Two Main Motions of the Earth
1. Rotation – Spinning on its axis (west to east)
2. Revolution – Movement around the Sun in elliptical orbit
Rotation (Daily Motion)
Earth rotates on its axis from west to east.
Time taken: 24 hours → causes day and night
Speed: ~1670 km/h at equator
Axis is inclined at 23.5° to the plane of orbit
Circle of Illumination
The imaginary line that separates the lighted half (day) from the dark half (night) of the Earth.
Revolution (Annual Motion)
Earth revolves around the Sun in an elliptical orbit.
Time taken: 365¼ days (one year)
Speed: ~29.8 km/second
Direction: Anti-clockwise (west to east)
Leap Year
Every 4th year has 366 days (February 29)
2024, 2028, 2032… are leap years
Summer Solstice – 21 June
• Longest day in Northern Hemisphere
• Shortest day in Southern Hemisphere
• Sun’s rays vertical over Tropic of Cancer (23.5°N)
• North Pole tilted towards Sun → 24-hour daylight
Winter Solstice – 22 December
• Shortest day in Northern Hemisphere
• Longest day in Southern Hemisphere
• Sun’s rays vertical over Tropic of Capricorn (23.5°S)
• South Pole gets 24-hour daylight
Equinoxes
21 March & 23 September
• Sun’s rays vertical over Equator
• Day and night equal all over the world (12 hours each)
UPSC/SSC One-Liners & Must-Know Facts
- Rotation → Day & Night
- Revolution → Seasons & Year
- 21 June → Longest day in India
- 22 December → Shortest day in India
- Equinox → 21 March & 23 September → Equal day-night
- Leap year every 4 years
- Axis tilted at 23.5° → Causes seasons
- Earth’s orbit is elliptical (not circular)
- Perihelion (closest to Sun) → 3 January
- Aphelion (farthest from Sun) → 4 July
Q1. Differentiate between Rotation and Revolution of the Earth.
Answer:
• Rotation: Spinning on axis → 24 hrs → Day & Night
• Revolution: Movement around Sun → 365¼ days → Seasons & Year
• Rotation axis tilted 23.5° → Revolution in elliptical orbit
Q2. What would happen if the Earth did not rotate on its axis?
Answer: One half would have continuous day, the other continuous night → extreme temperature differences → life impossible.
Q3. Explain Summer Solstice and Winter Solstice with dates and effects.
Answer:
• 21 June (Summer Solstice): Longest day in N-hemisphere, Sun vertical over Tropic of Cancer
• 22 Dec (Winter Solstice): Shortest day in N-hemisphere, Sun vertical over Tropic of Capricorn
Q4. Why do we have a leap year every four years?
Answer: Earth takes 365 days 6 hours to complete one revolution. Extra 6 hours × 4 = 24 hours → added as 29 Feb every 4th year.
Q5. What is an equinox? When do they occur?
Answer: When Sun is exactly above Equator → day & night equal everywhere.
Dates: 21 March (Vernal) & 23 September (Autumnal)
Q6. What is the Circle of Illumination?
Answer: The imaginary circle that divides the day (lighted) and night (dark) portions of the Earth at any given time.