GeographyICSE Class 9 · Geographic Grid: Latitudes and Longitudes

Latitudes, Longitudes and Calculating Time (ICSE Class 9 Geography)

Updated 17 October 2026 · Draft, not yet fact-checked

Latitudes are imaginary parallel circles measured in degrees north and south of the equator; longitudes are imaginary lines from pole to pole measured east and west of the Prime Meridian (0 degrees at Greenwich). Because the earth rotates 360 degrees in 24 hours, 15 degrees of longitude equal 1 hour and 1 degree equals 4 minutes; time is ahead in the east and behind in the west.

Latitudes are parallel circles, longitudes run pole to pole; every 15° east of Greenwich adds 1 hour, and every 15° west subtracts 1 hour.

Illustration drawn for QuestionBuddy.

Understand Latitudes, Longitudes and Calculating Time

1. Latitudes. Parallel circles round the earth: the Equator (0 degrees), Tropic of Cancer (23½ degrees N), Tropic of Capricorn (23½ degrees S), Arctic Circle (66½ degrees N), Antarctic Circle (66½ degrees S), and the poles (90 degrees). The equator is the longest; circles become smaller towards the poles. They define heat zones: torrid, temperate and frigid.

2. Longitudes. Imaginary semicircles from the north to the south pole, all of equal length. The *Prime Meridian* (0 degrees) passes through Greenwich, London; the opposite meridian is 180 degrees. They divide the earth into the Eastern and Western Hemispheres.

3. Longitude and time. The earth rotates 360 degrees in 24 hours, so 15 degrees equal 1 hour and 1 degree equals 4 minutes. A place east of Greenwich is ahead of Greenwich Mean Time (GMT); a place west is behind. Local time is based on the sun at noon for a meridian; *standard time* is the time of a chosen meridian used for the whole country: India uses the meridian 82½ degrees E (5 hours 30 minutes ahead of GMT).

4. International Date Line. Roughly along 180 degrees with bends; crossing it from west to east the date is put back by one day, and from east to west a day is added.

5. Great circle. A circle that divides the earth into two equal halves, such as the equator and any meridian pair; it gives the shortest distance between two places, hence used by aircraft.

6. Method. Difference in longitude (add if on opposite sides of 0 degrees, subtract if on the same side) x 4 minutes = time difference; add if east, subtract if west.

Latitudes, Longitudes and Calculating Time: key facts

Time: $15 d e g r ees = 1 h o u r ∣1 d e g r ee = 4 min u t es ∣ E a s t ah e a d, w es t b e hin d$
Main parallels: Equator 0
Tropic of Cancer 23.5 N
Tropic of Capricorn 23.5 S
Arctic Circle 66.5 N
Antarctic Circle 66.5 S
Standard times: Greenwich 0 degrees
India 82.5 degrees E
Ahead of GMT by 5 h 30 min
Date line: About 180 degrees
West to east subtract a day
East to west add a day

Latitudes, Longitudes and Calculating 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.

A video call

Suppose a student in Mumbai plans a call with a cousin in London when it is 4:00 pm in Mumbai, and worries about the hour in London.

Find London time using standard time (ignore daylight saving).

Try it first, then show the solutionSolution

India is 5 h 30 min ahead of GMT.
London on GMT is 5 h 30 min behind: 4:00 pm minus 5:30 = 10:30 am.

Answer: 10:30 am in London.

Memory hook: Behind in the west.

Tricks to remember Latitudes, Longitudes and Calculating Time

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

E-A-W-B
East Ahead, West Behind.
Use it when: Time direction.
15-1-4
15 degrees, 1 hour, 4 minutes per degree.
Use it when: Time conversion.

Latitudes, Longitudes and Calculating Time: solved examples

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

Example 1: Convert · 2 marks · Short answerBoard level

Convert 75 degrees longitude difference into time. [2]

Try it first, then show the solutionSolution

75 x 4 minutes = 300 minutes.
300 minutes = 5 hours.

Answer: 5 hours.

Example 2: Time at a place · 3 marks · Short answerHarder board level

A live concert starts at 8:00 pm GMT. Find the local start time at 60 degrees E and 45 degrees W. [3]

Try it first, then show the solutionSolution

60 degrees E: 60 x 4 = 240 min = 4 h ahead = 12:00 midnight.
45 degrees W: 45 x 4 = 180 min = 3 h behind = 5:00 pm.

Answer: 12:00 midnight (start of the next day) at 60 degrees E; 5:00 pm at 45 degrees W.

Example 3: IST to another place · 4 marks · Long answerHarder board level

When it is 8:00 pm at 82.5 degrees E (Indian Standard Time), find the time at 37.5 degrees W. [4]

Try it first, then show the solutionSolution

Difference of longitude = 82.5 + 37.5 = 120 degrees (opposite sides of 0).
Time difference = 120 x 4 = 480 min = 8 h.
Place is west, so behind: 8:00 pm - 8 h = 12:00 noon.
Same day.

Answer: 12:00 noon (same day).

