Page 127 - SST Class 06

North Pole North Pole four and has 366 days. For example,
6 months daylight 6 months daylight
N N
66½° the year 2016 was a leap year and the
23½° Short Long Short 66½°
Long next leap year will be 2020.
Night Day
0° Day Night 23½°
Sun
Equator
23½°
0° While moving round the sun, the earth
Long Short
Long Short
Night Day
66½° 23½°
Day Night
follows a fixed path known as orbit.
66½°
South Pole S South Pole S
6 months darkness 6 months darkness The earth’s orbit is elliptical in shape.
The earth moves through space at a
to experience fading light. When the sun is seen
speed of about 1,00,000 kilometers per hour.
over the horizon, there is again a period of
Because of the earth’s constant inclination during
twilight. This is known as dusk. When it is com-
its revolution around the sun, each of the two
pletely dark, it is known as night. At midnight,
spheres of the earth – the Northern Hemisphere
the 'Circle of Illumination' has moved exactly on
and the Southern Hemisphere – remain inclined
the opposite side.
towards the sun during one half of the year. Four
Because the earth rotates from west to east, the
points must be taken into consideration for
eastern parts experience the sunrise first. This is
understanding the phenomena caused by the
why Japan, the easternmost country, is known as
revolution of the earth. These are the inclination
the land of the rising sun. It is a sense of time, in
of the earth, a fixed orbit, the elliptical shape of
reality, that makes us experience the phenomena
the orbit and the rotation of the earth on its
of sunrise and sunset. If you turn the globe in
inclined axis.
front of a lighted candle, you will notice that the
The rotation of the earth, while being inclined on
angle of inclination rises gradually from sunrise,
its axis, along with the planet’s revolution around
reaches a maximum at noon and in afternoon it
the sun has resulted certain environmental
again begins to decrease. In the morning and in
problems. For example, the Tarbella Dam in
the afternoon, sun light is more intense because
Pakistan, the World’s largest earth and rock filled
rays of the sun at this time fall in a slanting
dam, face seasonal flooding and reduced water
position. At noon, rays of the sun are almost
flow on this account.
vertically overhead. They cover a shorter dis-
tance. Therefore, at noon sun light is very intense
and it is hotter than in the morning and the
Spillways
evening.
Revolution Tarbella Dam
The movement of the earth around the sun,
simultaneously, with its rotation is called revolu-
tion. The earth moves around the sun at a speed
of about 100,000 km per hour. The earth com-
pletes one revolution in 365 days and 6 hours
are adjusted into one complete day every four
years. This extra day is added to the month of
February, giving it 29 days instead of the normal
28 days. Hence, every four years the month of
February has 29 days. This year is known as the
Tarbella Dam, Pakistan
leap year. A leap year is generally divisible by

Social Science-6 127