MOTIONS OF THE EARTH

CLASS VI GEO. CH-3

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·         Earth has two types of motions namely rotation and revolution.

·         Rotation-Rotation is the movement of earth on its axis.

·         Revolution-Revolution is the movement of the earth around the sun in a fixed  path or orbit.

·         Orbital plane-The axis of the earth which is an imaginary line, makes an angle of 66.5­° with its orbital plane. The plane formed by the orbit is known as the orbital plane.

·         Due to the spherical shape of the earth, only half of it gets light from the sun at a time. The portion facing the sun experiences day while the other half away from the sun experiences night.

·         circle of illumination-The circle that divides the day from night on the globe is called the circle of illumination . The earth takes about 24 hours to complete one rotation around its axis. The period of rotation is known as the earthday. This is the daily motion of the earth.

·         What would happen if the earth did not rotate?

·         Ans-The portion of the earth facing the sun would always experience day, thus bringing continuous warmth to the region. The other half would remain in darkness and be freezing cold all the time. Life would not have been possible in such extreme conditions.

·         The second motion of the earth around the sun in its orbit is called revolution. It takes 365­ days (one year) to revolve around the sun. We consider a year as consisting of 365 days only and ignore six hours for the sake of convenience.

              Six hours saved every year are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February. Thus every fourth year, February is of 29                                                             days instead of 28 days. Such a year with 366 days is called a                                                                 leap year.

•      the earth is going around the sun in an elliptical orbit.
• ·         Since a large portion of the Northern Hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and the shortest night at these places occur on 21 st June. At this time in the Southern Hemisphere all these conditions are reversed. It is winter season there. The nights are longer than the days. This position of the earth is called the Summer Solstice.
• ·         On 22 nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn (23.5­° S), a larger portion of the Southern Hemisphere gets light. Therefore, it is summer in the Southern Hemisphere with longer days and shorter nights. The reverse happens in the Northern Hemisphere. This position of the earth is called the Winter Solstice.
• ·         On 21 st March and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox.On 23rd September, it is autumn season in the Northern Hemisphere and spring season in the Southern Hemisphere. The opposite is the case on 21st March, when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.