20-11-2012, 06:11 PM
SOLAR SYSTEM - STATISTICS
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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 66½O.
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 0O. On a globe
the meridians are numbered from 0O to 180O 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
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.
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;
© 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.