Example 4: Same side of Greenwich · 3 marks · Short answerHarder board level

A place A is at 30 degrees E and B at 105 degrees E. (a) Find the time difference. (b) If it is 6:30 am at A, find the time at B. [3]

Try it first, then show the solutionSolution

Difference = 105 - 30 = 75 degrees.
Time = 75 x 4 = 300 min = 5 h.
B is east, so ahead: 6:30 am + 5 h = 11:30 am.

Answer: 5 hours; 11:30 am.

Example 5: Case: the flight · 5 marks · Case-basedHOTS / competency

Case. A flight leaves Delhi (77.5 degrees E) at 10:00 am IST and reaches a city at 27.5 degrees W after 11 hours of flying. (i) Difference in longitudes between IST meridian (82.5 degrees E) and 27.5 degrees W. [1] (ii) Time difference. [1] (iii) Local time of arrival. [3]

Try it first, then show the solutionSolution

(i) 82.5 + 27.5 = 110 degrees.
(ii) 110 x 4 = 440 min = 7 h 20 min.
(iii) Arrival at 9:00 pm IST (10:00 am + 11 h); local time = 9:00 pm - 7 h 20 min = 1:40 pm.

Answer: (i) 110 degrees (ii) 7 h 20 min (iii) 1:40 pm local time. (Delhi time is taken as IST.)

Where students lose marks in Latitudes, Longitudes and Calculating Time

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

Mistake: Adding when places are on the same side of Greenwich
Longitudes are added incorrectly.
Do this: Subtract on the same side and add on opposite sides.
Usually costs: 1 mark.
Mistake: Using the wrong direction
East is subtracted.
Do this: East is ahead; west is behind.
Usually costs: 1 mark.
Mistake: Forgetting the unit
Hours/minutes missing.
Do this: Write the unit in each step.
Usually costs: 1 mark.

Exam tips

Show the difference in degrees, minutes and final time.
Mark east or west on every step.
Check if the date changes.
Draw a quick number line of longitudes.

Practice questions on Latitudes, Longitudes and Calculating 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
Which latitude is the longest?
(a) Tropic of Cancer
(b) Equator
(c) Arctic Circle
(d) 60 degrees N
Hint
Longest.
Answer
(b) Equator.
Q2 · 1 mark · Very short answerBoard level
What is the time difference for each degree of longitude? [1]
Hint
4 minutes.
Answer
4 minutes.
Q3 · 1 mark · Very short answerBoard level
Name the meridian on which Indian Standard Time is based. [1]
Hint
82.5.
Answer
82.5 degrees E.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The Equator is a great circle.
Reason (R): It divides the earth into two equal halves.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Definition.
Answer
(a).
Q5 · 3 marks · Short answerHarder board level
When it is 3:00 pm at Greenwich, find the time at 52.5 degrees E. [3]
Hint
Calculate.
Answer
52.5 x 4 = 210 min = 3 h 30 min; 3:00 pm + 3:30 = 6:30 pm.
Q6 · 3 marks · Short answerHarder board level
Find the local time at 135 degrees W when it is 9:00 am at 90 degrees W. [3]
Hint
Same side.
Answer
Difference = 45 degrees; 45 x 4 = 180 min = 3 h; 135 W is further west so behind: 9:00 am - 3 h = 6:00 am.
Q7 · 2 marks · Very short answerBoard level
Why are all meridians of equal length while parallels differ? [2]
Hint
Shape.
Answer
All meridians are semicircles from pole to pole, while parallels become smaller toward the poles.
Q8 · 3 marks · Short answerHarder board level
Why does the date change when crossing the International Date Line, and why is it bent? [3]
Hint
180 degrees.
Answer
Time is 12 h ahead of GMT on the western side of 180° (180°E) and 12 h behind GMT on the eastern side (180°W), so the two sides differ by 24 h (one full day) and the date must be adjusted. The line is bent to avoid dividing a country or island group.
Q9 · 3 marks · Case-basedHOTS / competency
Case. A ship crosses the International Date Line from west to east on a Wednesday. (i) What day will it be after crossing? [1] (ii) Reason. [1] (iii) What if it crosses east to west? [1]
Hint
Back one day.
Answer
(i) Tuesday. (ii) Moving east across the line subtracts a day. (iii) Thursday.
Q10 · 4 marks · Case-basedHOTS / competency
Case. A match in London is shown live at 7:30 pm GMT; the broadcast reaches Delhi (IST) and Los Angeles (120 degrees W). Find both local times (ignore daylight saving). [4]
Hint
Use difference.
Answer
Delhi: 7:30 pm + 5 h 30 min = 1:00 am next day; Los Angeles: 120 x 4 = 480 min = 8 h behind, so 11:30 am.

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.

1.1 degree longitude in time?Show answer
4 minutes.
2.Prime meridian passes through?Show answer
Greenwich.
3.IST meridian?Show answer
82.5 degrees E.
4.Great circle example?Show answer
Equator.
5.Parallel of the Tropic of Cancer?Show answer
23.5 degrees N.

Latitudes, Longitudes and Calculating Time: frequently asked questions

Why does India have one time?

A single standard meridian is used.

What is GMT?

Time at Greenwich.

Why do we have time zones?

Because the sun is overhead at different times.