World Geography

▸ Geography is the science that studies the lands, the features, the inhabitants and the phenomena of the Earth.
▸ The word geography adopted in the 2nd century BC by the Greek Scholar Eratosthenes.

Some Contributors to Geography

▸ Eratosthenes was the first person to calculate the circumference of the Earth and also calculate the tilt of the Earth’s axis.
▸ Anaximander created the first map of world.
▸ Ainville made the first map of India.
▸ Ptolemy first presented India on the world map.
▸ Anthropogeography was written by Friedrich Ratzel.

Branches of Geography

Geography is divided into two main branches i.e. physical geography and human geography.
Human Geography
Human geography is a branch of the geography that studies the world, its people, communities and cultures with an emphasis on relations of land across space and place. The fields of human geography are as follow:
▸ Cultural geography
▸ Development geography
▸ Economic geography
▸ Health geography
▸ Historical geography
▸ Political geography
▸ Population geography
▸ Settlement geography
Physical Geography
Physical geography deals with the physical environment and the various process that bring about changes in the physical environment on the Earth surface. The fields of physical geography are as follow:
Geomorphology It is the scientific study of landforms and processes that shape them.
Hydrology It is the study of the movement, distribution and quality of water on Earth, including the hydrological cycle, water resources and environmental watershed sustainability.
Climatology It is the study of climate, scientifically defined as weather conditions averaged over a period of time.
Pedology It is the study of soils in their natural environment.
Glaciology It is the study of glaciers and ice sheets.
Biogeography It is the study of relationships of organisms with their environment.


▸ The study of universe is known as Cosmology.
▸ The universe is commonly defined as the totality of everything that exists including all physical matter and energy, the planets, stars, galaxies and the contents of intergalactic space.
▸ The universe comprises of billions of galaxies. The galaxies are made up of millions of stars held together by the force of gravity and these stars account for most of the masses of the galaxy. Andromeda is our nearest galaxy.
▸ Our own galaxy is called the Milky Way (or the Akash Ganga) and it contains about 300 billion stars and one of these is our Sun. Planets and other objects revolve around the Sun and make up the solar system with the Sun at the centre.
▸ In AD 140, Ptolemy propounded the theory that the Earth was the centre of the universe and the Sun and the other heavenly bodies revolved around it.
▸ In 1543, Copernicus said that the Sun is the centre of universe and not the Earth.
Kepler supported Copernicus but said that the Sun is the centre of solar system and not the universe.
▸ In 1924, Edwin Hubble first demonstrated existence of galaxies beyond Milky Way.
▸ Structurally, the galaxies are found in three forms (i) Spiral have a central nucleus with great spiral arms. Milky Way and Andromeda are the examples. (ii) Elliptical without spiral arms. (iii) Irregular with no shape.

Evolution of Universe

The three main theories put forward to explain the origin and evolution of the universe are (i) Big Bang Theory (Proposed by Georges Lemaitre) Big Bang was an explosion that occurred 13.8 billion years ago, leading to the formation of galaxies of stars and other heavenly bodies. (ii) Steady State Theory Bondi, Gold and Fred Hoyle developed this theory and states that although the universe is expanding, it nevertheless does not change its appearance over time, it has no beginning and no end. (iii) The Pulsating Theory According to this theory, the universe is supposed to be expanding and contracting alternately i.e. pulsating. At present, the universe is expanding. NASA has launched the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anistropy Probe (WMAP) missions to study the radiation present in the universe.


▸ Stars are heavenly bodies made up of hot burning gases, thus shining by their own light.
▸ If the star is bigger than the Sun but not more than twice as big, it will turn into a Neutron star or Pulsar. They are formed due to novae or super novae explosion.
▸ A star’s colour indicates the temperature of its surface. Blue colour denotes maximum temperature, then comes yellow, then red etc.
Evolutionary Stages of a Star
1. Proto Star It is the stage, where the heliumcore become increasingly heavy, accompanied with expanding outer layers. 2. Red Giant This stage results into the swelling and reddening of the outer regions of the star. Such stars of gigantic dimension is called Red Star. 3. White Dwarf If the mass of the star is relatively small like that of our Sun, the gases that reach the outer layer are expelled. As these expelled gases cool and contract, the star becomes a White Dwarf.

