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Tuesday, January 3, 2012

THE WORLD



The seasons are due to the change of the Earth’s
position in the course of its revolution about the Sun,
and to the inclination of its axis. The Equator is an
imaginary line drawn round the Earth midway between
the Poles. There are two other lines, namely, Tropic of
Cancer (23½ 'N) and the Tropic of Capricon (23½' S).
The word tropic means, ‘turning place’. The inclination
of the Earth’s axis together with its revolution round the
Sun is the cause of the varying length of day and night in
different parts of the world. On March 21 (Vernal Equinox)
and September 23 (Autumnal Equinox) the Sun is overhead
at the Equator. On these dates, except at the Poles,
(a) days and nights are equal all over the world; and (b)
the Sun rises exactly due east and set exactly due west at
all places on the Earth’s surface. At the Equator itself
days and nights are equal throughout the year. Between
March 21 and September 23, when the North Pole is tilted
towards the Sun, the days are longer than the nights
throughout the Northern Hemisphere and there is
continuous daylight at the North Pole. Similar conditions
are experienced in the Southern Hemisphere and the
South Pole between September 25 and March 21.
Latitude and Longitude:
Latitude is distance, measured in degrees, north or
south of the Equator. Longitude is distance, measured in
degrees, east or west of any fixed meridian. The meridian
passing through Greenwich is numbered 0'. On a globe
the meridians are numbered from 0' to 180'E (East) or W
(West). At the equator the degrees are 69 to 70 miles
apart (25000÷360). Since earth completes one rotation on
its axis in 24 hours, 360 meridians pass under the Sun in
that time. Therefore, 1 degree passes under the Sun every
4 minutes.
International Date Line:
If we travelled westward to a place X on longitude
180o W, the time there would be 12 hours behind
Greenwich time (180 x 4 minutes = 720 minutes = 12 hours).
If we journeyed eastward to a place Y on longitude 180o
E, the time there would be 12 hours ahead of Greenwich
time. Thus X and Y both on 180o have the same time but
The Earth and the Solar System :
The Earth is a member of the Solar System. It is one
of nine major planets revolving round the Sun. Of these,
Mercury and Venus are nearer, and Mars, Jupiter, Saturn,
Uranus, Neptune and Pluto are farther away from the
Sun than the Earth.
The planets radiate no light of their own, but shine
with that reflected from the Sun. The Sun has a diameter
of 864,000 miles (1,390,000 Kms) and it is 93,000,000 miles
(150,000,000 Kms) from the Earth. The Sun is a star. The
stars, unlike the planets, are self-luminous bodies. The
other stars appear small because they are so far away;
the nearest star, Proxima Centauri, is 200,000 times more
distant from us than the Sun.
The Moon is a dead planet. It is about 240,000 miles
(386,000 Kms) from the Earth. The Moon revolves round
the Earth taking approximately 29 days to complete one
round. The phases of the Moon are the result of its
position in relation to the Earth and the Sun.
The Moon’s orbit is ecliptical and inclined at an
angle of 5o to the plane of the Earth’s orbit. This explains
why we do not have a total eclipse of the Sun every time
there is a new moon.
The Earth:
The Earth is a sphere but it is not a perfect sphere.
It is slightly flattened at the poles and bulges at the
equator. The circumference of the earth is approximately
25,000 miles (40,000 Kms).
It rotates on its axis once in every 24 hours, spinning
from west to east. Besides spinning on its axis, it also
moves round the Sun, called the revolution.
Its orbit round the Sun is oval or ecliptical. The
time taken to complete one revolution is approximately
365¼ days or one year. For convenience, one year is
taken as 365 days and the shortfall of ¼ day each
year is made good in the Leap Year which consists of
366 days. The Earth’s axis inclined to the plane of its
orbit at an angle of 66½'.


differ in date by a day (12 hours + 12 hours = 24 hours).
