UPSC Prelims 2002 Q128: Science & Technology – Space Technology & Astronomy
Assume that the moon takes exactly 30 days to complete the cycle and also assume that it
rises in the East exactly at 6.48 p.m. on the first day. On the fourth day, at what time will it rise?
- (a) 8.24 p.m.
- (b) 9.12 p.m.
- (c) 10.00 p.m.
- (d) 11.48 p.m.
Answer: (b) 9.12 p.m.
Explanation
Moon-rise time on the fourth day
| Quantity | Value |
|---|---|
| Cycle | 30 days for 24 hours of delay per cycle |
| Daily delay in moon-rise | 24 hours ÷ 30 = 48 minutes per day |
| Delay by the fourth day | 3 days × 48 minutes = 144 minutes = 2 hours 24 minutes |
| Time on the fourth day | 6.48 p.m. + 2 h 24 min = 9.12 p.m. |
Assuming the moon takes exactly 30 days for the cycle, it rises about 48 minutes later each day. From the first day to the fourth day there are three such delays, so 6.48 p.m. + 144 minutes = 9.12 p.m.
Answer: (b) 9.12 p.m.
Why each option is right or wrong
- (a) Incorrect. 8.24 p.m. is the time after only two days of delay.
- (b) CORRECT. 6.48 p.m. plus 3 × 48 minutes is 9.12 p.m.
- (c) Incorrect. 10.00 p.m. would need a delay of about 64 minutes a day.
- (d) Incorrect. 11.48 p.m. is five hours late, far too much.
Key point
Moon-rise comes about 50 minutes later each day in reality; the question simplifies to 48 minutes.
Remember
Moon rises 48 minutes later daily in a 30-day cycle.
Common mistakes
- Counting four delays instead of three.
- Using 24 minutes a day by halving the real delay.
- Adding hours and minutes incorrectly.
Exam tip
From day 1 to day 4 is three steps of 48 minutes.
Sources
- NASA — Moon Fact Sheet (Goddard Space Flight Center, Planetary Fact Sheets)
- United States Naval Observatory — Moon Phases and Moonrise/Moonset Data