GeographyCBSE Class 11 · India – Location

The Standard Meridian and Indian Standard Time (Class 11 Geography, India: Physical Environment, Chapter 1)

Updated 17 October 2026 · Fact-checked

India's longitudinal spread of nearly 30 degrees causes a sun-time difference of nearly two hours between the east and west (the earth turns 1 degree in 4 minutes). A single standard meridian, 82 degrees 30 minutes E, a multiple of 7 degrees 30 minutes, fixes Indian Standard Time at 5 hours 30 minutes ahead of GMT for the whole country.

India spans about 30° of longitude (2 hours of sun time), yet one meridian, 82°30′E, sets IST at GMT + 5:30 for everyone.

Illustration drawn for QuestionBuddy.

Understand Longitudinal Extent, Time Difference, the Standard Meridian and Indian Standard Time

Why this topic matters. Calculating local time from longitude and explaining why India uses one time zone are favourite numerical and reasoning questions.

1. Time and longitude. The earth rotates 360 degrees in 24 hours, so 1 degree of longitude equals 4 minutes of time and 15 degrees equals 1 hour. Time is ahead towards the east and behind towards the west.

2. India's spread. The mainland extends from about 68 degrees 7 minutes E to 97 degrees 25 minutes E, nearly 30 degrees. This gives a time difference of nearly two hours (30 x 4 = 120 minutes). The sun rises in the north-eastern states about two hours earlier than in Jaisalmer.

3. One time for all. Yet watches in Dibrugarh, Imphal, Jaisalmer, Bhopal and Chennai show the same time, because India uses a standard meridian.

4. Choice of meridian. Countries generally choose standard meridians in multiples of 7 degrees 30 minutes of longitude (which equals 30 minutes of time). India's standard meridian is 82 degrees 30 minutes E.

5. IST. Indian Standard Time is the local time of 82 degrees 30 minutes E: 82.5 degrees x 4 minutes = 330 minutes = 5 hours 30 minutes ahead of Greenwich Mean Time.

6. Several zones elsewhere. Countries with a vast east-west extent may use several standard meridians; the USA has several time zones (four in the contiguous states).

7. Local time at a place. Local (sun) time differs from IST by 4 minutes for each degree east or west of 82 degrees 30 minutes E. For example, when it is 6:00 am IST, a place at 92 degrees 30 minutes E has sun time 6:40 am (10 degrees x 4 = 40 min ahead), and a place at 72 degrees 30 minutes E has 5:20 am (40 min behind), derived from these rules.

Longitudinal Extent, Time Difference, the Standard Meridian and Indian Standard Time: key facts

Time and longitude: $360 d e g r ees = 24 h o u r s ∣1 d e g r ee = 4 min u t es ∣15 d e g r ees = 1 h o u r ∣7 d e g r ees 30 min u t es = 30 min u t es$; East is ahead, west is behind.
India's standard time: $S t an d a r d m er i d ian 82 d e g r ees 30 min u t es E ∣82.5 x 4 = 330 min u t es ∣ I S T = GM T + 5 h 30 min$; One time for the whole country.
Time spread of India: $L o n g i t u d es p r e a d n e a r l y 30 d e g r ees ∣30 x 4 = 120 min u t es ∣ N e a r l y tw o h o u r s$; East to west sun-time difference.
Local time rule: $L oc a l t im e = I S T + 4 min x (l o n g i t u d e - 82.5 d e g r ees E) ∣ A dd t o w a r d se a s t, s u b t r a c tt o w a r d s w es t$; Derived from the 4-minute rule.

How to answer Longitudinal Extent, Time Difference, the Standard Meridian and Indian Standard Time questions

Local-time questions use the 4 minutes per degree rule.

1Find the difference in longitude from 82 degrees 30 minutes E, converting minutes to decimal degrees.
2Multiply by 4 minutes to get the time difference.
3Add if the place is east of the meridian, subtract if west.
4State the answer in hours, minutes and the unit (am or pm).

Longitudinal Extent, Time Difference, the Standard Meridian and Indian Standard Time in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Early sunrise in the north-east

Suppose a schoolchild in the far north-east says it is already bright at 4:30 am while a child in the far west says it is dark until nearly 6:30 am, yet both clocks show the same time.

