UPSC Prelims 2005 Q83: Geography – Astronomy & Earth - Sun System
Assertion (A): The same face of the Moon is always presented to the Earth.
Reason (R): The Moon rotates about its own axis in 23½ days which is about the same time that it
takes to orbit the Earth.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (c) A is true but R is false
Explanation
Moon's rotation and the same face to Earth
| Part | Verdict |
|---|---|
| A. Same face of the Moon is always seen | True — the Moon is tidally locked |
| R. Moon rotates in 23.5 days | False: its rotation equals its orbital period of about 27.3 days |
Assertion A is true, but Reason R gives a wrong period (23.5 days), so R is false. The real period is about 27.3 days for both rotation and revolution.
Answer: (c) A is true but R is false.
Why each option is right or wrong
- (a) Incorrect. R is false, so it cannot explain A.
- (b) Incorrect. R is false, as 23.5 days is not the Moon's period.
- (c) CORRECT. A is true because rotation and revolution periods are equal, but R states a wrong figure of 23.5 days.
- (d) Incorrect. A is true, not false.
Key point
Moon: rotation period equals revolution period, about 27.3 days.
Remember
The Moon rotates once in about 27.3 days, equal to its orbit.
Common mistakes
- Accepting the 23.5 day figure, which is the Earth's axial tilt in degrees.
- Thinking the Moon does not rotate at all.
- Mixing the sidereal and synodic months (29.5 days).
Exam tip
Synchronous rotation keeps the near side facing Earth.
Sources
- NCERT Class XI Geography — Fundamentals of Physical Geography — The Origin and Evolution of the Earth
- NASA — Moon Fact Sheet (Goddard Space Flight Center, Planetary Fact Sheets)