BPSC 69th CCE Prelims General Studies · Question 94 of 144

Correct — A, 6:00 p.m. The whole question is one division and one sign, and both are worth doing on paper rather than recalling. The Earth completes one rotation of 360° in 24 hours, so it turns 360 ÷ 24 = 15° of longitude in every hour; invert that and one degree of longitude is worth 60 ÷ 15 = 4 minutes of time, one minute of arc is worth 4 seconds, and one second of arc is worth a fifteenth of a second. The stem places Thimphu at 90° East, so the gap from Greenwich is 90 ÷ 15 = 6 hours — or, by the per-degree route, 90 × 4 = 360 minutes, which is the same 6 hours. The sign comes from the direction of spin: the Earth rotates from west to east, so a place east of the prime meridian meets the Sun earlier in the day and its clock is ahead of Greenwich. Thimphu is east, so you add. 12:00 noon + 6 hours = 18:00 hours = 6:00 p.m., which is option (a). The frame of reference the question assumes was fixed at the International Meridian Conference held at Washington in October 1884, convened at the request of President Chester A. Arthur to settle 'a meridian to be employed as a common zero of longitude and standard of time reckoning throughout the world'. The resolution adopting the Greenwich meridian carried 22 votes to 1 — San Domingo, now the Dominican Republic, voting against, with France and Brazil abstaining — and the conference further resolved that longitude be counted east and west from that meridian up to 180°, and defined a universal day beginning at mean midnight at Greenwich. It pointedly did not adopt time zones; standard time was expressly exempted from the universal day, and no proposal on zones was even put to a vote. That is worth knowing here, because the exemption is exactly the distinction this question turns on. So be precise about what is being asked. 'Local time' is the mean solar time of the meridian named, an astronomical quantity, and not the legal clock a government chooses to run. For Bhutan the two land on the same reading, because Bhutan Time is UTC+06:00 with no daylight saving — an offset that corresponds to exactly 90° East — but that is a fact about Bhutan's statute book, not the reason this answer is right. Thimphu itself stands at 27°28′ N, 89°38′ E, roughly 22 minutes of arc west of the 90° meridian, so the city's true local mean solar time at 12:00 GMT works out closer to 5:58½ p.m. than to a clean 6:00. A sundial in Thimphu would disagree again, running as much as 16 minutes 33 seconds fast around 3 November and 14 minutes 6 seconds slow around 11 February, because apparent solar time and mean solar time part company through the year. Three defensible readings therefore sit within twenty minutes of each other; the stem rounds the city to its standard meridian, as every exam of this type does, so answer to the 90° figure it gives you.

• (b)4:00 p.m. — A four-hour lead belongs to 60° E, the standard meridian of the Gulf zone at UTC+04:00 used by the United Arab Emirates and Oman, not to 90° E. To reach it from this stem you would have to divide 90 by 22½ instead of by 15, which is not a rate the Earth's rotation supplies. The option catches the candidate who never performs the division at all and works from a vague memory that Bhutan is 'a few hours ahead of GMT' — a memory that is right in kind and wrong in size. It also catches a second slip, measuring from India instead of from Greenwich: Bhutan is only half an hour ahead of IST, but the stem's reference is the prime meridian, and half an hour is not four hours either.
• (c)7:00 p.m. — Seven hours ahead corresponds to 105° E, the standard meridian of Indochina Time, kept by Thailand, Cambodia, Laos and Vietnam and shared by Western Indonesian Time. This is the classic off-by-one: the candidate divides correctly to get six hours and then adds one more 'because it is east'. The eastward direction has already been carried by the plus sign in 12:00 + 6:00; applying it twice double-counts. A quick guard against it is to check the answer against a meridian you know — 82°30′ E is IST at UTC+05:30 and 90° E is only 7½° further east, which is half an hour of time, so the answer must sit half an hour past 5:30 p.m. and nowhere near 7:00.
• (d)6:00 a.m. — Right magnitude, wrong direction — and this is the trap the paper is really setting, because (a) and (d) are the same six-hour gap pointing opposite ways. Six hours behind Greenwich is 90° West, the meridian that underlies North American Central Standard Time and runs through the middle of the United States. Bhutan lies east of Greenwich, so its clock must read later than Greenwich's, never earlier; at the instant it is noon at Greenwich, 90° E reads 6:00 p.m. and its mirror at 90° W reads 6:00 a.m. UPSC built an entire question on that mirror case in 1998, giving the 6 a.m. reading and asking for the longitude, with 90° W as the key. The same pair of facts answers both papers; only the direction of travel changes.