Why do the clocks agree?

Try it first, then show the solutionSolution

Both places follow the standard meridian time.
The sun-time difference is about two hours.

Answer: India follows one standard time, IST, based on 82 degrees 30 minutes E.

Memory hook: One clock, many suns.

Cricket broadcast

Suppose a match is telecast at 9:00 am IST and a viewer asks the local time at a city whose longitude is 15 degrees west of the standard meridian.

What is the sun time there?

Try it first, then show the solutionSolution

15 degrees x 4 = 60 minutes.
It is west, so behind: 8:00 am.

Answer: Local sun time is 8:00 am.

Memory hook: West is behind.

Tricks to remember Longitudinal Extent, Time Difference, the Standard Meridian and Indian Standard Time

Short memory aids. Say each one aloud once, then use it on a question the same day.

4 minutes a degree
360 degrees in 24 hours so 1 degree is 4 minutes
Use it when: Core rule.
82.5 x 4 = 330
5 hours 30 minutes ahead of GMT
Use it when: IST.
East ahead, west behind
Add east, subtract west
Use it when: Direction.

Longitudinal Extent, Time Difference, the Standard Meridian and Indian Standard Time: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: IST offset · 1 mark · MCQBoard level

Indian Standard Time is ahead of Greenwich Mean Time by:
(A) 4 hours 30 minutes
(B) 5 hours
(C) 5 hours 30 minutes
(D) 6 hours 30 minutes

Try it first, then show the solutionSolution

82.5 x 4 = 330 minutes = 5 h 30 min (1 mark).

Answer: (C)

Example 2: Why one standard time · 2 marks · Very short answerBoard level

Why is a single standard time used in India although the east and west differ by nearly two hours of sun time? (2 marks)

Try it first, then show the solutionSolution

A standard meridian (82 degrees 30 minutes E) is chosen (1 mark).
All places follow its time to avoid confusion (1 mark).

Answer: India uses a standard meridian, 82 degrees 30 minutes E, and the whole country follows its time, so watches show the same time everywhere even though local sun time differs by nearly two hours.

Example 3: Local time calculation · 3 marks · Short answerHarder board level

When it is 8:15 pm IST, find the local sun time at a place on 94 degrees E and at one on 70 degrees E. (3 marks)

Try it first, then show the solutionSolution

94 degrees E is 94 - 82.5 = 11.5 degrees east (1 mark).
11.5 x 4 = 46 min ahead, so 8:15 pm + 46 min = 9:01 pm (1 mark).
70 degrees E is 12.5 degrees west; 12.5 x 4 = 50 min behind; 8:15 pm - 50 min = 7:25 pm (1 mark).

Answer: At 94 degrees E the place is 11.5 degrees east of the meridian, so it is 11.5 x 4 = 46 minutes ahead: 9:01 pm. At 70 degrees E the place is 12.5 degrees west, so it is 12.5 x 4 = 50 minutes behind: 7:25 pm.

Example 4: Assertion and reason: 7 degrees 30 minutes · 1 mark · Assertion-reasonHarder board level

Assertion (A): Countries usually select standard meridians that are multiples of 7 degrees 30 minutes.
Reason (R): 7 degrees 30 minutes of longitude equals exactly 30 minutes of time.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true

Try it first, then show the solutionSolution

7.5 x 4 = 30 minutes, so such meridians give times differing from GMT by whole half-hours (1 mark).

Answer: (A)

Example 5: HOTS: a separate time zone · 5 marks · Long answerHOTS / competency

A group suggests two time zones for India to match the sun in the north-east. Using the chapter, estimate the current sun-time gap, state one advantage of the proposal and one difficulty of splitting the time. (5 marks)

Try it first, then show the solutionSolution

Spread nearly 30 degrees x 4 = 120 minutes, about two hours (1 mark).
Advantage: working hours would match daylight better in the east (1 mark).
Difficulty: a single time avoids confusion in trains, communication and administration (1 mark).
Countries with vast east-west extent may use several meridians, e.g. the USA with several time zones (1 mark).
So it is a matter of balance between sun time and uniformity (1 mark).

