6.Chapter4.Geo.The Motion of the Earth

Chapter 4.Geography.

The Motion of the Earth 

(Day - Night and Seasons) Axis North Pole Tropic of Cancer Equator  

Page 30 We have learnt in the previous chapter that the earth is continuously spinning on its imaginary axis from west to east.  It completes one round in about 24 hours.  This motion of the earth is called Rotation.  The earth also revolves round the sun in its fixed path known as Orbit.  It revolves round the sun in 365% days, which makes one year.  This motion of the earth is called Revolution The axis of the earth is not perpendicular but tilted to one side.  It makes an angle of 23% from the vertical or perpendicular line.  In other words, the earth's axis makes an angle of 66% with its orbital plane.  This tilt of the axis of the earth is called the inclination of the earth's axis.  Now, let us know how the rotation of the earth causes day and night.  

ROTATION OF THE EARTH TROPIC OF Capricorn South Pole Earth and its axis The earth faces the sun while rotating from west to east.  Each part of the earth facing the sun is illuminated by sunlight turn by turn.  The lighted half part of the earth that faces the sun has day while the other part of the earth that is not facing the sun, experiences night.  

Earth day night rotation revolution Length of a day and night 

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Thus, as the earth rotates, the day follows the night and the night follows the day by turns.  Imagine, what would happen if the earth stops rotating?  Now, let us perform an experiment to understand the phenomenon of the formation of day and night.  For this, we require a globe and an electric lamp.  Keep the globe on a table and place a lamp in front of the globe in a dark room.  The globe represents the earth and the lamp represents the sun.  Mark a point 'P' on the globe to represent a 'place'.  Switch on the observe the lit half and the dark half parts of the globe.  First, place point 'P' of the globe in the dark side.  Rotate the globe from Experiment showing formation of day and night west to east.  See the 'P' is moving towards the light in the direction of the lamp representing the sun.  Notice 'P' first, in a diffused light.  It is the position before sunrise and the time is called dawn.  Move the globe further till the place 'P' receives the beam of light.  You will notice the first beam of the lamp illuminates the place 'P'.  This is the time which is called sunrise or morning.  Rotate the globe further eastward, now 'P' will face the lamp (sun) at right angle.  This position represents noon on the earth.  Turn the globe again and notice that the place 'P' moves towards darkness.  This is the position of sunset.  Although place 'P' moves out of the light, but still receives dim light.  This time is called dusk.  On further rotation, 'P' is in complete darkness and it is night again.  This explains how the day and night follow one after the other 

REVOLUTION OF THE EARTH Era) We already know that the earth completes one revolution around the sun in an elliptical orbitin one year.  To be specific, the earth completes one revolution in 365 days and about 6 hours) For our convenience, we consider 365 days in a year.  The remaining 6 hours are added over a period of four years till they become 24 hours or one day.  This 24 extra day is added to the month of February every four years to make it a month of 29 days instead of 28 days.  The year of 366 days is called leap year un a) 

Do You Know?  The distance between the earth and the sun changes in the course of its revolution on the elliptical orbit.  The minimum distance (147.5 million km) between the earth and the sun is on 4th January The maximum distance (152.6 million km) between the earth and the sun is on 4th July.  .  

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Revolution of the Earth and the Seasons We know that rotation and revolution of the earth are very essential for causing day and nigh and occurring of various seasons.  Let us find out the phenomenon of varying length of day ane night and the occurrence of different seasons on the earth with the help of a diagram.  Equinox March 21 Northern Hemisphere Spring Southern Hemisphere Autumn Solstice June 21 Northern Hemisphere Summer Southern Hemisphere Winter 23/2 Solstice December 22 Northern Hemisphere Winter Southern Hemisphere Summer Equinox September 23 Northern Hemisphere Autumn Southern Hemisphere Spring Revolution of the Earth and the Seasons The above diagram shows  the path of the earth's revolution and the four positions of the ear on particular dates.  The positions leading the four seasons and their change, one after the other, form a cycle of seasons.  Position on 21st June when the revolving earth reaches this position, the Northern Hemisphere is inclined towards th sun and the Southern Hemisphere is away from the sun.  The rays of the sun are vertical at the Trop of Cancer (23% 'N).  The larger portion of the Northern Hemisphere faces the sun.  As the lit part this hemisphere is more than the half, the length of the day is more than night.  The duration of de increases towards the North Pole.  The Arctic Circle faces the sun for 24 hours.  The region around the North Pole faces the su for a period of about six months.  As the sun rays are more vertical and have longer duration, t Northern Hemisphere receives more heat, therefore, it is summer season here.  This situation called summer solstice in this hemisphere.  The situation in the Southern Hemisphere is just th opposite.  The nights are longer than days.  The Antarctic Circle remains in complete darkness.  TH sun rays are more slanting, therefore, it is winter season in the Southern Hemisphere.  This situatic is called winter solstice.  

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Position on 23rd September nt 
After three months the revolving earth comes  to this position.The sun shines vertically over the Equator.The days and nights are of equal duration in both the hemispheres on this date.Both the hemispheres receive the same amount of heat.  This  situation is called Equinox.
Do You Know?   The North Polo has darkness for about six months from 23rd September to 21st March whereas the South Pole  has sunlight during this period. 

It is autumn season in the Northern Hemisphere.  Therefore, this position in Northern Hemisphere is known as autumnal equinox.  At the same time, the Southern Hemisphere has spring season called vernal equinox.  Position on 22nd December The earth continues to revolve and the Southern Hemisphere gradually comes towards the sup.  In this position, the sun rays are vertical at the Tropic of Capricorn (23% S).  The length of the day increases towards the south pole and the nights become shorter.  It is summer in the Southern Hemisphere and is called summer solstice for this hemisphere.  Opposite to it, the Northern Hemisphere, in this situation is away from the sun.  The days are shorter than nights.  The sun rays are more slanting.  It is winter in this hemisphere.  This position in the Northern Hemisphere is called winter solstice.  Position on 21st March (The earth continues to move further and comes to this position after three months. The sum rays are vertical again over the Equator. The duration of day and night becomes equal in both th hemispheres, ie 12 hours day and 12 hours  night (Both the hemispheres receive equal amount do you know? heat. It is spring in the Northern Hemisphere and the North Pole has sunlight for called vernal equinox. At the same time, Souther about six months from 21st March to Hemisphere experiences autumn season and  23rd September while the South Pole called autumnal equinox. Thus, the cycle of season has night during this period. Continuously goes on year after year.) Keywords axis: a line that joins the north and south pole about which the earth rotates. Inclination:  degree of sloping vertical rays: the rays which fall directly overhead. It covers minimum distance in the atmosphere.