The meridian through Greenwich was fixed as 0° longitude by the International Meridian Conference at Washington in October 1884, and the same conference tied the reckoning of the day to it, giving the world one zero for both position and time. Because the Earth turns through 360° in 24 hours, longitude and time are two readings of a single quantity: 15° = 1 hour, 1° = 4 minutes, 1 minute of arc = 4 seconds. Every place on a different meridian therefore has its own local, or mean solar, time. Running a country on true local time is unworkable — the sun rises and sets almost two hours earlier on India's eastern border than in the Rann of Kutch, because the country spans over 29° of longitude — so states adopt one standard meridian and apply its offset nationwide, usually choosing a whole multiple of 15° or 7½°. India settled on 82°30′ E, which passes near Vindhyachal in Mirzapur district of Uttar Pradesh; Indian Standard Time was adopted on 1 January 1906, replacing Madras or Railway Time, though Calcutta Time survived until 1948 and Bombay Time until 1955, and the National Physical Laboratory now maintains IST. Nepal uses 86°15′ E at UTC+05:45, Bhutan and Bangladesh 90° E at UTC+06:00, and Myanmar 97°30′ E at UTC+06:30. When a question quotes a meridian and a Greenwich time, it is asking for local mean solar time on that meridian, not for anyone's civil clock.

Do not try to recall Bhutan's time zone — compute it, because the computation is two decisions and both are visible in the options. The first is magnitude: 90 ÷ 15 = 6 hours, equivalently 90 × 4 minutes = 360 minutes. That single step kills (b), which needs a four-hour offset belonging to 60° E, and (c), which needs seven hours belonging to 105° E. The second decision is the sign, and it is the discriminating one, because (a) and (d) are the very same six-hour gap pointing in opposite directions. The rule behind the sign is physical rather than arbitrary: the Earth rotates west to east, so dawn sweeps across the globe from east to west and an eastern place has already had more of its day. East of Greenwich, add; west of Greenwich, subtract — 'east gain, west lose'. At the instant it is noon at Greenwich, 90° E reads 6:00 p.m. and its mirror at 90° W reads 6:00 a.m. Candidates lose this question on the sign far more often than on the division, which is precisely why the paper offers both readings. A second check, worth thirty seconds in the exam, is to anchor on a meridian you already carry: IST sits on 82°30′ E at UTC+05:30, and 90° E is 7½° further east, which at 4 minutes a degree is exactly 30 minutes more — so the answer has to be 6:00 p.m. and can be verified without repeating the first division. Finally, keep the vocabulary straight. Local time is astronomical and continuous, changing by four minutes for every degree you walk east; standard time is legal and steps in blocks; and Coordinated Universal Time is the atomic reference both are now quoted against. The stem says 'local time' and gives a meridian, so it wants the astronomical quantity — and the fact that Bhutan's civil clock happens to agree is a coincidence of that country's choice of standard meridian, not part of the answer.

