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Q128 (IAS/2002) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Astronomy and astrophysics Answer Verified

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?

Result
Your answer: —  ·  Correct: B
Explanation

Assuming a 30-day lunar cycle, the Moon’s rise time shifts later each day by 24 hours ÷ 30 days = 48 minutes per day [1]. Starting at 6:48 p.m. on day 1, the fourth day is three days later. Total delay = 3 × 48 minutes = 144 minutes = 2 hours 24 minutes. Add this to 6:48 p.m.: 6:48 p.m. + 2:24 = 9:12 p.m. This method aligns with the general observational rule that the Moon rises roughly an hour later each day (about 50 minutes on average), so using the specified 30-day assumption gives the exact 48-minute daily increment used above [2].

Sources

  1. [1] https://physics.weber.edu/schroeder/ua/MoonAndEclipses.html
  2. [2] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 11: Keeping Time with the Skies > A step further > p. 173
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