Motions of the Earth: Rotation and Revolution
Delhi Police & SSC Exams Preparation
1. Introduction
The Earth is constantly moving in space. Its two main motions are:
1. Rotation
Spinning on its own axis
2. Revolution
Orbiting around the Sun
Key Effects:
- Day and night
- Changes in seasons
- Variations in daylight
Exam Trick: "Rotation = Day; Revolution = Year."
2. Rotation
Key Facts:
- Definition: Spinning of the Earth on its axis
- Time Taken: 24 hours (1 day)
- Axis Tilt: 23½° (66½° to the plane of its orbit)
- Direction: West → East
Effects of Rotation:
Day and night
Time difference between longitudes
Apparent movement of celestial bodies
Deflection of winds and ocean currents (Coriolis Effect)
Exam Trick: "West to East – brings the Feast (Daylight)."
3. Revolution
Key Facts:
- Definition: Movement of the Earth around the Sun
- Shape of Orbit: Elliptical (oval-shaped)
- Time Taken: 365 days 6 hours (1 year)
- Direction: West → East (same as rotation)
Effects of Revolution:
Change of seasons
Variation in length of day and night
Change in apparent position of the Sun
Leap year correction
Exam Trick: "Revolve for the Year, Rotate for the Day."
4. Inclination of Earth's Axis (23½°)
Key Concept:
The Earth's axis is tilted at 23½° from the perpendicular to its orbital plane. This tilt, along with revolution, causes seasons and unequal daylight.
Important: Without this tilt, there would be no seasons!
Seasonal Impact:
Different parts receive varying sunlight intensity
Changes in temperature patterns
Variation in daylight hours
Exam Trick: "Tilt gives us seasons to build."
5. Solstices and Equinoxes
| Event | Date | Description | Hemisphere Effect |
|---|---|---|---|
| Summer Solstice | 21 June | Northern Hemisphere tilted towards Sun | Longest day in North |
| Winter Solstice | 22 December | Northern Hemisphere tilted away from Sun | Shortest day in North |
| Vernal (Spring) Equinox | 21 March | Sun directly over Equator | Equal day & night |
| Autumnal Equinox | 23 September | Sun directly over Equator | Equal day & night |
Exam Trick: "21-6 long, 22-12 short; 21-3 & 23-9 equal in sort."
6. Leap Year
Calculation:
- A year has 365 days and 6 hours
- After 4 years → 6 × 4 = 24 extra hours = 1 extra day
- Hence every 4th year is a leap year (Feb 29)
Key Points:
Leap years help synchronize the calendar year with the solar year
Exam Trick: "Once in four, Feb has more."
7. Quick Comparison – Rotation vs Revolution
| Feature | Rotation | Revolution |
|---|---|---|
| Meaning | Spinning of Earth on its axis | Movement around the Sun |
| Time Taken | 24 hours | 365 days 6 hours |
| Direction | West → East | West → East |
| Effect | Day & Night | Seasons |
| Axis Tilt Role | Not responsible | Responsible |
| Result | Sunrise, Sunset | Summer, Winter, etc. |
Exam Trick: "Rotation makes Day; Revolution makes May."
PYQs (Delhi Police + SSC + State Exams)
The Earth rotates on its axis from —
Options: (a) East to West (b) West to East (c) North to South (d) South to North
Category: Rotation Direction
Show Answer
b) West to East
The Earth takes how much time to complete one rotation?
Options: (a) 12 hours (b) 24 hours (c) 30 days (d) 365 days
Category: Rotation Time
Show Answer
b) 24 hours
Questions 3-11 continue with the same format. Complete all questions for comprehensive practice.
Final Exam Tips
"West to East" direction – Most repeated PYQ!
Remember 23½° tilt → reason for seasons
4 minutes = 1° rotation → helps in time questions
Dates: 21 June, 22 Dec, 21 Mar, 23 Sep → must memorize
Master Geography for Delhi Police Exam!
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