What is the local time of Thimphu (Bhutan) located at 90° East longitude when the time at Greenwich (0°) is 12:00 noon?

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (b) 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) 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) 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.

Concept: 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.

State PCS 2023 Q94 — Geography PYQ with Answer

State PCS 2023 Q94 — Geography PYQ with Answer