Stars: Quick Digest

▸ Brightest star outside solar system is Sirius, also called as Dog Star.
▸ Closest star to our solar system is Proxima Centauri (4.2 light years away). Followed by Alpha Centauri (4.3 light years away) and Barnard’s Star (5.9 light years away).
Concept of BlackHole and Chandrashekhar Limit
▸ There is an upper limit to the mass of stars, above which two things are possible 1. Explosion of the star to form neutron stars called Pulsar. 2. Collapse and compaction of the star to form Black Holes.
▸ Therefore, the black holes are formed due to collapse and compaction under gravity, at the end of the life cycle.
▸ A renowned Indian Physicist Chandrashekhar had predicted an upper limit to the mass of stars, which is called as Chandrashekhar limit. It is 1.44 times the mass of the Sun.
▸ Some of the units used for the calculation of these distances are as follows:

Units of Distance

Light year It is the distance that light can travel in 1 year. Our solar system is less than 1 light day across.
Astronomical unit It is the average distance between the Sun and the Earth. 1 light year = (almost) 60000 Aus
Parsec It is the distance from the Earth to a star that has parallax of 1 arc second. The actual length is about 3.262 light years.

Solar System

▸ The solar system comprises the Sun, 8 planets, their Moon and other non-stellar objects, which are believed to have been developed from the condensation of gases and other lesser bodies.
▸ The Sun is at the centre of the solar system and all the planets revolve around it in elliptical orbit. It is the nearest star to the Earth.
▸ The size of solar system has been estimated to be about 105 AU.

Components of the Solar System

Our Solar System consists of
▸ the Sun, Eight planets (excluding Pluto) and their respective satellites.
▸ interstellar debris such as asteroids, meteoroids, comets. The electrically charged gases, called Plasma.
▸ interplanetary dust particles.
▸ the components of solar system other than planets dwarf planets and satellites are called as Small Solar System Bodies (SSSB).

Origin of the Solar System

Various theories were given to explain the Origin of the Solar System.

Gaseous Hypothesis Kant
Nebular Hypothesis Laplace
Planetesimal Hypothesis Chamberline and Moulton
Tidal Hypothesis Sir James Jeans and Harold Jeffreys
Binary Star Hypothesis HN Russell
Supernova Hypothesis F Hoyle
Interstellar Dust Hypothesis Otto Schmidt
Electromagnetic Hypothesis H Alfven

The Sun

▸ The Sun accounts for more than 99 per cent of the mass of the Solar System and due to this, the Sun exerts immense gravitational pull to keep the planets rotating around it in definite elliptical orbit.
▸ The Sun is the major source of energy of the solar system. The energy is provided by the nuclear fusion reaction, that converts hydrogen into helium in the core of the Sun.
▸ The glowing surface of the Sun that we see is called as Photosphere. Above the photosphere is the red coloured Chromospheres and beyond it is the magnificent corona, which is visible during eclipses.
▸ Hydrogen and helium are the main gases present in the Sun.
▸ It has a surface temperature of about 6000°C.
▸ It takes 224 million years to complete one circle (revolution) around the galactic circle called cosmic year.
▸ The Sun is 1300000 times bigger than the Earth in terms of volume.
▸ Superimposed on Sun’s white light are hundred of dark lines called Fraunhofer lines. Each line indicates some elements present in the Solar System.

Specifics of the Sun

Average distance from the Earth 149598900 km
Diametre 1391980 km
Temperature of the core 15000000°C
Rotation speed 25.38 days (with respect to equator); 33 days (with respect to poles)
Time taken by sunlight to reach the Earth 8 min and 16.6 sec

Concepts Associated with the Sun
Solar Winds The Sun is continuously emitting streams of photon in all directions either as spiral streams called Solar Wind or bouts of incandescent material called Solar Flares. Solar flares being hot ionised gases pose danger to satellite communication.
Aurora The constituent particles of the solar wind are trapped by the Earth’s magnetic field and enter the Earth’s upper atmosphere as Aurora. It is described as Aurora Borealis in the Northern hemisphere and Aurora Australis in Southern hemisphere.
Plages and Sunspots The surface of the Sun is continuously changing. Bright spots are called Plages and dark spots are called Sunspots. The Sunspots are cold and dark regions on the Sun’s surface with a periodicity of 11 years. These spots greatly influence the global climate.


