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Class 11 India — Location: cheat sheet
CBSE Social Science. India’s size and setting on the Indian Ocean, its standard meridian and Indian Standard Time, local time and longitude, its coastline, islands and territorial waters.
Key ideas
- India covers about 3.28 million sq km, about 2.4% of the world’s land, and is the seventh largest country.
- The earth turns 360° in 24 hours, so every degree of longitude makes 4 minutes of difference in local (sun) time.
- Places east of a meridian see the sun earlier, so their local time is ahead; places to the west are behind.
- India spreads over nearly 30° of longitude, a gap of about two hours in local time, so one standard time keeps all clocks in step.
- Standard meridians are chosen in multiples of 7°30′ (30 minutes of time); India’s is 82°30′ E, so IST is 5 hours 30 minutes ahead of GMT.
- A long coastline and two island groups (Andaman and Nicobar in the Bay of Bengal, Lakshadweep in the Arabian Sea) link India to the world by sea.
Remember
| Time per degree | 4 minutes of time for 1° of longitude |
| Local time from IST | local time = IST + (longitude − 82.5) × 4 minutes (east of 82°30′ E: ahead) |
| IST and GMT | IST = GMT + 5 h 30 min (82.5 × 4 = 330 minutes) |
| A standard meridian | longitude × 4 minutes ahead of GMT; 7°30′ = 30 minutes |
| Nautical miles | 1 nautical mile ≈ 1.852 km: km = nautical miles × 1.852 |
| Territorial waters | up to 12 nautical miles (about 21.9 km) from the coast |
| Coastline | about 6,100 km on the mainland; 7,517 km with the islands |
Watch out
- Subtracting time for a place to the east: places east of 82°30′ E are ahead of IST.
- Going from IST to GMT by adding 5 h 30 min (subtract it).
- Multiplying by 15 instead of 4: 15° makes one hour, so each degree is 4 minutes.
Worked example
Suppose the sun rises at 05:00 local time at two places on the same latitude, one on 94° E and one on 84°30′ E. (a) How many minutes apart are their local times? (b) Express each sunrise in IST (82°30′ E). (c) Why does all of India use one time?
Working: local time − (longitude − 82.5) × 4 min = IST
Answer: (a) 38 minutes (b) 94° E: 04:14 IST; 84°30′ E: 04:52 IST (c) One standard time keeps all clocks, transport and offices in step across the country.
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