NCERT Universe

1. Chapter 6_1: Geo6_1_The Earth in the Solar System
The sun, the moon and all those objects shining in the night sky are called celestial bodies.
 These celestial bodies are called stars. The sun is a star.
 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 recognisable 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 help of the Saptarishi.
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’.
Jupiter, 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.

2. THE SOLAR SYSTEM
The sun, nine planets, satellites and some other celestial bodies known as asteroids and meteoroids
form the solar system
 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
MY VERY EFFICIENT MOTHER JUST SHOWED US NINE PLANETS.
Aryabhata was a famous astronomer of ancient India.
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 UB313 has been discovered recently in our solar system. It is bigger than
Pluto and farthest from the Sun.
 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.
 Our earth has only one satellite, that is, the moon. It is about 3, 84, 400 km away from us.
 Neil Armstrong was the first man to step on the surface of the moon on 29 July 1969.
 Human-made Satellite is an artificial body. Some of the Indian satellites in space are INSAT,
IRS, EDUSAT, etc.
Scientists are of the view that asteroids are parts of a planet which exploded many years back. These
bodies are called asteroids. They are found between the orbits of Mars and Jupiter. The largest
asteroid is the Ceres.
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.
 Like a white glowing path across the sky on a clear starry night, it is a cluster of millions of
stars. This band is the Milky Way galaxy. Our solar system is a part of this galaxy.
 In ancient India, it was imagined to be a river of light flowing in the sky. Thus, it was named
Akash Ganga. A galaxy is a huge system of billions of stars, and clouds of dust and gases.
There are millions of such galaxies that make the Universe.

3. Chapter11_2: Geo11_2_The Origin and Evolution 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.
 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.
 Sir James Jeans and later Sir Harold Jeffrey supported this argument. 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.
Modern Theories
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.
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 “ti ny
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.

4. The Star Formation
 A galaxy contains a large number of stars. Galaxies spread over vast distances that are
measured in thousands of light-years.
 The diameters of individual galaxies range from 80,000-150,000 light years. A galaxy starts to
form by accumulation of hydrogen gas in the form of a very large cloud called nebula.
 Eventually, growing nebula develops localised clumps of gas.
 The formation of stars is believed to have taken place some 5-6 billion years ago
 A light year is a measure of distance and not of time. Light travels at a speed of 300,000
km/second. Considering this, the distances the light will travel in one year is taken to be one
light year. This equals to 9.461×10 12km. The mean distance between the sun and the earth
is 149,598,000 km. In terms of light years, it is 8.311 minutes of a year.
Formation of Planets
 Our Solar system consists of nine planets. The tenth planet 2003 UB 313 has also been
recently sighted.
 The nebula, from which our Solar system is supposed to have been formed, started its
collapse and core formation some time 5-5.6 billion years ago and the planets were formed
about 4.6 billion years ago.
 Our solar system consists of the sun (the star), 9 planets, 63 moons, millions of smaller
bodies like asteroids and comets and huge quantity of dust-grains and gases.
 Out of the nine planets, mercury, Venus, earth and mars are called as the inner planets as
they lay between the sun and the belt of asteroids the other five planets are called the outer
planets.
 Alternatively, the first four are called Terrestrial, meaning earth-like as they are made up of
rock and metals, and have relatively high densities.
 The rest five are called Jovian or Gas Giant planets. Jovian means jupiter-like.
The difference between terrestrial and Jovian planets can be attributed to the following conditions:
o The terrestrial planets were formed in the close vicinity of the parent star where it
was too warm for gases to condense to solid particles. Jovian planets were formed
at quite a distant location.
o The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust
from the terrestrial planets. The solar winds were not all that intense to cause
similar removal of gases from the Jovian planets.
o The terrestrial planets are smaller and their lower gravity could not hold the
escaping gases
In 1838, Sir George Darwin suggested that initially, the earth and the moon formed a single rapidly
rotating body.
EVOLUTION OF THE EARTH
The planet earth initially was a barren, rocky and hot object with a thin atmosphere of hydroge n and
helium

5. Development of Lithosphere
 The earth was mostly in a volatile state during its primordial stage. Due to gradual increase
in density the temperature inside has increased.
 With passage of time it cooled further and solidified and condensed into a smaller size.
 This later led to the development of the outer surface in the form of a crust
 During the formation of the moon, due to the giant impact, the earth was further heated up.
Evolution of Atmosphere and Hydrosphere
 The present composition of earth’s atmosphere is chiefly contributed by nitrogen and
oxygen.
 There are three stages in the evolution of the present atmosphere.
o The first stage is marked by the loss of primordial atmosphere.
o In the second stage, the hot interior of the earth contributed to the evolution of the
atmosphere.
o Finally, the composition of the atmosphere was modified by the living world through
the process of photosynthesis
 The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a
result of the solar winds.
 The early atmosphere largely contained water vapor, nitrogen, carbon dioxide, methane,
ammonia and very little of free oxygen. The process through which the gases were
outpoured from the interior is called degassing.

