Page 126 - SST Class 06

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Rotation and Revolution

The earth is in constant motion, and these MOTIONS OF THE EARTH
movements are some of the earth’s most impor- The earth basically has two types of motions. They
tant characteristics. The earth rotates on its axis are rotation and revolution. These two motions are
everyday and orbits the sun every year. We don’t responsible for different kinds of phenomena. Let
feel the motion of the earth because we move us look into them in further details.
right along with it. However, we can see the Rotation
results of its motion in our everyday lives.
Each day, the earth rotates around its axis, which
INCLINATION is an imaginary line that passes through the

The earth is tilted on its axis. This tilting of the earth’s north and south poles. It takes the earth
earth on its axis is known as the inclination of the just under 24 hours to make one complete
Earth’s axis. rotation. The earth rotates from the west to the
It is because of the tilted axis of the earth that east. As seen from Polaris, the North Star, the
days and nights are of varying lengths in different earth rotates in a counterclockwise direction. The
parts of the earth. Four main seasons are also speed of the earth’s rotation is about 1,040 miles
caused due to the inclination of the earth. If the per hour as measured on the earth’s surface at
axis of the earth was vertical, then days and the equator. The following phenomena are
nights would be of equal lengths at all places, caused as a result of this motion :
and the earth would experience only one season Day and Night : The part of the earth that
throughout the year. faces the sun is lighted. Being spherical in shape,
the earth completes one motion in 24 hours and
The vertical line from the
Earth’s orbital plane moves from west to east. Therefore, no place on
Axis
earth receives sunlight constantly. At any time
North Pole
23°30’N
during the course of the motion, the part oppo-
site the lighted part remains in darkness. The
Arctic C
irc le
circle which divides the earth into lit-half is
Tropic of C
a n c e r
known as circle of illumination. This circle of
illumination moves gradually from the east
Equator
towards the west (because the earth rotates from
The Plane of
r T o p ic o the Earth’s orbit west to east). As the area in the zone of darkness
A n a t rc f Capricorn comes nearer to the zone of illumination, there is
23°30’S diffused light. This early period of the day is
tic Circle
known as Dawn. As the day advances, the sun is
South Pole
seen overhead. This period is called midday or
The Inclination of noon. The lighted portion then gradually begins
the Earth on Its Axis

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