|
|
 |
| |
| |
|
|
|
In the vastness of the Universe, the
Earth, the Sun and planets are tiny dots. The Sun is a
single star in a Galaxy comprising 100,000 million stars.
The Solar System is centred on the Sun. It consists of a
star called the Sun and all the objects that travel around
it. The Solar System includes : 9 planets (Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto),
along with the numerous satellites that travel around most
of them; planet-like objects called asteroids (hundreds of
asteroids); chunks of iron and stone called meteoroids;
bodies of dust and foreign gases called comets (thousands of
comets); and drifting particles called interplanetary dust
and electrically charged gas called plasma that together
make up the interplanetary medium.
The whole solar system by volume appears to be an empty
void. This vacuum of ‘space’ comprises the interplanetary
medium. The speed of the solar wind is about 400 kilometer
per second in the vicinity of Earths' orbit.
The Solar System originated in a primitive solar nebula–a
rotating disc of gas and dust. It is from this rotating disc
that the planets and the rest of the Solar System evolved.
The Solar System is also tucked away in a corner of the
Milky Way at a distance of about 30,000 to 33,000 light
years from the centre of the galaxy.
The Sun contains 99.85% of all the matter in the Solar
System. The planets which condensed out of the same disk of
material that formed the Sun, contains only 0.135% of the
mass of the Solar System.
Jupiter contains more them twice the matter of all the other
planets combined. Satellites of the planets, comets,
asteroids, meteoroids, and the interplanetary medium
constitute the remaining 0.015%. |
|
|
|
THE PLANETS |
|
The bodies revolving around the sun (at
the same time rotating on their imaginary axis) are called
planets. They have no light of their own but shine by
radiating the fight they receive from the sun. They all
revolve around the sun in elliptical orbits. Until about 200
years ago only six planets were known. Three more planets
were discovered later, the latest being Pluto (discovered in
1930). Nine planets can now be identified.
Mercury
Mercury is the planet nearest to the sun. It rotates on its
own axis in 56.65 earth days. It takes 88 days to complete
one revolution round the sun. Thus it is the fastest planet
in our solar system.
Venus
Also known as the evening star and morning star, is the
brightest object in the sky after the sun and the moon. It
is slightly smaller than the earth and is the planet closest
to the earth. It is also the hottest planet in our solar
system and has a weak magnetic belt.
Mars
Mars is the fourth planet from the sun and is the next
planet after the earth. Being favorably situated, it is
brighter than most of the stars and, is therefore, known as
the Red Planet. It has two small satellites called Phobos
(Fear) and Deimos (Terror).
Jupiter
Jupiter is the largest planet in our solar system. It is
about eleven times larger than the earth. Its volume is one
and half times the volume of all the planets combined
together. The most conspicuous aspect about Jupiter is its
Great Red Spot. It is also known as the giant planet because
of its huge size.
Saturn
Saturn is an outer planet visible to the naked eye. Second
in size to Jupiter, it is the least dense of all the
planets. The most spectacular feature of Saturn is its
system of rings. The ring system is made up of a variety of
separate particles which move independently in circular
orbits. It has 46 satellites. Titan is its biggest.
Uranus
Uranus is the seventh planet from the sun and is not visible
to the naked eye. It was identified as a planet in 1781 by
William Herchel. It has completed only two revolutions round
the sun since its discovery, and takes about 84 terrestrial
years to circle round the sun. It has 27 satellites.
Neptune
Neptune is not visible to the naked eye but can be seen
through a small telescope as a greenish star. It is eighth
in position from the sun. This planet was discovered by J.G.
Galle of Berlin in 1846. Till 1930, it was believed to be
the farthest planet from the sun and the outermost in our
solar system. It has eight satellites, and Triton and Nereid
are the most conspicuous of them.