Answer: The longitudinal spread of nearly 30 degrees gives 30 x 4 = 120 minutes, so the sun-time gap is nearly two hours. A second zone could align working hours with daylight in the north-east. The chapter explains why one standard meridian is used: it gives uniform time for administration and communication (the difficulty of splitting is an inference, not stated). Countries with a vast east-west spread, such as the USA with several time zones, do use several meridians, so the choice balances sun time against uniformity.

Where students lose marks in Longitudinal Extent, Time Difference, the Standard Meridian and Indian Standard Time

The slips examiners see most often, and what to write instead.

Mistake: Adding for a place in the west
Direction forgotten.
Wrong working
West place: 6:00 + 57 min.
Right working
West is behind, so subtract: 6:00 - 57 min.
Do this: East add, west subtract.
Usually costs: 1 mark.
Mistake: Using 1 degree = 1 hour
Confusing 15 degrees and 1 degree.
Wrong working
1 degree equals 1 hour.
Right working
15 degrees = 1 hour; 1 degree = 4 minutes.
Do this: Memorise 360 / 24 = 15.
Usually costs: 1 mark.
Mistake: Not converting minutes of arc to degrees
Mix of 82 degrees 30 minutes with 82.30.
Wrong working
82 degrees 30 minutes = 82.30 degrees.
Right working
30 minutes = 0.5 degree, so 82.5 degrees.
Do this: Divide minutes by 60.
Usually costs: 1 mark.

Exam tips

Always show the 4 minutes per degree step.
Convert minutes of arc to decimal degrees before multiplying.
State east ahead or west behind in the final line.

Practice questions on Longitudinal Extent, Time Difference, the Standard Meridian and Indian Standard Time

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The standard meridian of India is:
(A) 68 degrees 7 minutes E
(B) 75 degrees E
(C) 82 degrees 30 minutes E
(D) 97 degrees 25 minutes E
Hint
Multiple of 7 degrees 30 minutes.
Answer
(C)
Q2 · 2 marks · Very short answerBoard level
How much time does the earth take to rotate through 1 degree of longitude? (2 marks)
Hint
360 over 24.
Answer
The earth turns 360 degrees in 24 hours, so 1 degree takes 24 x 60 / 360 = 4 minutes.
Q3 · 3 marks · Short answerHarder board level
A place lies at 72 degrees 30 minutes E. Find its local sun time when IST reads 12:00 noon. (3 marks)
Hint
82.5 x 4.
Answer
The place is 82.5 - 72.5 = 10 degrees west of the standard meridian. 10 x 4 = 40 minutes. West is behind, so local time = 12:00 - 0:40 = 11:20 am.
Q4 · 3 marks · Short answerHarder board level
When it is 10:00 am IST, what is the sun time at a place 7 degrees 30 minutes west of the standard meridian? (3 marks)
Hint
West is behind.
Answer
7 degrees 30 minutes = 7.5 degrees; 7.5 x 4 = 30 minutes. The place is west, so it is behind: local time is 9:30 am.
Q5 · 4 marks · Short answerHOTS / competency
Town A lies at 87 degrees 30 minutes E and town B at 77 degrees 30 minutes E. Find their local sun times when IST reads 3:00 pm and the time gap between them, and say which has the earlier sunrise. (4 marks)
Hint
Both relative to 82.5.
Answer
Town A is 87.5 - 82.5 = 5 degrees east: 5 x 4 = 20 min ahead, so 3:20 pm. Town B is 82.5 - 77.5 = 5 degrees west: 20 min behind, so 2:40 pm. The gap is 10 degrees x 4 = 40 minutes. Town A, being further east, gets the earlier sunrise.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What time is IST relative to GMT?Show answer
5 hours 30 minutes ahead.
2.Which meridian is India's standard meridian?Show answer
82 degrees 30 minutes E.
3.Name a country with several time zones.Show answer
The USA (Russia is another example).
4.How much time does 15 degrees represent?Show answer
1 hour.

Longitudinal Extent, Time Difference, the Standard Meridian and Indian Standard Time: frequently asked questions

Is the standard meridian in India Greenwich?

No; it is 82 degrees 30 minutes E.

Why multiples of 7 degrees 30 minutes?

They correspond to whole half-hours of time.

Does India have local sun time differences?

Yes, nearly two hours between east and west.