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Old Friday, November 18, 2011
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Roshan wadhwani Roshan wadhwani is offline
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Earth:


Features Of Earth:
i) The earth is the fifth largest planet of the solar system, where life exists.
ii) Its equatorial diameter is 7,927 miles.
iii) Its polar diameter is 7900 miles.
iv) The earth has average density of 5 ½ g/cc.
v) the rotation period of the earth is 23 hours, 56 minutes and 4 seconds.
vi) the revolution period of the earth is 365 days.
vii) The total mass of the earth is 6*1021 tons.
viii) The earth is composed of shells or layers, which are, the crust, mantle and core.
ix) It is surrounded by a blanket of gases, which is known as the atmosphere, mainly composed of Nitrogen and Oxygen.
x) The surface of the earth is rich in oxygen, silicon, iron, magnesium, sodium etc.

Structure Of The Earth:
The earth comprises of the following layers:
a) The crust
b) Moho discontinuity
c) The mantle
d) The Core

a) The Crust:
i) It is the outermost layer of the earth.
ii) It extends to about 25 miles (40kms) and comprises of rocks.
iii) The crust is divided into the oceanic and the continental crust.
iv) Out of these the oceanic crust is 808 meter thick and consists of sedimentary mud.
v) The continental crust is divided into upper continental and lower continental crust.

b) Moho discontinuity:
i) The sharp boundary between the crust and mantle is called Moho Discontinuity.

c) The Mantle:
i) The layer of the earth lying below the crust and above the core is known as the mantle.
ii) It is almost 2900 kms (1800 miles) thick and comprises about 80% of the volume of the earth.
iii) The chemical composition of the entire mantle is fairly homogenous.
iv) However, temperature and pressure increases with depth.
v) The behaviour of the earthquake waves as they travel through the mantle further tells us that it consists of several layers and they are:
a) Lithosphere
b) Asthenosphere
c) Mesosphere

d) The Core:
i) The innermost part of the earth is known as the core.
ii) It extends from the base of the mantle to the centre of the earth.
vi) This portion consists of melted iron and nickel that is why it is known as Nife.
vii) The density of this molten mass of the core is 345 pounds per cubic feet.

Types Of Movements Of Earth:
• There are two types of motions of the earth.
• One is around its own axis which is called Rotation. One rotation completes in 23 hours, 56 minutes and 4 seconds.
• The other motion of earth is round the sun, and is called revolution of the earth.
• One revolution completes in 365 days.
• Northern end of the earth’s axis is called the North Pole and the Southern end is called South Pole.
• An imaginary line drawn round the earth midway between the poles is called equator.
• The path which the earth takes round the sun is called earth’s orbit.

Effects Of These Movements:
• Rotation of earth round its axis cause days and nights.
• That portion of earth, which is within the circle of illumination caused by the rays of sun, has its day.
• The other side earth, which is away from sun, remains dark and therefore has its night.

Longer And Shorter Days:
• The circular areas near the North and South Pole of the Earth are called Arctic and Antarctic circles, respectively.
• These are situated at 66 ½0 North and 66 ½0 South of the equator, respectively and form limits of polar region.
• On June, the earth is in position A. north pole is inclined towards the sun and south pole is away from it.
• Obviously any place in Northern Hemisphere will have longer days because it remains in light for more than half the time of earth’s rotation.
• Places on equator remains in light for half the time making days and nights equal in this region.
• The southern hemisphere remains in light for less time than half the rotation of earth so the days are shorter here.
• Positions of days and nights in the northern and southern hemisphere are reversed on 22nd December, when the earth completes its half revolution around the sun, so days are longer in southern hemisphere than those in the north.

Change Of Season:
• The second type of motion of earth is round the sun and is called revolution of the earth.
• This is the cause of change of seasons.
• On June 21, the earth is in position A, when the north pole is inclined towards sun while the south pole is away from it.
• As the sun rays fall vertically and for a longer period in the northern hemisphere, it is summer here but in the southern hemisphere it is winter.
• On December 22, when the earth completes half revolution, the conditions become reversed; it is summer in the southern hemisphere and winter in the northern hemisphere.
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