Introduction to Physical Geography - Physical Geography, UPSC, IAS. UPSC Notes | EduRev

Geography (Prelims) by Valor Academy

UPSC : Introduction to Physical Geography - Physical Geography, UPSC, IAS. UPSC Notes | EduRev

The document Introduction to Physical Geography - Physical Geography, UPSC, IAS. UPSC Notes | EduRev is a part of the UPSC Course Geography (Prelims) by Valor Academy.
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Introduction to Physical Geography



While watching the night sky, you may notice various patterns formed by different groups of stars. These are called constellations. Ursa Major or Big Bear is one such constellation. One of the most easily recognizable constellations is the small bear or Saptarishi (Sapta-seven, rishi-sages). It is a group of seven stars that forms a part of the large Ursa Major Constellation.

In ancient times, people used to determine directions during the night with the help of stars. The North Star indicates the north direction. It is also called the Pole Star. It always remains in the same position in the sky. We can locate the position of the Pole Star with the Pole Star.

Some celestial bodies do not have their own heat and light. They are lit by the light of the stars. Such bodies are called planets. The word ‘planet’ comes from the Greek word “Planetai” which means ‘wonderers’.

The earth on which we live is a planet. It gets all its heat and light from the sun, which is our nearest star.

The moon that we see in the sky is a satellite. It is a companion of our earth and moves round it. Like our earth, there are eight other planets that get heat and light from the sun. Some of them have their moons too.


Introduction to Physical Geography - Physical Geography, UPSC, IAS. UPSC Notes | EduRev

The Sun

The sun is in the centre of the solar system. It is huge and made up of extremely hot gases. It provides the pulling force that binds the solar system. The sun is the ultimate source of heat and light for the solar system. But that tremendous heat is not felt so much by us because despite being our nearest star, it is far away from us. The sun is about 150 million km away from the earth.


There are nine planets in our solar system. In order of their distance from the sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

An easy way to memorize the name of the planets in order of their distance from the sun is: MY VERY EFFICIENT MOTHER JUST SHOWED US NINE PLANETS.

All the nine planets of the solar system move around the sun in fixed paths. These paths are elongated. They are called orbits. Mercury is nearest to the sun. It takes only about 88 days to complete one round along its orbit. Pluto is supposed to be farthest till now. So it takes about 248 years to complete one round. Venus is considered as ‘Earth’s-twin’ because its size and shape are very much similar to that of the earth.

A new planet 2003 UB has been discovered recently in our solar system. It is bigger than Pluto and farthest from the Sun. But other details about this planet are not yet available.

Sun  -











(Distance from the sun in million Kms)











Interesting Facts:

Saturn and Uranus have rings around them. These are belts of small debris. These rings may be seen from the earth with the help of powerful telescopes.

Jupiter is the largest of all planets.

All the nine planets revolve around the sun anti clockwise in elliptical paths known as Orbits.

Except Venus and Uranus, all other planets rotate (on their own axes) in the same direction in which they revolve.

Time to complete one revolution-

88 days                               Mercury

255 days                           Venus

365days, 6 hours          Earth (with speed100,000 km/hr).

11 years                               Jupiter

164 years                           Neptune

248 years                           Pluto

On size Earth ranks 5th, in size and shape the earth is almost identical to Venus. Only Mercury and Venus have no satellite.

The Earth

The earth is the third nearest planet to the Sun. In size, it is the fifth largest planet. It is slightly flattened at the poles. That is why, its shape is described as a Geoid. Geoid means an earth-like shape.

Conditions favourable to support life are probably found only on the earth. The earth is neither too hot nor too cold. It has water and air, which are very essential for our survival. The air has life-supporting gases like oxygen. Because of these reasons, the earth is a unique planet in the solar system. From the outer space, the earth appears blue because its two-thirds surface is covered by water. It is, therefore, called a blue planet.


Our earth has only one satellite, that is, the moon. Its diametre is only one-quarter that of the earth. It appears so big because it is nearer to our planet than other celestial bodies. The moon does not have conditions favourable for life. It has neither water nor air. It has mountains,• 

  • Its diameter is only 1/4th of that of the earth.
  • Distance from earth – 385000km.
  • The moon revolves around the earth in about 27 days and 8 hours. It takes exactly the same time for it to complete one rotation about its axis. As such we always see only one side of the moon while the other side always remains away from us.
  • The moon is very hot during the day and very cold during the night.

Light travels at the speed of about 300,000 km per second. Yet, even with this speed, the light of the sun takes about eight minutes to reach the earth.


A swarm of small bodies in between the orbits of Mars and Jupiter, which revolve around the sun. The largest asteroid is the Ceres. Scientists are of the view that asteroids are parts of a planet which exploded many years back.


The small pieces of rocks which move around the sun are called meteoroids. Sometimes these meteoroids come near the earth and tend to drop upon it. During this process due to friction with the air they get heated up and burn. It causes a flash of light. Sometimes, a meteor without being completely burnt falls on the earth and creates a hollow.


STAR:-A celestial body, having its own heat and light.

PLANET:-A celestial body which revolves around the sun and receives heat and light from it.

SATELLITE:-A celestial body which revolves around a planet just as planet revolves around the sun. (So far 49 Satellite have been discovered).



Origin of Earth

Early Theories

A large number of hypotheses were put forth by different philosophers and scientists regarding the origin of the earth. One of the earlier and popular arguments was by German philosopher Immanuel Kant. Mathematician Laplace revised it in 1796. It is known as Nebular Hypothesis. The hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating. Later in 1900, Chamberlain and Moulton considered that a wandering star approached the sun. As a result, a cigar-shaped extension of material was separated from the solar surface. As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and it slowly condensed into planets. Sir James Jeans and later Sir Harold Jeffrey supported this argument. At a later date, the arguments considered of a companion to the sun to have been coexisting. These arguments are called binary theories. In 1950, Otto Schmidt in Russia and Carl Weizascar in Germany somewhat revised the ‘nebular hypothesis’, though differing in details. They considered that the sun was surrounded by solar nebula containing mostly the hydrogen and helium along with what may be termed as dust. The friction and collision of particles led to formation of a disk-shaped cloud and the planets were formed through the process of accretion.

Modern Theories

However, scientists in later period took up the problems of origin of universe rather than that of just the earth or the planets. The most popular argument regarding the origin of the universe is the Big Bang Theory. It is also called expanding universe hypothesis. Edwin Hubble, in 1920,

provided evidence that the universe is expanding. As time passes, galaxies move further and further apart. The distance between the galaxies is also found to be increasing and thereby, the universe is considered to be expanding. Scientists believe that though the space between the galaxies is increasing, observations do not support the expansion of galaxies.

The Big Bang Theory considers the following stages in the development of the universe.


In the beginning, all matter forming the universe existed in one place in the form of a


“tiny ball” (singular atom) with an unimaginably small volume, infinite temperature and


infinite density.


At the Big Bang the “tiny ball” exploded violently. This led to a huge expansion. It is now


generally accepted that the event of big bang took place 13.7 billion years before the


present. The expansion continues even to the present day. As it grew, some energy was


converted into matter. There was particularly rapid expansion within fractions of a second


after the bang. Thereafter, the expansion has slowed down. Within first three minutes


from the Big Bang event, the first atom began to form.

Within 300,000 years from the Big Bang, temperature dropped to 4,500 K and gave rise to atomic matter. The universe became transparent. The expansion of universe means increase in space between the galaxies. An alternative to this was Hoyle’s concept of steady state. It considered the universe to be roughly the same at any point of time. However, with greater evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe


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