Motions of the Earth

The Earth has two motions: rotation and revolution. Rotation is the spinning of the Earth on its axis, an imaginary line making an angle of 66.5 degrees with the plane of its orbit. One rotation takes about 24 hours and causes day and night. The circle dividing the lighted half of the Earth from the dark half is the circle of illumination. Without rotation, one half of the Earth would always be extremely hot and the other half dark and freezing. Revolution is the movement of the Earth around the Sun in a fixed orbit, taking 365 and 1/4 days. The extra quarter day is added every four years as 29 February, giving a leap year of 366 days. The tilted axis and revolution together cause seasons. On 21 June, the summer solstice, the sun's rays fall vertically on the Tropic of Cancer; on 22 December, the winter solstice, on the Tropic of Capricorn. On the equinoxes, 21 March and 23 September, day and night are equal everywhere.

Detailed Worksheet: Motions of the Earth Section A - Definitions (10 marks) 1. Define rotation and revolution of the Earth. (2 marks) 2. What is the axis of the Earth? What angle does it make with the orbital plane? (2 marks) 3. What is the circle of illumination? (2 marks) 4. What is a leap year? Why is it needed? (2 marks) 5. What are solstices and equinoxes? Give their dates. (2 marks) Section B - Calculations and Applications (15 marks) 6. The Earth takes 365 and 1/4 days to complete one revolution. (i) How many extra hours are there each year beyond 365 days? (ii) How many extra days collect in 4 years? (iii) Which of the years 2024, 2026, 2028 and 2030 are leap years? (3 marks) 7. The Earth rotates once in about 24 hours. (i) How many rotations does it complete in a leap year? (ii) How many hours are there in a non-leap year? (iii) How many degrees does it turn in 6 hours? (3 marks) 8. Name the season in the Northern Hemisphere and in the Southern Hemisphere on (i) 21 June (ii) 22 December. (iii) What happens to day length at the North Pole on 21 June? (3 marks) 9. On 21 June, the sun's rays fall vertically on the Tropic of Cancer at 23.5 degrees N. (i) What is the latitude of the place receiving vertical rays on 22 December? (ii) What is the difference in degrees between these two latitudes? (iii) On which dates do vertical rays fall on the Equator? (3 marks) 10. Explain what would happen if (i) the Earth did not rotate (ii) the Earth's axis was not tilted (iii) the Earth did not revolve around the Sun. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the revolution of the Earth around the Sun, showing the Earth's positions on 21 June, 23 September, 22 December and 21 March, and the seasons in the Northern Hemisphere. (4 marks) 12. Draw a labelled diagram showing day and night on the Earth with the circle of illumination, the axis and the direction of rotation. (3 marks) 13. Draw a diagram showing the inclination of the Earth's axis at 66.5 degrees to its orbital plane and 23.5 degrees from the vertical. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. When it is summer in India, it is winter in Australia. (i) Explain why. (ii) In which months does Australia celebrate Christmas in summer? (iii) Which hemisphere is tilted towards the Sun in June? (iv) Why are days longer in summer? (5 marks) 15. Use a globe and a torch in a dark room. (i) What happens to the part of the globe facing the torch? (ii) What happens when you slowly rotate the globe? (iii) What does the torch represent? (iv) How does this activity explain day and night? (5 marks) 16. The North Pole has six months of continuous daylight and six months of night. (i) Explain this using the tilt of the axis. (ii) When does the North Pole have continuous daylight? (iii) What happens at the South Pole at the same time? (iv) Why are polar regions cold even in summer? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil, mark the dates clearly and show the direction of motion with arrows.