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

Some Important Definitions

The Axis of the Earth: An imaginary line makes an angle of $66 \dfrac{1} {2}^{\circ}$ with its orbital plane.

The Orbital Plane: The plane formed by the orbit is known as the orbital plane.

Circle of illumination: The imaginary circle on the earth which divides the earth into day and night halves is known as the circle of illumination. It is not the same as the axis of the earth and doesn’t coincide with it. The circle of illumination makes a $23\dfrac{1}{2}$ degrees angle from the earth’s axis.

Rotation of Earth

● The phenomenon of day and night is because of the rotation of the earth.Had there been no rotation, one side of the earth would perpetually have night, and temperatures would have dropped in this half. The other half would forever have a day, and in this half, temperatures would have risen. In such cases, it would not be possible to sustain life on earth. 

● Earth takes 24 hours to complete one rotation, and this is known as earth day. 

● 23 hr, 56 min, 4 sec is the actual time taken to complete one rotation, but for convenience purposes, it is taken to be 24 hours. 

Revolution of the Earth

● The movement of the Earth on its own axis around the sun is termed a revolution. 

● The Earth moves around the sun in an elliptical-shaped orbit. 

●  It takes 365.25 days to complete one revolution. This is the time taken by the Earth to go round an elliptical orbit with the sun at a focus. 

● We consider 365 days as one year, and the rest one-fourth of a day is added up for every four years to give us one extra day. This day is added up to the fourth year, and the year has 366 days. This year is called a leap year. This day is added up to the month of February.

● In a leap year, February has 29 days instead of 28 days. 

Seasons

● Seasons change due to change in the position of the Earth around the sun.

● Because of the change in the position of Earth, a year is divided into summer, winter, spring, and autumn.

Summer Solstice

● Summer solstice begins on 21st June in the Northern Hemisphere. 

● On this day, the Northern Hemisphere is tilted towards the sun.

● Sun rays fall on the Tropic of Cancer.

● The North Pole is inclined towards the sun, and areas beyond the Arctic Circle experience continuous daylight for about six months.

● The longest day and shortest night occur on 21st June, the Summer Solstice.

● The conditions in the Southern Hemisphere are reversed, and it is winter season there.

Winter Solstice

● Winter solstice occurs on the 22nd of December in the Northern Hemisphere.

● The Southern Hemisphere at this time is tilted towards the sun. 

● The Sunrays fall on the Tropic of Capricorn in the Southern Hemisphere.

● The Southern Hemisphere has longer days during this period and shorter nights. 

● The conditions are reversed in the Northern Hemisphere.

Equinox

● The direct rays of the sun fall on the equator on 21st March and 23rd September.

● The whole Earth experiences equal days and equal nights because neither of the poles is tilted towards the sun.

● 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.

Class 6 Geography Chapter 3 - Motions of the Earth Notes

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.