▸ Planets are opaque bodies, which continuously revolve around and are lighted by the Sun. There are eight planets in the solar system.
▸ The sequence of planets according to their size (in descending order) is Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars and Mercury.
▸ The sequence of planets according to their distance from the Sun is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Classification of Planets
1. Inner Planets Include Mercury, Venus, Earth and Mars. 2. Outer Planets Include Jupiter, Saturn, Uranus and Neptune.

They are called as Terrestrial or Rock planets. They are nearer to the Sun. They are called as Jovian or Gaseous planets. They are far away from the Sun.
They have very few natural satellites (or moons) or no satellites. They have a large number of natural satellites (or moons).
They have a core of molten metals. They have ring system around the Sun.
They move faster and have a shorter period of revolution. They move rather slowly and have a longer period of revolution.

Specifics of the Planets

Hottest Planet Venus
Densest Planet Earth
Fastest Rotation in Solar System Jupiter
Morning Star Venus
Nearest Planet to Earth Venus
Nearest Planet to Sun Mercury
Red Planet Mars
Biggest Satellite Ganymede
Blue Planet Earth
Green Planet Uranus
Brightest Planet Venus
Slowest Revolution in Solar System Neptune
Slowest Rotation in Solar System Venus
Smallest Planet Mercury
Smallest Satellite Phobos
Earth’s Twin Venus
Only Satellite with an Atmosphere Like Earth Titan

A Comparative Study of the Planets of the Solar System

Planets Special Characteristics Rotation and Revolution Time Important Physical Properties Satellite Systems
Mercury Smallest and the innermost planet. It has no atmosphere. It has a cratered surface, much like the Moon. Rotation : 58.65 days Revolution: 88 days (Fastest Revolution in the Solar System). It has the maximum diurnal range of temperature. No satellite
Venus Also called as the veiled planet. known as (Evening and Morning star) as it is seen in the East in morning and in the West in the evening. It is the brightest object in solar system because of almost 70% albedo. It contains 90 to 95%CO2. The night and day temperature almost the same. It has the slowest rotational speed. It has almost equal rotation and revolution. Rotation (Clockwise) 257 days and Revolution: 224.7 days. Rotates from East to West unlike the other planets. It is the hottest planet. No satellite
Earth The Earth is neither too hot nor too cold.It is called as the Blue Planet due to the presence of water. Rotation :24 hours; Revolution : 365 days and 6 hours. It is the densest of all and is unique for the presence of higher forms of life. Moon is the only natural satellite.
Mars Called as Red Planet. It has a thin atmosphere comprising of nitrogen and argon. Rotation 24.6 hour. (almost equal to Earth) Revolution: 687 days. It is marked by dormant volcanoes. Nix Olympia is the highest mountain, which is three times higher than the Mount Everest. Two satellites : Phobos and Deimos .
Jupiter It is the largest planet in the solar system with a mass 2.5 times greater than the combined mass of all the remaining planets, satellites and asteroids put together. In contains hydrogen, helium, methane and ammonia. A great red spot is detected on it. Fastest rotational velocity (9.8 hrs) It is too massive to solidify as a planet but not massive enough to develop nuclear fusion and become a star. It gives off more energy than it receives from the Sun, because of the heat inside. It has 67 satellites. Some of the prominent satellites are: Europa, Callisto and Ganymede. These are called as Galileon Moons.
Uranus It is unique as its axis of rotation is inclined at 98° to its orbital plane. Unlike the others, which spin on their axis, Uranus actually rolls, apparently from North to South. Surrounded by a system of 9 faint rings. It has 27 satellites .The prominent are Miranda Ariel etc.
Neptune It is a penultimate planet, has a dynamic atmosphere, which contains an the Earth sized blemish called the Great Dark Spot that is reminiscent of Jupiter’s Great Red Spot . Rotation 15.7 days and Revolution 165 years. It has 5 faint rings .It appears as Greenish Star. It has 14 satellites. The prominent are Triton and Nereid.

