the Sun. These bodies are called Asteroids. Each asteroid has its own orbit and the orbits of all of them are spread over a large distance forming a band. A few asteroids have elongated orbits going beyond the orbit of Jupiter. The size of asteroids varies from barely a pebble to a few hundred kilometres. The largest asteroid is Cere, has a diameter of about 1,000 km. Scientists believe that asteroids are the pieces of matter that somehow could not get assembled in the form of a planet. The asteroids are small rocky bodies. While making their rounds the asteroids often collide and break into smaller pieces. These smaller pieces or Meteorites sometime fall on to the earth. They crash to the ground and create craters. Meteor crater in Arizona, USA and Lonar Lake in the state of Maharashtra, India are good examples of that.

Meteors and Meteorites

A Meteorite from Mars

Meteors are very small stone-like objects that are revolving around the sun. Their existence becomes known only when some of them occasionally enter the earth’s atmosphere. At that time it has a very high speed. Due to the friction in the atmosphere it heats up. It glows and evaporates quickly. The path of the meteor, therefore, appears as bright streaks of light in the sky. The bright steak lasts for a very short time. These are commonly known as shooting stars, although they are not stars. 

Some meteors are large and so they can reach the earth before they evaporate completely. The body that reaches the earth is called a meteorite. They can be examined in the laboratory. They give us an idea of the composition of the material from which the solar system was formed.

Comets

A Comet

Comets revolve around the sun in highly elliptical orbits. However, their period of revolution round the sun is usually very long. They become visible from the earth only when they come close to the sun. A comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail disappears again when the comet move away from the Sun. The tail of a comet is always directed away from the sun. Many comets are known to appear periodically. One such comet is Halley’s comet, which appears after nearly every 76 years. It was last seen in 1986. The study of comet tails has shown the existence of molecules of carbon, nitrogen and hydrogen. Since these molecules can help to form complex molecules necessary for the origin of life, some scientists have suggested that the seeds of life on the earth were brought by comets from the outer space.

Meteor Showers

Meteor Showers

When the earth crosses the tail of a comet, swarms of meteors are seen. They are known as meteor showers. Some meteor showers occur at regular intervals each year.

The Universe: Still Expanding

The Universe

(Broadly, it is the celestial bodies and the space together comprise the universe)

Universe

We all must know what the term Universe denotes. But do we know what comprises of Universe? All objects that are a part of sky are called celestial bodies and these celestial bodies or heavenly bodies in a vast emptiness called Space. Objects like stars, planets, satellites, asteroids, meteors, comets, dust and gases are called celestial bodies. All these heavenly bodies within the space is termed as Universe.

The Stars

Stars are huge bodies of glowing gases. They give out huge flames and radiate a tremendous amount of heat and light. So they have heat and light of their own. The distances of stars are expressed in terms of light years. One light year is the distance traveled by light in one year at a speed of light which is about 300,000 km/s. Light year is a unit of distance and is equal to 300,000365246060 km, which is equal to 9,460,000,000,000 km or 9.461012 km. The color of a star is determined by its surface temperature. Stars which have low temperature appear red. Those with higher temperature appear white and those with very high temperature are blue. 

Stars

Stars are not scattered regularly in space: they occur in clusters, better described as galaxies. So, all stars including our sun move around some celestial body or a group of bodies with high speeds. However, when viewed from the earth the distance between any two stars does not seem to change in spite of their great speeds. This is because the stars are so far away from us that any changes in distance between them do not become perceptible in a few years. The stars appear to move from east to west. This is because the earth rotates from west to east about an imaginary axis that passes through its center. The earth completes one revolution in 24 hours, so the stars. A star, therefore, travel an angle of about one degree in four minutes. Since the stars rises four minutes earlier everyday, they are one degree higher in the sky than the previous day. In a month, they rise 30 degrees higher. In 6 months they move 180 degrees and will be close to the western horizon. Thus the constellations also move westwards.

However, there is one star, which appears stationary to us known as pole star or Dhruv tara. The North Star or Polaris indicates the north because it is seen directly above the North Pole. It lies close to the axis of rotation of the earth. So it does not appear to move like all other stars. In fact, all the stars appear to revolve around the Pole Star. The pole star is not visible from the southern hemisphere. Some of the northern constellations like Ursa Major may also not be visible from some points in the southern hemisphere. 

Constellations

Constellations

A group of stars forming various shapes is called constellation i.e. star patterns. Laghu saptarshi or Ursa Minor is one such constellation. Vrihat Saptarshi also known as Ursa Major, a group of seven stars is another constellation. It forms a part of the constellation of the Big Bear. It can be seen during summer time in the early part of the night. Orion or Mriga is another well-known constellation that can be seen during winter in the late evenings. The star Sirius, which is the brightest star in the sky, is located close to Orion. Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night. A constellation does not have only 5-10 stars. It has a large number of stars. However we can see only the bright stars in a constellation with our naked eye. All the stars which make up a constellation are not at the same distance. They are just in the same line of sight in the sky.