Pluto
Pluto is the youngest planet to be discovered in our solar
system. It was discovered photographically by C.W. Tombaugh
(USA) in 1930. It is the smallest planet in our solar
system; slightly smaller than Mercury and visible only
through a telescope. The duration of its revolution round
the sun is the longest and it is, therefore, the slowest
planet in our solar system. |
|
|
|
SATELLITES |
|
Satellite are bodies which
revolve around the planets. All planets have one or more
satellites, except Mercury and Venus. The moon is the
earth's natural satellite. There are approximately 62
satellites in our solar system.
In August 1989, the US Space probes Voyager-1 and Voyager-2
revealed six new satellites around Neptune which was earlier
believed to have only two satellites.
The Moon
The moon is the earth's natural satellite and is its nearest
neighbour in space. It revolves around the earth while
rotating on its own axis. Only 59% of its surface is
directly visible from the earth. Of all satellites in the
solar system, the moon is the largest in proportion to its
primary body, that is, the earth. All other satellites have
sizes below 1/8 the size of the mother planet. The moon is
about 1/4 the size of its mother planet, the earth. It takes
about 1.3 seconds for moonlight to reach the earth, whereas
sunlight takes about 8 minutes and 16.6 seconds to reach the
earth.
The moon takes 27 days 7 hours 43 minutes and 11.47 seconds
to complete one revolution of the earth. It rotates on its
axis in exactly the same time. Hence, we see only one side
of the moon. |
|
|
|
THE EARTH |
|
Modern theories on the formation of the
Earth and other planets are of course based on the
Copernican theory.
The age of the Earth was a matter of speculation till very
recent times. It was only about 200 years ago, that
scientific enquiries were started by geologists. According
to their deductions, based on the study of rocks, the age of
the Earth is 4.6 billion years.
Our knowledge of the internal structure of the Earth is
derived from studies of earthquakes. The shock waves sent
out by an earthquake indicate the physical nature of the
regions through which they pass. These studies show that the
centre of the Earth is a solid core–the Inner Core. The
density of this core is about 13 g to the cubic centimeter.
The Inner Core is about 1,370 km thick and is surrounded by
an Outer Core of around 2,080 km. The Outer Core appears to
be molten.
The Outer Core is surrounded by the Mantle which has a
thickness of around 2,900 km. The Mantle is topped by the
crust of the Earth, which varies widely in thickness–from 12
to 60 km. At the centre or the Inner Core, that is at a
depth of some 6,370 km, temperature goes upto some 4,000°C
and pressure reaches nearly 4 million at mospheres.
The mantle is important in many ways. It accounts for nearly
half the radius of the Earth (2,900 km), 83% of its volume
and 67% of its mass. The dynamic processes which determine
the movements of the crust plates are powered by the mantle.
Starting at an average depth of from 45 to 56 km below the
top surface of the Earth, the mantle continues to a depth of
2,900 km where it joins the outer core. The mantle is a
shell of red hot rock and separates the Earth's metallic and
partly melted core (both the inner and the outer cores) from
the cooler rocks of the Earth's crust.
It is composed of sllicate minerals rich in magnesium and
Iron. The density of the mantle increases with depth from
about 3.5 gram per cubic centimetre to around 5.5 gram, near
the outer core.
The outer surface of the Earth is divided into 4 spheres:
Lithosphere means the entire top crust of the Earth and
includes not only the land surface but also the ocean floor.
Hydrosphere is the water surface which includes the oceans,
lakes and rivers.
Atmosphere is the blanket of air that
envelops the Earth. It covers both the land surface and the
water surface.
Biosphere is this sphere of life which spreads over all the
three other spheres.
Earth's Movements
The earth has two types of movements, viz. rotation or
daily motion and revolution or annual motion.
The earth spins on its own imaginary axis from west to east
once in 24 h (in precisely 23 h 56 min and 40.91 s). It is
also called diurnal or daily motion. The axis is an
imaginary line which runs form north to south and passes
through the centre of the earth. It always remains inclined
at an angle of 66½° to the plane of the earth's orbit.
Effects of Rotation:
(i) Occurrence of day and night.
(ii) The position of a place on earth can be fixed.
