Discussion
How many degrees of longitude does the Earth rotate through in one hour?
Answer: A
Correct answer: 15°
The Earth turns through 360° in 24 hours, so in one hour it turns \[ \frac{360}{24} = 15 \text{ degrees}. \] That single figure underlies the whole system of time zones: each standard zone is nominally 15° of longitude wide, and local time changes by one hour for every 15° travelled east or west. Four minutes correspond to one degree. It also explains India’s single time zone. Indian Standard Time is fixed to the 82.5°E meridian near Mirzapur, giving an offset of \( 82.5 / 15 = 5.5 \) hours ahead of Greenwich - the half hour comes from the meridian falling midway between two whole multiples of 15. Before the railways forced standardisation in the nineteenth century, each town simply kept its own solar time.
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