Pluto is not a Planet Now

▸ Pluto was discovered by Clyde Tombaugh in 1930.
▸ The redefinition of planet by the International Astronomical Union (IAU) on 24th August, 2006 states that, in the solar system, a planet is a celestial body that
▸ it is in orbit around the Sun.
▸ it has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape.
▸ it has cleared the neighbourhood around its orbit.
▸ A non-satellite body fulfilling the first two rule is classified as a Dwarf planet. So, Pluto is considered as Dwarf planet.

The Moon

The study of Moon is called Selenology.

Specifics of the Moon

Rotation Period day, 7 h, 43 min and 11.47s
Atmosphere Absent
Part of Moon not visible from Earth 41%
Maximum distance from Earth (Apogee) km
Minimum distance from Earth (Perigee) km
Circumference km
Diameter km
Mass (with respect to Earth) : 8.1
Ratio of Gravitational Pull of Moon and Earth : 6
Highest Mountain ft (Leibnitz Mountain)
Time Taken by Moonlight to Reach Earth 1.3 s
Rotation speed kmph
Speed of Revolution around Earth kmph
Revolution Period around Earth days, 7 h, 43 min and 11.47s

▸ Moon is also known as the fossil planet.
▸ The Moon is the only natural satellite of the Earth.
▸ The Moon has no atmosphere, no twilight and no sound.
▸ The size of the Moon is one-fourth (1/4th) the size of the Earth.
▸ Gravitational pull of the Moon is one-sixth (1/6th) that of the Earth.
▸ Silicon, iron, magnesium etc elements are found mainly on the Moon’s surface.

Interstellar Debirs :

Asteroids, Meteoroids, Comets

Various Aspects Asteroids or Planetoids Comets Meteoroids/Meteors
Constitutents and Genesis Composed of rocks, dust and metal. They cannot retain their atmosphere due to small size. Comets may originate in a huge cloud called the Oort cloud that is thought to surround the solar system. It is composed of frozen gases and dust. Meteoroids are small fragments of rocks and metal. Under the Earth’s gravitational field, they become white hot through friction as they fall through the atmosphere and are seen as the Meteors or Shooting stars.
Shape and Size No definite shape, rather same as of Planetoids or small planet. Comets have a head and tail, where the tail always points away from the Sun because of the solar wind and the radiation pressure. No definite shape.
Orbit They orbit the Sun in the asteroid belt, which lies between the orbits of Mars and Jupiter. They have an extremely eccentric orbit but with definite periodicity. Meteoroids travel through space. Meteors are scattered in the interplanetary space of the solar system.

Composition of Whole Earth

1. Iron—35%
2. Oxygen—30%
3. Silicon—15%
4. Magnesium—13%
5. Nickel—2.4%
6. Sulphur—1.9%
7. Calcium—1.1%
8. Aluminium—1.4%
9. Others—0.5%

The Earth

▸ Earth is the third planet from the Sun and the densest and the fifth-largest of the eight planets in the solar system.It is also the largest of the solar system’s four terrestrial planets. Earth is also called as Blue Planet.
▸ The age of the Earth is estimated at about 4.6 billion years. The history of the Earth is studied in terms of geological eras, periods and epochs. The whole history is divided into three Eras-Palaeozoic, Mesozoic and Cenozoic.