(iii) Change in the direction of wind and ocean currents. |
| |
|
ECLIPSES |
|
When the light of the sun or the moon is
obscured by another body the sun or moon is said to be in
eclipse.
Lunar Eclipse: The moon is said to be in eclipse when the
earth comes between the moon and the sun, and this is called
Lunar eclipse. The shadow cast by the earth on the moon is
called an eclipse.
Lunar eclipse occurs only on a full moon
day. However, it does not occur on every full moon day
because the moon is not in the same position in relation to
the earth and the sun on every full moon day.
Solar Eclipse: The sun is said- to be in eclipse when the
moon comes between the sun and the earth. This is called
Solar eclipse. There is either a partial or total
obstruction of the sun's light when viewed from the earth. A
solar eclipse occurs on a new moon day when the moon is in
line with the sun. However, due to the inclination of the
moon's orbit, a solar eclipse does not occur on every new
moon day. |
| |
|
ATMOSPHERE |
|
The atmosphere is a gaseous envelope
that surrounds a celestial body. The terrestrial atmosphere,
by nature of its composition, control of temperature and
shielding effect against solar radiation, makes life
possible on earth. It covers both the land and the water
surface. It is bound to the earth by the gravitational pull
of the earth. The composition of the atmosphere changes as
we go higher from the earth's surface. Upto about a height
of 50 km from the earth, the atmosphere is composed of:
Nitrogen 78.09%
Oxygen 20.95%
Argon 0.93%
Minor gases (Carbon dioxide, hydrogen, neon, helium,
methane, xenon, krypton, etc.) 0.03%
After a height of 50 km above the earth's surface the
atmosphere is made up of atomic oxygen (O2), ozone (O3),
helium and hydrogen.
Atmospheric Layers
These are the layers of air that lie above the earth's
surface. The atmosphere of the earth is arranged into layers
as accrued below, viz.
Troposphere: The troposphere is the layer nearest to the
earth's surface and extends from sea-level to a height of
about 15 km. This region is the densest of all the
atmospheric layers and contains water vapour, moisture and
dust. In this region the temperature decreases as the height
increases from the earth.
Tropopause: Tropopause is the layer which separates the
troposphere (lowest layer) from the stratosphere (upper
layer).
Stratosphere: This is the region of uniform temperature
extending from an altitude of about 15km above the earth to
a height of about 50 kill. It is free from water vapour,
clouds and dust.
Mesosphere: This is a very cold region and lies above the
ozone-rich layer of the stratosphere. It extends from 50 or
80 km above the earth's surface.
Menopause: The Menopause separates the mesosphere from the
next layer called the ionosphere.
Ionosphere: The ionosphere lies immediately above the
mesosphere and extends from 60 to 400 km above the earth's
surface. This layer contains ionised (or electrically
charged) air which protects the earth from the falling
meteorites (shooting stars) as most of them burn out in this
region. It also protects the earth from the harmful
radiations of the sun. The ionosphere consists of ‘D’, ‘E’
and ‘F’ layers and includes the thermosphere and exosphere.
Thermosphere: This is the middle layer of the ionosphere. It
is the region of the atmosphere where the temperature is
above 100°C.
Exosphere: The exosphere is the uppermost region of the
ionosphere and makes up the outer limits of the atmosphere.
Here the gravity of the earth is exceedingly weak. The
magnetic belt of the earth which is known as Magnetosphere,
extends to about 64,000 km above the earth's surface. The
exosphere is now considered as part of the magnetosphere.
The outer boundary of the magnetosphere or the final
boundary between the earth and outer space is known its
magnetopause.
The land surface of the earth is made up of immense land
masses divided into seven continents and a great number of
islands. Together, they cover about one quarter of the
earth's surface.
It is believed that originally there was only one land mass
called Pangaea. This large land mass split into a northern
mass Laurasia and a southern one called Gondwana Land. From
these two land masses, the continents gradually drifted to
where they are now located and the process is still
continuing.
|
| |
|
|
|
|
