What is Indian Standard Time (IST)?

Why time depends on longitude

The Earth spins on its axis from west to east. Place a lamp beside a globe and turn the globe slowly eastward: some places are in morning light, others at midday, others in evening or night. When it is breakfast time in one country, it is lunchtime in another, and in a third people are fast asleep. So measuring the longitude of a place also tells us about its time.

The Earth makes one full turn, 360°, in about 24 hours. That is:

• 360° ÷ 24 = 15° every hour, and
• 60 minutes ÷ 15 = 4 minutes for every 1° of longitude.

Local time

Mark the meridians every 15° from Greenwich. Each one eastward adds an hour of local time; each one westward takes an hour away.

If it is 12 noon at Greenwich, it is 1 pm at 15°E, 2 pm at 30°E and 3 pm at 45°E. Going west it is the other way round: 11 am at 15°W, 10 am at 30°W and 9 am at 45°W.

To compare two places, multiply their difference in longitude by 4 minutes. The place to the east is ahead.

Example. Two friends talk on the phone late one afternoon, one in Tinsukia (Assam) and the other in Porbandar (Gujarat). The one in Tinsukia says the sun has set and it is dark; the one in Porbandar still has full daylight. Taking them to be about 30° apart, the difference in local time is 30 × 4 = 120 minutes, or 2 hours. The sun reaches Assam, far to the east, much earlier. (Extra: their real longitudes, about 95.4°E and 69.6°E, are nearly 26° apart: a difference of about 1 hour 43 minutes.)

Standard time and IST

Every meridian has its own local time, so a country using all of them would be confused: trains, offices and phone calls could not agree. That is why most countries adopt one standard time, based on a meridian passing through them.

India’s standard time, Indian Standard Time (IST), is 5 hours 30 minutes, or 5.5 hours, ahead of the local time at Greenwich, called Greenwich Mean Time (GMT). It is the local time of the meridian at 82°30′E, which passes near Mirzapur in Uttar Pradesh: 82.5 × 4 = 330 minutes, or 5 hours 30 minutes.

India stretches from about 68°E to 97°E, about 29° of longitude, which is nearly 2 hours of local time. When clocks everywhere show 12 noon IST, the sun time in the far west of Gujarat is close to 11 am, and in the far east of Arunachal Pradesh nearly 1 pm. In Assam, local time is ahead of IST, which is why the sun rises and sets early there.

Time zones of the world

Standard times are organised into time zones, which broadly follow 15° bands. The lines between them are not straight, though: they follow international borders so that each country keeps its own standard time. On a time zone map, a number like +5:30 (India) is added to GMT, while a number with a minus sign is subtracted from it.

Not every country has a single standard time. Russia, Canada and the USA are too large for one. The USA has six time zones and Russia has eleven, so travelling across Russia from east to west you would reset your watch 10 times.

The International Date Line

Opposite the Prime Meridian, at about 180° longitude, runs the International Date Line. The +12 and −12 time zones touch along it. Crossing it by ship or plane changes the date:

• travelling eastward, you subtract a day (Monday becomes Sunday);
• travelling westward, you add a day (Sunday becomes Monday).

The line is only ‘approximately’ at 180° because it bends in places so that no country is divided into two different days. For latitudes and longitudes, see latitudes and longitudes; for every textbook question, see the Chapter 1 NCERT solutions.