UPSC Prelims 2006 Q18: Science & Technology – Space Technology & Astronomy
Assertion (A): To orbit around the Sun, the planet Mars takes lesser time than the time taken
by the Earth.
Reason (R): The diameter of the planet Mars is less than that of the Earth.
- (a) Both A and R are true and R is the correct explanation of A
- (b) Both A and R are true but R is not a correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (d) A is false but R is true
Explanation
Mars and Earth: orbital period and size
| Planet | Orbital period around the Sun | Diameter |
|---|---|---|
| Earth | about 365 days | about 12,742 km |
| Mars | about 687 Earth days | about 6,779 km |
Assertion (A): Mars takes less time than Earth to orbit the Sun. False, since Mars, being farther out, takes almost twice as long.
Reason (R): Mars's diameter is less than Earth's. True.
Orbital period depends on distance from the Sun (Kepler's third law), not on the planet's size.
Answer: (d) A is false but R is true.
Why each option is right or wrong
- (a) Incorrect. A is false, so this option fails at once.
- (b) Incorrect. A is false; Mars's year is longer than Earth's.
- (c) Incorrect. A is false and R is true, the opposite of this option.
- (d) CORRECT. Mars takes about 687 days to orbit, longer than Earth, while its diameter is indeed smaller.
Key point
Mars is farther from the Sun and takes about 687 days per orbit, longer than Earth, though it is smaller in diameter.
Remember
Mars's year is about 687 Earth days; it takes longer than Earth to orbit.
Common mistakes
- Assuming a smaller planet orbits faster.
- Forgetting Kepler's third law: farther planets take longer.
- Confusing Mars's day (about 24.6 hours) with its year.
Exam tip
Farther from the Sun means a longer year, regardless of planet size.
Sources
- NCERT Class VIII Science — Stars and the Solar System
- NASA — Mars Fact Sheet (Goddard Space Flight Center, Planetary Fact Sheets)