MOTIONS OF THE EARTH
The earth, like other planets has two motions. It spins continuously on its
axis once in about twenty four hours, which is called ROTATION. The earth’s axis
is tilted at an angle of 231/20 from a perpendicular to the orbital plane. In other
words, the earth’s axis makes an angle of 661/20 with the plane of the earth’s orbit.
This tilting of the earth’s axis is referred to as the INCLINATION OF THE
EARTH’S AXIS.
The northern and southern points on the earth through which this axis
passes, are known as the North Pole and the South Pole respectively.
While rotating on its axis, the earth also moves round the sun at a speed of
about 100, 000 kilometre per hour. It completes one circle in about 365 days and 6
hours. This annual motion of the earth is called REVOLUTION.
ROTATION
The earth receives its light and heat from the sun. When the earth rotates on
its axis, one half of it, facing the sun, is lit. The other half remains in darkness.
Thus each part of the earth’s surface comes into the sunlight for a certain period
and then turns away from it in 24 hours. The lighted part of the earth has day, and
the other half, in darkness, has night. Imagine what will happen if the earth stops
rotating on its axis! In fact, it is because of the rotation of the earth that day and
night follows each other in regular succession in all parts of the earth.
The sun appears to move in the sky from east to west. Actually it is not the
sun but the earth which moves. Try to remember what happens when you look
outside a running train. You feel that trees, poles, house and other objects are
moving away in opposite direction. In fact it is the train which is moving and not
the trees. Similarly, it is not the sun but the earth which moves from west to east.
REVOLUTION
While rotating on its axis, the earth also moves round the sun. The earth
completes one revolution in about 365 days and 6 hours. For the sake of our
convenience, we consider a year as consisting of only 365 days, and ignore 6
hours. Thus in four years the difference becomes as much as 24 hours or one day.
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Hence, every fourth year we add one day to the year. The year to which one day is
added has 366 days and it called a LEAP YEAR. The extra day is added to the
month of February, making the number of days in this month 29 instead of 28.
On its path around the sun, the earth’s axis always remains inclined to one
side. Because of this constant inclination in one direction, the Northern
Hemisphere remains inclined towards the sun or faces the sun during one half of
the year. Therefore, a larger part of this hemisphere receives sunlight. Every point
in this hemisphere takes a longer time to go out of the sunlight, with the result that
the days are longer. In contrast, the Southern Hemisphere is away from the sun. It,
therefore, has shorter days and longer nights.
If you study in the below given figure carefully, you will understand this. In
this figure you see a circle running parallel to the equator in the Northern
Hemisphere. Measure the length of that part of the circle which is in light and also
that part which is in darkness. You will find that even if the earth rotates on its
axis, more than half of this circle will always be in light and less than half will
always be in darkness. Now imagine a point on this circle steadily moving as the
earth rotates. This point will complete one circle in 24 hours but the journey of this
point through sunlight will be no longer than its journey through darkness, which
means longer days and shorter nights. Similarly, all points or places in the
Northern Hemisphere would have longer days and shorter nights when this
hemisphere is inclined towards the sun.
Inclination of the earth’s axis and unequal length of days and nights
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In the opposite case, when the Southern Hemisphere is inclined towards the
sun, all places in the Southern Hemisphere will have longer days and shorter
nights.
Draw a small circle around the North Pole as shown in the figure. You will
find that this circle will always be in sunlight which means that it will have a 24
hour day with no night at all. The exact opposite would be the case for a small
circle around the South Pole. This circle will have a 24 hour night with no day at
all. When the Southern Hemisphere is inclined towards the sun, the South Pole will
have no night and the North Pole will have no day.
It is only on the equator that the day and the night are always of equal
length. As we move away from the equator, either northward or southward, the
difference between the length of day and that of night generally goes on increasing.
THE SLANTING AND DIRECT RAYS OF THE SUN
It is our common experience that the rays of the sun at noon give more heat
than the rays of the sun in morning and in the evening. This is because the rays of
the sun are always vertical at noon and slanting in the morning and in the evening.
Vertical rays of the sun fall over a small area giving it a greater amount of heat.
Slanting rays on the other hand spreads over a wide area and give less heat.
THE CYCLE OF THE SEASONS
Let us follow the earth in elliptical orbit around the sun as shown in the
below given figure.
This figure shows four position of earth, each at an interval of about three
months. These dates are less fixed. In all these positions, the axis of the earth
makes an angle 661/2 0 with the plane of its orbit. The direction of its inclination
always remains the same.
Study the position of the earth on 21 June. You will find that the North Pole
is inclined towards the sun, while the South Pole is away from it. The rays of the
sun are falling vertically not on the equator but on 231/2 0 north of it, on the Tropic
of Cancer. You will also notice that the greater part of the Northern Hemisphere is
receiving the rays of the sun. Here the days are longer, nights shorter and the rays
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of the sun at noon are vertical. It receives more heat than it loses. Therefore, it is
having the summer season.
Revolution of the earth and the seasons
In comparison, the rays of the sun are slating in the Southern Hemisphere. A
relatively small part of this hemisphere is receiving the sun rays. Hence, days are
shorter and the nights are longer. This is the winter season the Southern
Hemisphere.
Now look at the position of earth on 22 December. It is the Southern
Hemisphere which is inclined towards the sun, while in Northern Hemisphere is
away from. The vertical rays of the sun are the falling vertically on places at south
of the equator, i.e., the Tropic of Capricorn. The days are longer and nights are
shorter in the Southern Hemisphere. It is therefore, summer in the Southern
Hemisphere and winter in the Northern Hemisphere.
Let us look at the two other positions of the earth. On 23 September and 21
March the rays of the sun fall vertically on the equator at noon. Both the poles
receive the rays of the sun. As a result, exactly one-half portion of each hemisphere
receives the sun’s rays, making day and night equal all over the world on these two
days.
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On 23 September, it is the autumn season in the Northern Hemisphere and
the spring season in the Southern Hemisphere. The opposite is the case on 21
March, when it is spring in the Northern Hemisphere and autumn in the Southern
Hemisphere.
Thus you find that we have days and nights and change in the seasons
because of the rotation and revolution of the earth.
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