Motions of The Earth - Geography NCERT Notes For UPSC

Rotation and associated terms

• 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 circle that divides the day from night on the globe is called the circle of illumination.
• The period of rotation is known as Earth day which is the daily motion of the earth.

Important points related to Revolution

• Every fourth year, February is 29 days instead of 28 days. Such a year with 366 days is called a leap year.
• In this revolution time, the Earth goes around the sun in an elliptical orbit. Throughout its orbit, the Earth is inclined in the same direction.
• Seasonal changes are due to changes in the earth’s position around the Sun.

Important dates in Earth’s revolution

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 and areas near poles receive less heat due to slanting rays. Places around the Arctic Circle receive continuous daylight for 6 months. Therefore it is called the summer season in the northern hemisphere. 
• The longest day and the shortest night occur at these places on 21st June. The reverse happens in the southern hemisphere. 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 (23½° 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 21stMarch and 23rdSeptember

• 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 23^rdSeptember, it is the 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. 

Thus, the rotation and revolution of the Earth is responsible for formation of days and nights and seasonal changes respectively.

Interesting Points

• The ancient Indian astronomer Aryabhata had stated that ‘the Earth is round and rotates on its own axis’.

If you are targeting UPSC CSE 2023, download the PrepLadder app and transform your UPSC CSE preparation from the Beginner level to the Advanced level. You can also join our Telegram channel to stay updated with the latest information about the exam.