To overcome the confusion that would otherwise arise,
the International Date Line has been established. It runs
along 180o E or W. Westward-bound vessels crossing
the Date Line drop a day from the calendar, while those
going eastward add a day by giving the same date to two
consecutive days. Instead of changing the time exactly
according to change in degrees at the rate of 4 minutes
per degree, certain time zones have been established. All
places in the same area or time zone or time belt, use
what is called Standard Time. Thus we have the
Greenwich Mean Time (G.M.T) and the Indian Standard
Time (I.S.T). There are five time-belts in Canada and four
in United States.
The Lithosphere:
The mass of the Earth is generally divided into three
layers, namely, Crust, Mantle and Core. The Lithosphere
is the name given to the outer Crust which is not more
than 10 miles thick. It is made up of a great variety of
rocks, soils, etc.
Rocks:
1. Sedimentary Rocks:These rocks are made up of
deposits laid down on the floor of river beds, lakes
and seas.
Examples:Sand and sandstone, clay, lime stone,
chalk and carbonaceous rocks, such as lignite, coal
and anthracite.
2. Igneous Rocks:These are primary rocks which are
formed by cooling and solidification of molten lava.
When such rocks are poured out on the surface
they are known as Volcanic rocks, e.g. basalt. When
the molten material solidifies at considerable depth,
plutonic rocks are formed, e.g. granite.
3. Metamorphic Rocks:These rocks are formed as a
result of alteration by extreme heat and or pressure
of igneous or sedimentary rocks. Example, slate,
gneiss, schist etc.
Soil:
The upper layers of rocks weather to form the soil.
There are three distinct layers of soil. The uppermost
layer forms the top soil. The second layer is called the
subsoil. The third layer is made up of decomposing and
much-broken rock, known as mantle- rock. The type of
soil depends on a number of factors, namely, climatic
conditions, the nature of the parent rock, relief, vegetation
and the period over which it has been worked by man.
Soils may by broadly classified as (a) Forest, (b) Grassland
and (c) Desert types.
Mountains:
In past geological ages disturbances in the Earth’s
interior have caused crumpling and cracking of the crust.
This has resulted in great upholds forming Fold-
Mountains which are mainly made up of folded strata of
sedimentary rocks, e.g. the Alps, the Rockies, the Andes,
and the Himalayas. The mountain structures worn down
by prolonged denudation are known as Residual
Mountains, e.g. Highlands of Scotland and Scandinavia.
Denudation:
The process known as denudation or the wearing
away of the land is continually going on. The chief causes
of such erosion are (a) changes in temperature; (b) frost;
(c) winds; (d) water, including rivers; (e) ice; and (f) the
action of the sea. Steps to combat soil erosion include (i)
terracing; (ii) contour ploughing; (iii) strip cropping (iv)
planting shelter belts of trees; and (v) plugging the gullies
by building small dams etc.
The Atmosphere:
The air is composed mainly of nitrogen (78%) and
oxygen (21%) with small proportions of carbon dioxide,
water vapour and rarer gases like argon and neon.
Atmosphere is 200 miles thick, but nine-tenths of the air
composing it is found within 12 miles, and half within 3½
miles of the earth’s surface. We are concerned mainly
with the lower layer of troposphere. The upper layers in
the ascending order are Stratosphere, Mesosphere and
Ionosphere. Troposphere extends to a distance of about
ten kilometres. Stratosphere is a region extending from
an altitude of about 11 Km to 50 Km above the earth. The
upper part of stratosphere has plenty of ozone which
protects us from the fatal effects of Sun’s ultraviolet
radiation. Mesosphere is the next layer extending from 50
to 80 Kms above the earth. It is a very cold region.
Ionosphere extends from about 60 Kms upwards. It
includes Thermosphere and Exosphere which marks the
outer limits of the earth’s atmosphere.
