UPSC Prelims 2007 Q73: Geography – Astronomy & Earth - Sun System
What is the average distance (approximate) between the Sun and the Earth?
- (a) 70 × 10^5 km
- (b) 100 × 10^5 km
- (c) 110 × 10^7 km
- (d) 150 × 10^6 km
Answer: (d) 150 × 10^6 km
Explanation
Average Sun-Earth distance
| Measure | Value |
|---|---|
| Mean distance (1 AU) | ~149.6 million km ≈ 150 × 10⁶ km |
| Perihelion (closest, ~3 Jan) | ~147 million km |
| Aphelion (farthest, ~4 July) | ~152 million km |
| Light travel time | ~8 minutes 20 seconds |
Quick check: light travels 3 × 10⁵ km/s × 500 s ≈ 1.5 × 10⁸ km.
Any value of 7 × 10⁶ km, 7 × 10⁸ km or 7 × 10⁹ km is far off.
Answer: (d) 150 × 10⁶ km.
Why each option is right or wrong
- (a) Incorrect. 70 × 10⁵ km = 7 million km — about 20 times too small.
- (b) Incorrect. 100 × 10⁵ km = 10 million km — far too small.
- (c) Incorrect. 110 × 10⁷ km = 1.1 billion km — roughly Saturn's distance from the Sun.
- (d) CORRECT. The mean Sun-Earth distance is about 150 million km (1 AU).
Key point
1 AU ≈ 150 million km (1.5 × 10⁸ km); sunlight takes about 8 min 20 s to reach Earth.
Remember
Sun-Earth distance ≈ 150 million km = 1 AU.
Common mistakes
- Miscounting powers of ten — 150 × 10⁶ and 1.5 × 10⁸ are the same number.
- Thinking Earth is closest to the Sun in summer. Perihelion is in January; seasons come from the tilt, not distance.
- Confusing 150 million km with 93 million miles — both are 1 AU.
Exam tip
Use light: 8 min × 60 × 3 lakh km/s ≈ 15 crore km. That rebuilds 1 AU in seconds.
Sources
- NCERT Class VI — The Earth in the Solar System
- NCERT Class XI — Fundamentals of Physical Geography, Ch 2
- IAU — Astronomical Unit (2012 definition)