NDA (II) 2017 General Ability Test · Question 141 of 150

Correct — B, about two hours. India stretches from roughly 68°7' E at its western edge in Gujarat to about 97°25' E at its eastern edge in Arunachal Pradesh, a span of a little over 29 degrees of longitude. The Earth turns 360 degrees in 24 hours, which is 15 degrees an hour or 4 minutes for every degree. Multiplying 29 degrees by 4 minutes gives about 117 minutes — one hour fifty-seven minutes, which rounds to two hours. Since the Earth rotates from west to east, the eastern edge meets the Sun first, so the day begins in Arunachal Pradesh roughly two hours before it does in Kachchh.

• (a)One hour — This would need a longitudinal spread of only 15 degrees. India's east-west spread is about twice that, so one hour understates the gap by half.
• (c)Three hours — Three hours corresponds to 45 degrees of longitude, far more than India's 29 degrees. A country would have to be as wide as Australia or the contiguous United States to open a gap of that size.
• (d)Four hours — Four hours means 60 degrees of longitude — roughly the width of Russia's European section or of Brazil. India is nowhere near that wide, and it is precisely because the gap is only about two hours that a single standard time works reasonably well.

Local time is set by the Sun's position and therefore by longitude. Because the Earth completes one rotation of 360 degrees in 24 hours, every 15 degrees of longitude is worth one hour and every single degree is worth 4 minutes. India runs from about 68°7' E to about 97°25' E, so its local times differ by roughly two hours from end to end. To avoid running two clocks, India uses a single Indian Standard Time based on the 82°30' E meridian, which passes near Mirzapur in Uttar Pradesh and is 5 hours 30 minutes ahead of Greenwich.

Every question in this family reduces to one conversion: 1 degree equals 4 minutes, 15 degrees equals 1 hour. Learn the two extreme longitudes of India and the arithmetic follows. The consequence is worth knowing too — because the whole country runs on a meridian well to the west of Arunachal Pradesh, the sun rises there while clocks still read very early, which is the practical basis of the long-running demand for a separate time zone for the north-east.

• India extends from about 68°7' E to about 97°25' E, a span of roughly 29 degrees of longitude.
• The Earth rotates 15 degrees of longitude per hour, so one degree corresponds to 4 minutes of time.
• The local-time difference between the eastern and western extremities of India is therefore close to two hours.
• Indian Standard Time is fixed on the 82°30' E meridian and is 5 hours 30 minutes ahead of Greenwich Mean Time.
• The Earth rotates west to east, so places further east see the sunrise earlier.

• Using latitude instead of longitude; only longitude governs time.
• Forgetting the direction of rotation and reversing which end sees the sunrise first.
• Confusing the 15-degrees-per-hour rule with the 1-degree-equals-111-kilometre rule for distance.

This concept comes as a time-difference estimate like this one, as a local-time calculation for a named city, or as an identification of the IST meridian.

• practice — not a real PYQ

  Indian Standard Time is based on which meridian?

  • (a)75°30' E
  • (b)82°30' E
  • (c)88°30' E
  • (d)90°00' E

  Answer(b) 82°30' E — passing near Mirzapur, making IST 5 hours 30 minutes ahead of GMT.

• practice — not a real PYQ

  Two places differ in longitude by 20 degrees. The difference in their local times is

  • (a)40 minutes
  • (b)1 hour
  • (c)1 hour 20 minutes
  • (d)2 hours

  Answer(c) 1 hour 20 minutes — 20 degrees at 4 minutes per degree gives 80 minutes.