If it is 12.00 Noon in India, on which meridian will it be 7.00 am of the same day?
- (a)7.5 degree E. Longitude
- (b)7.5 degree W. Longitude
- (c)75 degree E. Longitude
- (d)75 degree W. Longitude
Correct — A, 7.5 degree E. Longitude. Indian Standard Time is fixed on the 82.5° E meridian, where it is 12:00 noon. A place showing 7:00 am is 5 hours earlier, so it lies to the west. Since 1 hour of time equals 15° of longitude, 5 hours equals 5 × 15° = 75°. Moving 75° west of 82.5° E gives 82.5° − 75° = 7.5° E — so it is 7:00 am on the 7.5° E meridian.
- (b)7.5 degree W. Longitude — 7.5° W is 90° west of the 82.5° E IST meridian, which is a 6-hour difference — it would be 6:00 am there, not 7:00 am.
- (c)75 degree E. Longitude — 75° E is only 7.5° west of 82.5° E, a difference of just 30 minutes, so the time there would be about 11:30 am, not 7:00 am.
- (d)75 degree W. Longitude — 75° W is 157.5° west of the IST meridian — a difference of about 10.5 hours — giving roughly 1:30 am, far from 7:00 am.
Local time depends on longitude. The Earth turns 360° in 24 hours, so 15° of longitude equals 1 hour, and 1° equals 4 minutes. Places to the east are ahead in time; places to the west are behind. Indian Standard Time is based on the 82.5° E meridian (passing near Mirzapur).
Convert the time difference into a longitude difference: 5 hours behind × 15° per hour = 75° to the west. Subtracting from the IST meridian (82.5° E) lands on 7.5° E. The trap options either mislabel the direction (E vs W) or use 75° instead of the difference from 82.5° E.
- Indian Standard Time is based on the 82.5° E meridian.
- 1 hour of time = 15° of longitude; 1° = 4 minutes.
- Places east of a meridian are ahead in time; places west are behind.
- 5 hours earlier ⇒ 75° to the west of 82.5° E ⇒ 7.5° E.
5 hours west of the 82.5° E IST meridian is 7.5° E — option (a).
- Getting the east/west direction wrong — an earlier time means a westward meridian.
- Using 75° as a longitude value instead of subtracting the 75° difference from 82.5° E.
Asked as 'if it is time T at longitude X, what is the time/longitude at Y', using the 1 hour = 15° conversion.
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. — Shillong (92° E) is 9.5° east of the 82.5° E IST meridian, i.e., 38 minutes ahead.
Same computation — converting a longitude difference from the 82.5° E IST meridian into a time difference (4 minutes per degree / 15° per hour).
Consider the following statements regarding the International Date Line: 1. It is roughly 180° meridian, which deviates slightly East and West to avoid land areas surrounded by the Atlantic Ocean. 2. The date to the East of this line is (24 hours) earlier than to the West. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(b) 2 only
Same theme — longitude, time and date across meridians, extending the local-time reasoning of this question to the change of date at the 180° line.
- practice — not a real PYQ
Indian Standard Time is calculated on the basis of which meridian?
- (a)75° E
- (b)82.5° E
- (c)90° E
- (d)0° (Prime Meridian)
Answer(b) 82.5° E — the standard meridian for IST, passing near Mirzapur.
- practice — not a real PYQ
How many degrees of longitude does the Earth rotate through in one hour?
- (a)1°
- (b)4°
- (c)15°
- (d)24°
Answer(c) 15° — the Earth turns 360° in 24 hours, i.e., 15° per hour.