If it is 10:00 a.m. at Greenwich Mean Time (GMT), then what will be the time at 90° E longitude?
- A.5:00 a.m.
- B.4:00 a.m.
- C.3:00 p.m.
- D.4:00 p.m.
Correct answer
Explanation
The correct answer is D, 4:00 p.m. A place at 90° E is six hours ahead of Greenwich, so 10:00 a.m. GMT becomes 4:00 p.m. there. The Earth turns through 360° of longitude in 24 hours, which means 15° in one hour, or 1° in four minutes. Places east of the Prime Meridian see the Sun earlier, so their local time is ahead of Greenwich; places to the west are behind. Here the difference is 90° multiplied by four minutes, which is 360 minutes or six hours, and adding six hours to 10:00 a.m. gives 4:00 p.m. The same rule explains Indian Standard Time: its meridian of 82°30′ E puts India five and a half hours ahead of GMT. Option A is wrong because 5:00 a.m. would mean subtracting five hours, as for a place west of Greenwich. Option B is wrong because 4:00 a.m. is six hours behind, which fits 90° W, not 90° E. Option C is wrong because 3:00 p.m. is only five hours ahead, the time at 75° E. Exam tip: add four minutes per degree going east, subtract going west; 15° equals one hour.
Practice Questions
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