Geological History of the Earth

Cenozoic Era Quaternary Period Holocene Epoch 10000 Modern man
Pleistocene Epoch 2 million Homo Sapiens
Tertiary Period Pliocene Epoch 5 million Early human ancestor
Miocene Epoch 24 million Flowering plants and trees
Oligocene Epoch 38 million Early horses, cats, dogs, camel
Eocene Epoch 55 million Rabbits, hare
Palaeocene Epoch 63 million Small mammals : Rats, mice
Mesozoic Era Cretaceous Period 138 million Extinction of dinosaurs
Jurassic Period 205 million Age of dinosaurs
Triassic Period 240 million Frogs and turtles
Palaeozoic Era Permian Period 290 million Reptile dominate, Replace amphibians
Lower Carboniferous Period 330 million 1st Reptiles
Upper Carboniferous Period 360 million Fish
Devonian Period 410 million Amphibians
Silurian Period 435 million Corals
Ordovician Period 500 million Graptolites
Cambrian Period 570 million Trilobites
Pre-Cambrian Time 4.5 billion Bacteria

Specifics of the Earth

Mass 5.9 × 1024 kg
Volume 1083 × 1012 Km3
Mean Density 5.513 g/cm3
Shape An oblate spheroid or a geoid
Radius of Earth 6400 km
Total surface area 509700000 sq km
Land area (29%) 148400000 sq km
Water area (71%) 361300000 sq km
Rotation time 23 hours 56 minute and 4.09 seconds
Revolution time 365 days, 5 hours, 48 minutes and 45.51 seconds
Orbit speed about the Sun 29.8 km/second
Mean surface temperature 14°C
Highest temperature recorded 58°C Al-Aziziyah
Mean Distance from the Sun 149598500 km
Inclination of polar axis from orbital plane 23° 26 min and 59 s
Deepest ocean point 11034 m, Mariana Trench (Pacific Oceans)
Time coordinate of Earth Longitude
Temperature coordinate of Earth Latitudes

The Earth’s Movement

▸ The Earth moves in space in two distinct ways:Rotation and Revolution. (i) It rotates on its own axis from West to East once in every 24 hours. It causes day and night. (ii) It revolves around the Sun in an orbit once in every 365¼ days. It causes the seasons and the year.

Rotation of Earth

▸ Spins on its imaginary axis from West to East in 23 hours, 56 minutes and 40.91 seconds.
▸ The rotational speed at equator is maximum (1667 km/hr) and then decreases towards the poles, where it is zero.
▸ The days and the nights are equal at the equator. The rotation of the Earth has the following implications such as:
▸ Causation of day and night.
▸ Change in the direction of winds and ocean currents.
▸ Rise and fall of tides everyday.
▸ A difference of one hour between the two meridians which are 15° apart.

Revolution of Earth

▸ It is the Earth’s motion in elliptical orbit around the Sun. It takes 365 days, 5 hours, 48 minutes and 45.51 seconds. It leads to one extra day in every fourth year. The revolution of the Earth results in
▸ changes of season.
▸ variation of the length of the days and nights at different times of the year. Shifting of the wind belts. Below figure shows the revolution of the Earth and its effects on seasons and the variations of lengths of day and night.

Major Difference Between Rotation and Revolution of Earth

Rotation Revolution
Turning around the Earth on its own axis. Movement of the Earth around the Sun.
Earth takes 24 hours to complete one rotation. The Earth takes 365.25 days to complete revolution.
Rotation causes day and night. Revolution along with inclination of the Earth on its axis causes change in seasons.

Earth’s Position wrt Moon

Apogee: The period of the farthest distance between the Moon and the Earth is called Apogee. It is about 407000 km.
Perigee: The period of the nearest distance between the Moon and the Earth is called Perigee. It is about 364000 km.

Earth’s Position wrt Sun

Perihelion: The period of the nearest distance between the Earth and the Sun is Perihelion. It happens on 3rd January and the distance is 147 million km.
Aphelion: The period of the farthest distance between the Earth and the Sun is Aphelion. It happens on 4th July and the distance is 152 million km.

Tilt of the Earth’s Axis

▸ The axis of the Earth is inclined to the plane of ecliptic (the plane, in which the Earth orbits round the Sun) at an angle of 66½° giving rise to different seasons and varying lengths of day and night.
▸ If the axis were perpendicular to this plane, all parts of the globe would have equal days and nights at all times of the year.
Sun 22nd December 21st June Equator NP SP Summer in N hemisphere Winter in S hemisphereNP SP Autumn in S hemisphere NP SP Winter in N hemisphere Equator NP SP 23rd September 21st March Summer in S hemisphere
Revolution of the Earth
▸ The Earth is tilted about 23.5° from a line perpendicular to ecliptic plane.