• The Earth rotates 360° in 24 hours, so 15° of longitude = 1 hour of time, 1° = 4 minutes, 1 minute of arc = 4 seconds and 1 second of arc = one-fifteenth of a second
• 90° E is exactly 90 ÷ 15 = 6 hours ahead of Greenwich, so 12:00 noon GMT is 18:00 hours — 6:00 p.m. — on that meridian; the mirror meridian 90° W is 6 hours behind and underlies North American Central Standard Time, which is exactly the reading offered by option (d)
• Greenwich became the world's prime meridian at the International Meridian Conference, Washington, October 1884; the resolution passed 22 votes to 1 with San Domingo against and France and Brazil abstaining, and France did not adopt it until 1911, calling it 'Paris mean time, retarded by 9 minutes and 21 seconds' until 1978
• The 1884 conference fixed the zero of longitude and a universal day beginning at mean midnight at Greenwich, but it deliberately did NOT create time zones — standard time was expressly exempted and no zone proposal was put to a vote
• Bhutan Time is UTC+06:00, observes no daylight saving and carries the IANA zone name Asia/Thimphu; Bangladesh keeps the same +06:00 offset, so Bhutan's civil clock and the 90° E local time agree — a coincidence of Bhutan's chosen standard meridian, not the reason for the answer
• Thimphu's actual position is 27°28′ N, 89°38′ E — about 22′ of arc west of the 90° meridian — so its true local mean solar time at 12:00 GMT is nearer 5:58½ p.m. than 6:00 p.m.
• Local mean solar time is itself an average: a sundial runs as much as 16 minutes 33 seconds fast around 3 November and 14 minutes 6 seconds slow around 11 February, the equation of time being zero near 15 April, 13 June, 1 September and 25 December
• Indian Standard Time is calculated from 82°30′ E, passing near Vindhyachal in Mirzapur district, Uttar Pradesh; it was adopted on 1 January 1906 in place of Madras (Railway) Time, while Calcutta Time survived to 1948 and Bombay Time to 1955, and the National Physical Laboratory maintains it today
• India spans more than 29° of longitude and about 2,933 km east to west, so the sun rises almost two hours earlier on the eastern border than in the Rann of Kutch — the standing argument for a second Indian time zone
• Neighbouring standard meridians for comparison: Nepal 86°15′ E (UTC+05:45), Bhutan and Bangladesh 90° E (UTC+06:00), Myanmar 97°30′ E (UTC+06:30) — the quarter- and half-hour offsets that make a whole-degree division insufficient on their own

• Reversing the sign. East of Greenwich is ahead, west is behind; option (d) is the correct six hours applied the wrong way round, and it is UPSC's own 1998 answer transplanted into a BPSC distractor set.
• Reading 'local time' as a country's civil clock. The exam always means the mean solar time of the meridian quoted — for Bhutan the two coincide, but for most places they do not, and for a city at 89°38′ E even the astronomical figure is not a round number.
• Mis-scaling the conversion: 1° of longitude equals 4 minutes of time, while 1 minute of arc equals 4 seconds. Mixing the two turns a 38-minute answer into a 38-second one.
• Measuring from India instead of from Greenwich. Bhutan is only 30 minutes ahead of IST but six hours ahead of GMT, and a stem that names Greenwich wants the second figure.

BPSC keeps this as a one-step numerical — a meridian, a reference time, four clock readings — and the marks sit in the sign rather than in the arithmetic, which is why the option set contains a matched pair six hours either side of noon. UPSC has asked the identical conversion in every direction: give the clock reading and ask for the longitude (1998), give IST and ask for the local time at a named Indian city on a stated meridian (1999), even reverse the Earth's spin and ask what IST would then read (1997). The practical preparation is therefore not to memorise offsets but to be able to run the conversion both ways in under a minute, and to carry two anchors — 82°30′ E for IST and 15° per hour — from which every other meridian on an Indian paper can be derived.

• practice — not a real PYQ

  When it is 12:00 noon at Greenwich, what is the Indian Standard Time?

  • (a)5:00 p.m.
  • (b)5:30 p.m.
  • (c)6:00 p.m.
  • (d)4:30 p.m.

  Answer(b) 5:30 p.m. — IST is fixed on 82°30′ E, and 82.5 ÷ 15 = 5 hours 30 minutes ahead of Greenwich.

• practice — not a real PYQ

  Two places lie on the 30° E and 45° W meridians respectively. What is the difference between their local times?

  • (a)1 hour
  • (b)3 hours
  • (c)5 hours
  • (d)7 hours

  Answer(c) 5 hours — the meridians lie on opposite sides of Greenwich, so the separation is 30 + 45 = 75°, and 75 ÷ 15 = 5 hours.