CAPF AC 2025 Q71: Geography – Astronomy & Earth - Sun System
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer: (a) 1 only
Explanation
Longitude, time and the International Date Line
How to think: every 15° of longitude is 1 hour, so 180° is 12 hours from the Prime Meridian whichever way you go.
The date, though, changes at the International Date Line (IDL).
| Statement | Fact | Verdict |
|---|---|---|
| 1. Time at 180° is exactly 12 hours from 0°, whether one goes west or east | 180° ÷ 15° = 12 hours ahead or behind; the clock difference is 12 hours both ways | Correct |
| 2. A person moving eastward on Tuesday counts the day as Wednesday after crossing the IDL | Travelling east across the IDL you lose a day (Tuesday becomes Monday). Travelling west you gain a day | Wrong |
Only statement 1 is correct.
Bank key corrected: the stored key was (c) both; the right answer is (a) 1 only.
Answer: (a) 1 only.
Why each option is right or wrong
- (a) CORRECT. Statement 1 is true; statement 2 reverses the rule for eastward travel.
- (b) Incorrect. Statement 2 is wrong: going east across the IDL the date goes back a day.
- (c) Incorrect. Statement 2 is wrong, so 'both' is not right.
- (d) Incorrect. Statement 1 is true: 180° is 12 hours from 0° both ways.
Key point
15° = 1 hour; 180° = 12 hours. IDL along about 180°: crossing eastward, subtract a day; crossing westward, add a day. Travelling east, local time moves ahead; travelling west, it falls behind.
Remember
East across the IDL: minus a day. West: plus a day.
Common mistakes
- Adding a day while crossing the IDL eastward; the date goes back.
- Thinking 180° is 12 hours ahead only when travelling east.
- Confusing the Prime Meridian with the International Date Line.
Exam tip
Educated guess: eastward you gain hours (time moves ahead), so at the IDL you must correct by taking a day off. A statement that adds a day eastward is the trap.
Sources
- NCERT Class VI Social Science, The Earth: Our Habitat, Chapter 2 (Globe: Latitudes and Longitudes; local time and standard time)
- Savindra Singh, Physical Geography (Prayag Pustak Bhawan), chapter on Latitudes, Longitudes and Time