MOTIONS OF THE EARTH



→ The earth has two types of motions, namely rotation and revolution. 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 66*1⁄2° with its orbital plane. The plane formed by the orbit is known as the orbital plane.

→ 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.

Day and Night on the Earth due to rotation.

-->The second motion of the earth around the sun in its orbit is called revolution. It takes 365*1⁄4 days (one year) to revolve around the sun.

Revolution of the Earth and Seasons.

→ a year with 366 days is called a leap year. Where February is of 29 days instead of 28 days.

→ the earth is going around the sun in an elliptical orbit.

→ 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.

→ on 21st June, 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 it. As the sun’s rays fall vertically at the Tropic of Capricorn (231⁄2° 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 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 Hemisphere. The opposite is the case on 21 st March,when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

Tick the correct answers.

The movement of the earth around the sun is known as
(i) Rotation
(ii) Revolution
(iii) Inclination

(b) Direct rays of the sun fall on the equator on
(i) 21 March
(ii) 21 June
(iii) 22 December

(c) Christmas is celebrated in summer in
(i) Japan
(ii) India
(iii) Australia

(d) Cycle of the seasons is caused due to
(i) Rotation
(ii) Revolution
(iii) Gravitation
Fill in the blanks.
(a) A leap year has _______________ number of days.
(b) The daily motion of the earth is _______________.
(c) The earth travels around the sun in ______________ orbit.
(d) The sun’s rays fall vertically on the Tropic of ___________ on 21 st June.
(e) Days are shorter during ___________ season.