Wind is air in motion:
The chief cause of wind is difference in atmosphere
pressure. One of the main reasons for differences in
pressure is unequal heating of the air. From the high
pressure belts the air flows outwards to the regions of
low pressure. Owing to the rotation of the earth, the winds
do not blow due north and south, but are deflected. In
this deflection they obey Ferrel’s Law which states, “Any
moving body on the earth surface including a current of
air, tends to be deflected, the deflection being to the right
in the northern hemisphere and to the left in southern
hemisphere.” Land and seabreezed are local winds caused
by the unequal heating of land and water. During the day
the land becomes very much hotter than the sea, with the
result that there is marked low pressure over the land.
Thus the air over the sea flows rapidly loses heat, but the
sea remains warm for a longer time. Thus at night, heavy
cool air blows from the land to take the place of warm air
rising over the sea. The monsoon or seasonal winds may
be regarded as land and sea breezes on a large scale, in
which the time-frame is a year instead of a day. This
phenomenon is to be found in south-east Asia, but is
especially marked in the subcontinent of India. A cyclone
is a portion of the atmosphere in which the pressure is
lowest in the centre. The winds blow inwards in anticlockwise
direction in the Northern Hemisphere. In the
Southern Hemisphere cyclonic winds blow in a clockwise
direction in accordance with Ferrel’s Law. An anticyclone
is a portion of the atmosphere in which the pressure is
highest in the centre. The winds blow outwards in a
clockwise direction in the Northern Hemisphere and in
an anti-clockwise direction in the Southern Hemisphere.
The Oceans:
It is estimated that 72% of the surface of the globe
is covered with water. The Pacific, which is the greatest
of all oceans, covers a third of the earth’s surface, its
total area being greater than that of all the dry land.
Atlantic is slightly less than half the size of the Pacific,
yet so many great rivers flow into it that it receives half
the drainage of the world. The other oceans are Indian,
Mediterranean, Antarctic and Arctic. The average depth
of the ocean is 12,500 feet, compared with the average
height of the land which is about 2,500 feet. The greatest
known depth is that of the Marianas Trench in the Pacific,
where a depth of 35,800 feet has been recorded.
Tides:
Tides are caused by the gravitational attraction -
the ‘pull’ of the moon on the earth, sometimes assisted
by and sometimes hindered by the Sun.At new and full
moon, when the Sun, Moon and Earth are practically in a
straight line the attractive force of the Sun increases that
of the Moon. Such high tides are called spring tides.
Weather and Climates:
Weather may be defined as the condition of the
atmosphere at any place at a particular time. The average
weather conditions determine the climate. Humidity,
temperature, elevation, distance from the sea, ocean
currents, atmosphere pressure and prevailing winds
combine to affect the climate of a region.
Humidity:
Humidity, the term used to express the dampness
of the atmosphere, is due to the pressure of water vapour.
Evaporation is the mean by which water is drawn off as
invisible water vapour from oceans, lakes, rivers, etc.,
when the air contains as much water vapour as it can
hold is said to be saturated. When saturated air is cooled,
condensation takes place and some of the water which
form the clouds grow larger, they precipitate i.e., fall to
the earth as rain. The proportion of water vapour in the
air, compared with the maximum it can hold at the same
temperature, is known as Relative Humidity. Dew is
caused by the condensation of water vapour on the cold
ground during the night. The temperature at which such
condensation takes place is called the Dew Point. When
condensation takes place near the surface (and not on
the ground) the result is usually mist or fog. When
condensation takes place at some distance from the
ground, clouds are formed. When water vapour is
condensed at a temperature below freezing point it forms
snow. There are various kinds of clouds. The layer like
clouds often seen on the horizon at sunrise and sunset
are called stratus clouds. The light wispy clouds formed
high in the sky are called cirrus clouds. The heaped up
clouds, looking rather like masses of cotton wool are called
cumulus clouds. The black rain clouds are known as
nimbus.
Temperature:
Of all climatic factors, temperature is the most
important. It affects man’s food, crops, dress and the
type of dwelling he builds. Temperature decreases 1oF
for every 300 feet above the sea-level. This decrease
is largely due to the fact that the rarified air, found
in elevated regions, absorbs less heat than the denser
air at lower levels.


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