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 — D, 4:00 p.m. The earth turns through 360 degrees in 24 hours, so 15 degrees of longitude correspond to one hour and one degree to four minutes of time. A place at 90 degrees east lies 90 divided by 15, that is 6 hours, ahead of the Greenwich meridian, because the sun reaches an eastern meridian earlier. Add six hours to 10:00 a.m. and the local time at 90 degrees east is 4:00 p.m. The two morning options in the list are what you would get by subtracting instead of adding, and the third afternoon option comes from using 5 hours rather than 6.
- (a)5:00 a.m. — This subtracts five hours instead of adding six. Places east of Greenwich are ahead of it, not behind, so any morning answer is going in the wrong direction.
- (b)4:00 a.m. — The same error of direction, with the correct six-hour interval subtracted rather than added. It is the answer for 90 degrees west, not 90 degrees east.
- (c)3:00 p.m. — The right direction but the wrong interval — five hours instead of six. Dividing 90 by 15 gives 6, and this option comes from a slip in that division or from importing the five-and-a-half-hour Indian Standard Time offset.
Longitude and time are two ways of describing the same rotation. A full turn of 360 degrees takes 24 hours, so 15 degrees equals one hour and one degree equals four minutes. Local time therefore runs ahead of Greenwich to the east and behind it to the west. Standard time is a separate idea layered on top: a country picks one meridian and applies its local time across the whole territory, which is why India runs on 82 degrees 30 minutes east and is five and a half hours ahead of Greenwich, rather than letting Gujarat and Arunachal Pradesh keep different clocks.
Every question of this family is solved by two decisions — how many hours, and which way. The interval is the longitude difference divided by 15. The direction is east means ahead, west means behind, which is worth fixing with a physical image rather than a rule: the sun rises over the eastern meridian first, so the eastern clock has already got further into the day. Here the difference from Greenwich is the full 90 degrees, giving a clean 6 hours, and the direction is east, so you add. The examiners build the wrong options out of exactly the two mistakes available — a sign error and an arithmetic slip — so a candidate who checks both is safe. Note that 90 degrees east is close to the meridian of Bangladesh's standard time, six hours ahead of Greenwich, which is a good sanity check on the answer.
- The earth rotates 360 degrees in 24 hours, so 15 degrees of longitude equal one hour and one degree equals four minutes.
- Places east of the Greenwich meridian are ahead of Greenwich Mean Time; places west of it are behind.
- 90 degrees east is exactly 6 hours ahead of Greenwich, so 10:00 a.m. GMT is 4:00 p.m. there.
- India's standard meridian is 82 degrees 30 minutes east, which puts Indian Standard Time five and a half hours ahead of Greenwich.
- India keeps a single standard time for the whole country despite spanning roughly 30 degrees of longitude, about two hours of local time.
Two decisions decide every item of this type — the interval, and the sign.
- Subtracting when the place is east of Greenwich.
- Importing the five-and-a-half-hour Indian Standard Time offset into a Greenwich calculation.
- Confusing local time, which varies continuously with longitude, with standard time, which is fixed by law for a whole country.
As a two-step arithmetic item — divide the longitude difference by 15, then add or subtract — sometimes with the Date Line added to make a third step.
If it is 10.00 a.m. I.S.T., then what would be the local time at Shillong on 92°E longitude?
- (a) 9.38 a.m.
- (b) 10.38 a.m.
- (c) 10.22 a.m.
- (d) 9.22 a.m.
Answer(b) 10.38 a.m.
The identical calculation with a harder reference meridian. There the starting point is India's 82 degrees 30 minutes east rather than Greenwich, so the difference is 9.5 degrees and the answer runs in minutes, but the four-minutes-per-degree rule and the east-means-ahead rule are the same.
When there is noon at the G.M.T. meridian, people at another place of the Earth are taking their 6 o’clock morning tea. The longitude of the place is
- (a) 17° 30′ E
- (b) 7° 30′ W
- (c) 172° 30′ E
- (d) 90° W
Answer(d) 90° W
The same relation run backwards — a six-hour gap converted into 90 degrees of longitude, with the morning time fixing the direction as west. Being able to work it in either direction is what makes this family of questions safe.
CDS_GK_2021_I_Q702021Which one of the following longitudes is the standard meridian of India?
- (a) 83°30' E
- (b) 82°30' E
- (c) 82°30' W
- (d) 83°30' W
Answer(b) 82°30' E
The fact that turns this arithmetic into Indian geography. Once the standard meridian is fixed, the five-and-a-half-hour offset from Greenwich follows from the same fifteen-degrees-per-hour rule.
CDS_GK_2021_II_Q632021Which of the following statements is/are correct with respect to Time Zone in India? 1. There is one standard time for the whole country. 2. Andaman and Nicobar Islands and Lakshadweep Islands have different Time Zones. 3. Indian Standard Time (IST) is five and half hours behind GMT. Select the correct answer using the code given below:
- (a) 1 only
- (b) 2 only
- (c) 1 and 2 only
- (d) 1 and 3 only
Answer(a) 1 only
Statement 3 there fails on the very point that decides this question — India lies east of Greenwich, so Indian Standard Time is ahead of it, not behind. The sign is what both items are really about.
- practice — not a real PYQ
If it is 12 noon at Greenwich Mean Time, what is the local time at 45 degrees west longitude?
- (a)9:00 a.m.
- (b)3:00 p.m.
- (c)9:00 p.m.
- (d)6:00 a.m.
Answer(a) 9:00 a.m. — 45 divided by 15 gives 3 hours, and a western meridian is behind Greenwich, so subtract.
- practice — not a real PYQ
Indian Standard Time is calculated from which meridian?
- (a)80 degrees east
- (b)82 degrees 30 minutes east
- (c)85 degrees east
- (d)90 degrees east
Answer(b) 82 degrees 30 minutes east — the meridian passing near Mirzapur, which puts India five and a half hours ahead of Greenwich.