Discussion
When it is 1:00 p.m. at Greenwich, what time will it be at 60° West and 60° East respectively ?
Answer: C
It will be 9:00 a.m. and 5:00 p.m. respectively.
The earth turns 360° in 24 hours, so local time changes by 4 minutes for each degree of longitude: later towards the east and earlier towards the west.
\[ \begin{aligned} 60 \times 4\ \text{min} &= 240\ \text{min} \\ &= 4\ \text{hours} \end{aligned} \]
At 60° West the time is 4 hours behind Greenwich: 1:00 p.m. minus 4 hours is 9:00 a.m. At 60° East it is 4 hours ahead: 1:00 p.m. plus 4 hours is 5:00 p.m.
'5:00 p.m. and 9:00 a.m.' puts the west ahead of Greenwich and the east behind it, the wrong way round. '5:00 a.m. and 9:00 p.m.' uses 8 minutes a degree, doubling the difference, and '9:00 p.m. and 5:00 a.m.' makes both mistakes at once.
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