6. Chapter 8_17: Sci8_17_Stars And The solar system
 The objects which do not twinkle are planets.
 The moon is the brightest object in the night sky. The stars, the planets, the moon and many
other objects in the sky are called celestial objects
 The various shapes of the bright part of the moon as seen during a month are called phases
of the moon
 The time period between one full moon to the next full moon is slightly longer than 29 days.
 The moon completes one rotation on its axis as it completes one revolution around the
Earth.
 On July 21, 1969 (Indian time) the American astronaut Neil Armstrong landed on the moon
for the first time followed by Edwin Aldrin
The Sun is nearly 150,000,000 kilometers (150 million km) away from the Earth. The next nearest
star is Alpha Centauri. It is at a distance of about 40,000,000,000,000 km from the Earth.
There is actually a star, the pole star, which is situated in the direction of the earth’s axis. It does not
appear to move
The stars forming a group that has a recognisable shape is called a constellation
 Constellations were devised by ancient people to be able to recognise stars in the sky.
 The shapes of constellations resemble objects familiar to those people.
 One of the most famous constellations which you can see during summer time in the early
part of the night is Ursa Major
 It is also known as the Big Dipper, the Great Bear or the Saptarishi.
 In fact, all the stars appear to revolve around the Pole Star.
Note that 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.
Orion is another well-known constellation that can be seen during winter in the late evenings.
 It also has seven or eight bright stars. Orion is also called the Hunter.

7.  The three middle stars represent the belt of the hunter. The four bright stars appear to be
arranged in the form of a quadrilateral
 The star Sirius, which is the brightest star in the sky, is located close to Orion. To locate
Sirius, imagine a straight line passing through the three middle stars of Orion.
Cassiopeia is another prominent constellation in the northern sky. It is visible during winter i n the
early part of the night. It looks like a distorted letter W or M
The Solar System
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 does
not fit this definition. It is no longer a planet of the solar system.
 The simplest method of identifying planets from stars is that stars twinkle, whereas planets
do not.
 A planet has a definite path in which it revolves around the Sun. This path is called an orbit.
 The time taken by a planet to complete one revolution is called its period of revolution. The
period of revolution increases as the distance of the planet increases from the sun
 Besides revolving around the Sun, a planet also rotates on its own axis like a top. The time
taken by a planet to complete one rotation is called its period of rotation
 Some planets are known to have moons/satellites revolving round them. Any celestial body
revolving around another celestial body is called its satellite.
Mercury (Budh)
The planet mercury is nearest to the Sun. It is the smallest planet of our solar system.
Venus (Shukra)
Venus is earths nearest planetary neighbor. It is the brightest planet in the night sky.
The Earth
The Earth is the only planet in the solar system on which life is known to exist.
 From space, the Earth appears blue green due to the reflection of light from water and
landmass on its surface. The axis of rotation of the Earth is not perpendicular to the plane of
its orbit. The tilt is responsible for the change of seasons on the Earth. The Earth has only
one moon.
 About 500 years ago, a Polish priest and astronomer, named Nicolaus Copernicus (1473 –
1543), stated that the Sun was at the centre of the solar system and the planets revolved

8. around it. It was a revolutionary idea. Even Copernicus hesitated to publish his work. His
work was published in the year of his death in 1543.
 In 1609, Galileo designed his own telescope. Through this telescope Galileo observed
moons of Jupiter, phases of Venus and rings of Saturn. He argued that all the planets must
orbit the Sun and not the Earth.
Only on two days in a year, March 21 and September 23, the Sun rises exactly in the east. On all
other days, the Sun raises either north of east or south of east.
 From summer solstice (around June 21) the point of sunrise gradually shifts towards the
south. The Sun is then said to be in dakshinayan (moving south). It keeps moving towards
south till winter solstice (around December 22).
 Thereafter, the point of sunrise changes direction and starts moving towards north. The Sun
is then said to be in uttarayan (moving north)
The plane of the equator is called the equatorial plane. The plane in which the Earth revolves round
the Sun is called the orbital plane of the Earth.
These two planes are inclined to each other at an angle of 23.5º. This means that the axis of the
Earth is inclined to its orbital plane at an angle of 66.5º
Mars (Mangal)
The next planet, the first outside the orbit of the Earth is Mars. It appears slightly reddish and,
therefore, it is also called the red planet. Mars has two small natural satellites.
Jupiter (Brihaspati)
Jupiter is the largest planet of the solar system. It is so large that about 1300 earths can be placed
inside this giant planet. However, the mass of Jupiter is about 318 times that of our Earth. It rotates
very rapidly on its axis.
Saturn (Shani)
Beyond Jupiter is Saturn which appears yellowish in colour. What makes it unique in the solar system
is its beautiful rings
One interesting thing about Saturn is that it is the least dense among all the planets. Its density is
less than that of water
Uranus and Neptune
 These are the outermost planets of the solar system.
 Like Venus, Uranus also rotates from east to west. The most remarkable feature of Uranus is
that it has highly tilted rotational axis
The first four planets, Mercury, Venus, Earth and Mars are much nearer The Sun than the other four
planets. They are called the inner planets. The inner planets have very few moons.

9. The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are much farther
off than the inner planets. They are called the outer planets. They have a ring system around them.
The outer planets have large number of moons
Asteroids
There is a large gap in between the orbits of Mars and Jupiter. This gap is occupied by a large
number of small objects that revolve around the Sun. These are called asteroids.
Comets
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.
 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
 Superstitions about the Comets Some people think that comets are messengers of disasters,
such as wars, epidemics and floods. But these are all myths and superstitions. Appearance of
a comet is a natural phenomenon. We have no reason to be afraid of it
India has built and launched several artificial satellites. Aryabhata was the first Indian satellite. Some
other Indian satellites are INSAT, IRS, Kalpana-1, EDUSAT, etc.