CBSE Class 6 Geography Chapter 3 Notes - Motions of the Earth

Important Terms Used to Describe Motions of The Earth

To learn the motions of the earth in detail, it is essential to understand some important terms.

• The axis of the earth or Earth’s axis is an imaginary pole or line passing through the center of the earth and makes an angle of 66½° with its orbital plane. It runs from the North Pole to the South Pole.

• The orbital plane is the plane formed by the orbit.

• The circle of illumination is the circle that divides day and night on the globe. The sun gives light on the earth, which is spherical in shape. Due to its shape, only half of the earth gets sunlight at a time, i.e. the portion of earth facing the sun experiences day while the other half, which is away from the sun, experiences night. This circle of illumination does not coincide with the earth’s axis. 

• Earthday is the daily motion of the earth and the period of rotation which the planet earth takes to make one complete rotation around its axis, which is 24 hours. 

What Will Happen If The Earth Stops Rotating?

In such a case, the portion of the earth facing the sun would always experience day, thus bringing continuous warmth to that region. The other half would remain dark and freezing cold all the time. The existence of life would not be possible in such extreme conditions. Thus, the motion of the earth is important for life to exist.

How Long Does The Earth Take to Revolve?

The earth takes 365¼ days (one year) to revolve around the sun. For convenience, we consider a year of 365 days only and ignore six hours. Over a span of four years, these six hours saved every year are added to make one complete day of 24 hours. This surplus day then is added to the month of February of that year. Thus, every fourth year, February has 29 days instead of 28 days and such a year with an additional one day by making it 366 days is called a leap year. 

The earth revolves around the sun in an elliptical orbit, and throughout its orbit, the earth is inclined in the same direction. This revolution gives four seasons in the year: summer, autumn, winter, and spring. Seasons change according to the change in position of the earth around the sun.

Positions of the Sun:

• Summer Solstice: On 21st June, the Northern Hemisphere is tilted maximum towards the sun. The sun rays fall directly on the Tropic of Cancer, and as a result, these areas receive more heat. Since the sun rays are slanting, the areas near the poles receive less heat. The area beyond the Arctic Circle experiences continuous daylight for about six months as the North Pole is inclined towards the sun. Since a majority portion of the Northern Hemisphere receives light from the sun, it is summer season in the regions north of the equator. On 21st June there is the longest day and the shortest night at these places. At this time, all conditions are reversed in the Southern Hemisphere. They experience the winter season, and the nights are longer than the days. 

• Winter Solstice: On 22nd December, the South Pole is tilted maximum towards the sun, and the Tropic of Capricorn receives direct sun rays. As the rays of sun fall vertically at the Tropic of Capricorn at 23½° S, a major portion of the Southern Hemisphere receives light. Therefore, the Southern Hemisphere experiences summer with longer days and shorter nights. The opposite happens in the Northern Hemisphere at this time. 22nd December experiences the longest night and the shortest day.

• Equinox: On 21st March and September 23rd, the sun rays fall directly on the equator. At this unique position, neither of the poles is tilted towards the sun; the entire earth experiences equal days and equal nights. This position is called an equinox. On 23rd September, while the Northern Hemisphere experiences autumn, the Southern Hemisphere has spring. The contrary happens on 21st March when the Northern Hemisphere has spring season, and the Southern Hemisphere has autumn.

Thus, based on the earth’s rotation and revolution around the sun, we see days/nights and changes in the seasons.