- Rotation is the movement of the earth on its axis.
- The movement of the earth around the sun in a fixed path or orbit is called Revolution.
- The axis of the earth which is an imaginary line, makes an angle of 66½° with its orbital plane.
- The plane formed by the orbit is known as the orbital plane.
- The earth receives light from the sun.
- 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.
- This circle does not coincide with the axis
- 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?
- The portion of the earth facing the sun would always experience day, thus bringing continuous warmth to
- 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 365¼ days (one year) to revolve around the sun.
- A year 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.
- The earth is inclined in the same direction.
- A year is usually divided into summer, winter, spring and autumn seasons.
- Seasons change due to the change in the position of the earth around the sun
- the Northern Hemisphere is tilted towards the sun.
- The rays of the sun fall directly on the Tropic of Cancer.
- As a result, these areas receive more heat.
- The areas near the poles receive less heat as the rays of the sun are slanting.
- The North Pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months.
- 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 21st 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 22nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards
- As the sun’s rays fall vertically at the Tropic of Capricorn (23½° S), a larger portion of the Southern Hemisphere gets light.
- It is summer in the Southern Hemisphere with longer days and shorter nights.
- The reverse happens in the Northern Hemisphere.
- The position of the earth is called the Winter Solstice
- On 21st 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
- The opposite is the case on 21st March when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
- The days and nights and changes in the seasons because of the rotation and revolution of the earth respectively.