▸ These are the days, when days and nights are equal. Under this situation, the Sun is vertically overhead at the equator. It happens on two days of the year i.e. 21st March and 23rd September.
▸ 21st March: Vernal Equinox.
▸ 23rd September: Autumnal Equinox.

Solstice Summer Solstice

▸ After the March equinox, the Sun appears to move Northward and is vertically overhead at the Tropic of Cancer on 21st June. This is known as Summer Solstice.
▸ On 21st June, the Northern hemisphere will have its longest day and shortest night. The Southern hemisphere will have shortest day and longest night.
Winter Solstice
▸ On 22nd December, the Sun is overhead at the Tropic of Capricorn. This is the winter solstice, when the Southern hemisphere will have its longest day and shortest night. A summary of daylight hours in the Northern and Southern hemisphere is as follows:

Winter Solstice

Longest day in the Northern 21st June
Shortest day in the Northern hemisphere 22nd December
Equal day and night in the Northern hemisphere 21st March and 23rd September
Longest day in the Southern hemisphere 22nd December
Shortest day in the Southern hemisphere 21st June
Equal day and night in the Southern hemisphere 21st March and 23rd September


▸ They are the periods into which the year can be divided as a result of the climatic conditions, mainly due to the changes in duration and intensity of solar radiation.
▸ There are four seasons such as:

Spring Summer Autumn Winter
When the Sun is directly overhead the equator. (21st March) When the Sun is directly overhead the Tropic of Cancer -the North temperate zone experiences summer. (21st June) When the Sun returns to the equator and the North temperate zone experiences the season of autumn. (23rd September) The Sun is at the Tropic of Capricorn and the North temperate zone experiences winter. (22nd December)


▸ Latitude is the angular distance of a point on the Earth surface from the centre of the Earth, measured in degree. These lines are called parallels of latitude and on the globe they are circles. The circumference of the circles decreases from equator to pole and at the pole it converses to a point.
▸ The distance between any two parallels of latitude is always equal. One degree latitude = Approx 111 km. The most important lines of latitudes are Equator (0°), the Tropic of Cancer (23½°N), the Tropic of Capricorn (23½°S), the Arctic Circle (66½°N) and the Antarctic Circle (66½°S).


▸ Longitude is the angular distance of a point on the Earth surface along the equator, East or West from the Prime meridian. On the globe, they form semi-circles from pole to pole passing through the equator.

▸ Prime meridian is the semi-circle from pole to pole, from which all the other meridians radiate Eastwards and Westwards up to 180°. In 1884, it has been decided that the zero meridian is one that passes through the Royal Astronomical Observatory at Greenwich near London. 180° meridian (International Date Line) is exactly opposite to the prime meridian. Such points are called antipodal points.
Universal Time (Standard time) and Time Zones
▸ To avoid confusion about having many local times within one country, a particular meridian is chosen for the whole country, whose time is known as standard time.
▸ The Indian Government has accepted the meridian of 82.5 degree East for standard time, which is 5 hrs 30 mins ahead of the Greenwich Mean Time.
▸ The Earth is divided in 24 longitudinal zones, each being 15 degree or 1 hour apart in time (360 degree = 24 hours, 360/24 = 15 degree in 1 hour) or 1 degree in 4 minute are called Standard Time Zones.
▸ Larger countries such as USA, Russia and Canada, which have greater East-West stretch have to adopt several time zones for practical purposes.
▸ Russia has asmany as 11 time zones.
▸ Both USA and Canada have five time zones, viz, the Atlantic, Eastern, Central, Mountain and Pacific time zones.
▸ A simple memory aid for time is East-Gain-Add (EGA) and West- Lose-Subtract (WLS).

International Date Line

▸ It is the 180 degree meridian running over the Pacific Ocean, deviating at Aleutian Island, Fiji, Samoa and Gilbert Island.
▸ This is the International Date Line, where the date changes by exactly one day, when it is crossed.
▸ Samoa and Tokelau shifted his position back to West of the date line on 30th December, 2011.

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