Galaxies

Milky Way Galaxy

A galaxy is a system of stars that also contains a large number of gas clouds (nebula), some of which are quite huge. It is in these gas clouds that new stars are born. They are bound to one another by gravitational forces. A galaxy has billions of stars very close to one another. There are billions of such galaxies in the Universe. Some galaxies are spiral in shape and others are elliptical. However, there are galaxies which have no regular shape.

The galaxy in which our sun is located is called Milky Way galaxy. It contains several hundred billion stars of all ages, sizes and masses. It appears like a river of light running from one corner of the sky to the other. That is why it is called the Milky Way Galaxy. It is spiral in shape. In India it is known as Akash Ganga. The Milky Way Galaxy is rotating slowly about its centre. Its center is at an inestimable distance from us. The sun too, along with the solar system, rotates about the centre of the galaxy. It takes about 250 million years to complete one revolution. Our sun is just one of the hundred thousand million (1011) stars in the Milky Way Galaxy. The sun is located about 30,000 light years away from the center of the galaxy. 

Big Bang Theory

Big Bang explosion

In the earlier part of the twentieth century, Edwin Hubble (1889-1953), an American astronomer, showed that all other galaxies are rushing away from our galaxy. The velocity with which they are rushing away from us, increases as their distances increase from us. The motion of other galaxies away from us gives rise to the idea of the expanding universe. From this, if we take inference then we can say, If all the galaxies are moving away from one another at this moment, there must have been a time in the past when all the galaxies were together at one place. At that time, the whole universe occupied a small volume. It is believed that a huge explosion took place at that time, and since then, the universe is expanding. It has been estimated that this event occurred about 15 billion years ago. This means, the universe is about 15 billion years old. This event occurred that time and ultimately led to the present movement of galaxies away from one another is explained by the Big Bang theory.  Scientists believe this gigantic explosion formed small atoms which initiated the formation of the Universe. Then the atoms lead to the formation of molecules. Eventually a Nebula was formed.

Nebula

A Nebula

A nebula is a huge cloud floating in the Universe. It is made up of gases and dust. Temperature at the centre of the nebula is very high because the atoms and molecules of the gases attracted by the gravity of each other keep colliding. When the temperature rises over 10,000,000 Centigrade, nuclear chain reactions take place and a new star is born. Thus, a nebula is a birthplace of stars.

Stars too Die

White Dwarf Star

As we read above, stars are formed in giant clouds of dust and gas, and progress through their normal life as balls of gas heated by thermonuclear reactions in their cores. The ages of the stars range from millions to billions of years. A typical star, such as our Sun, radiates small amounts of X-rays continuously and larger bursts of X-rays during a solar flare. The Sun and other stars shine as a result of nuclear reactions deep in their interiors. These reactions change light elements into heavier ones and release energy in the process. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight. When a star has used all hydrogen at its center, it starts fusing helium into carbon. If the mass of the star is only a few times the solar mass, the fusion process does not go beyond this. As the energy production inside the star stops, its core contracts under its own weight. Nuclear reactions outside the core cause the dying star to expand outward in the "red giant" phase before it begins its inevitable collapse. Depending on their mass, they reach the end of their evolution as a white dwarf, neutron star or black hole. 

A Neutron Star

If the star is about the same mass as the Sun and density becomes very high, it will turn into a white dwarf star. The size of a white dwarf star is comparable to that of a planet. The mass of such stars is always less than about 1.44 times the mass of the sun. This result was proved by S. Chandrasekhar. The maximum mass is, therefore, called the Chandrasekhar Limiting Mass. A white dwarf is considered as a dead star because it does not produce its own energy by fusion process. It shines by radiating the heat stored by it during its lifetime.

A Massive Black Hole

If it is somewhat more massive, it may undergo a supernova explosion and depending on their size, ends up as Neutron stars or Black holes. Generally a supernova explosion leave behind a neutron star. But if the collapsing core of the star is very great i.e. at least three times the mass of the Sun then nothing can stop the collapse. The star implodes to form an infinite gravitational warp in space i.e. a black hole (in which the gravitational pull is so high that not even light can escape out of them).

New Cycle

A Brown Dwarf Star

The cycle begins anew as an expanding super shell from one or more supernovas trigger the formation of a new generation of stars. The life cycles of stars follow patterns based mostly on their initial mass. These include

• intermediate-mass stars such as the sun, 
• with half to eight times the mass of the sun, 
• high-mass stars that are more than eight solar masses, and 
• low-mass stars a tenth to half a solar mass in size. 

The greater a star's mass, the shorter its lifespan is. Objects smaller than a tenth of a solar mass do not have enough gravitational pull to ignite nuclear fusion. They might become failed stars and known as brown dwarfs, so they never evolve.