Latitude, Longitude and Time
Latitude, longitude, local and standard time, IST at 82°30′E, time zones and the International Date Line, with worked time calculations for UPSC.
SyllabusGS I · Salient features of world physical geography
Prerequisites: The Universe and the Earth · Earth’s Revolution and the Seasons
The Hook: The Country That Skipped a Day
On the night of 29 December 2011, Samoa went to sleep and woke up on 31 December. The Pacific island nation had moved itself from the eastern to the western side of the International Date Line, so that its working week matched its main trading partners, Australia and New Zealand, instead of the United States. For Samoans, 30 December 2011 never happened.
How can a country simply choose its date? Because time on Earth is a human convention built on one physical fact: the Earth turns 360° in about 24 hours. Latitude, longitude, time zones and the date line all follow from that.
The Core Concept
The Earth is close to a sphere (more precisely an oblate spheroid or geoid — slightly flattened at the poles). To locate any point on it we use a grid of two sets of imaginary lines.
| Feature | Latitude (parallels) | Longitude (meridians) |
|---|---|---|
| Definition | Angular distance north or south of the Equator, measured from the Earth’s centre | Angular distance east or west of the Prime Meridian |
| Range | 0° (Equator) to 90° N and 90° S | 0° to 180° E and 180° W |
| Shape of lines | Full circles, parallel to each other; get smaller towards the poles | Semicircles from pole to pole; all of equal length |
| Great circle? | Only the Equator | Every meridian (with its opposite) forms a great circle |
| Spacing on the ground | Nearly constant: 1° ≈ 111 km | 1° ≈ 111 km at the Equator, shrinking to zero at the poles |
| Number at 1° interval | 89 in each hemisphere + the Equator = 179 (the poles are points) | 360 |
| Main use | Temperature zones, climate, day length | Time |
Important parallels
| Parallel | Approx. latitude | Why it exists |
|---|---|---|
| Equator | 0° | Divides the globe into Northern and Southern Hemispheres; a great circle |
| Tropic of Cancer | 23.5° N | Northernmost latitude where the Sun can be overhead (June solstice) |
| Tropic of Capricorn | 23.5° S | Southernmost latitude where the Sun can be overhead (December solstice) |
| Arctic Circle | 66.5° N | Southern limit of the 24-hour day/night in the north |
| Antarctic Circle | 66.5° S | Northern limit of the 24-hour day/night in the south |
The tropics and polar circles are not arbitrary — they are set by the 23.5° tilt of the Earth’s axis (see Earth’s Revolution and the Seasons).
Heat zones
- Torrid zone — between the two tropics; the Sun is overhead at least once a year.
- Temperate zones — between each tropic and its polar circle; the Sun is never overhead.
- Frigid zones — between each polar circle and its pole; at least one day of 24-hour daylight and one of 24-hour night each year.
Longitude and time
The Prime Meridian (0°) passes through the Royal Observatory at Greenwich, near London. It was adopted as the world’s reference meridian at the International Meridian Conference held in Washington in 1884.
Because the Earth rotates 360° in 24 hours:
| Rotation | Time taken |
|---|---|
| 360° | 24 hours |
| 15° | 1 hour |
| 1° | 4 minutes |
| 1′ (minute of arc) | 4 seconds |
The Earth rotates from west to east, so places to the east see the Sun first and are ahead in time.
- Local time is set by the Sun: it is noon at a place when the Sun crosses that place’s meridian. Every meridian has its own local time.
- Standard time is the local time of one chosen meridian (the standard meridian), used across a whole country or zone so that clocks do not differ from town to town.
graph LR
A[Earth rotates 360 degrees in 24 h] --> B[15 degrees = 1 hour]
B --> C[1 degree = 4 minutes]
C --> D[Local time: Sun overhead at each meridian]
D --> E[Standard time: one meridian chosen per zone]
E --> F[Time zones roughly 15 degrees wide]
F --> G[International Date Line near 180 degrees]Indian Standard Time
- Mainland India extends from about 68°7′ E to 97°25′ E — nearly 30° of longitude, which means a local-time difference of about two hours between the easternmost parts of Arunachal Pradesh and the western edge of Gujarat (NCERT, Class IX).
- IST is the local time of 82°30′ E, a meridian that passes near Mirzapur in Uttar Pradesh.
- 82°30′ E × 4 minutes per degree = 330 minutes = 5 hours 30 minutes ahead of Greenwich (UTC + 5:30).
- Mainland India extends in latitude from about 8°4′ N to 37°6′ N. The Tropic of Cancer divides the country almost in half and passes through eight states: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram.
The two-hour spread is why there has been a long-running debate on a separate time zone for the North-East, where the Sun rises and sets much earlier than clock time suggests. India continues to follow a single time zone.
Time zones around the world
- The world is divided into time zones based roughly on 15° bands, but boundaries bend to follow national and provincial borders.
- Large countries choose differently: Russia uses eleven time zones; the contiguous United States uses four main zones; China, despite its east–west spread, uses a single national time (Beijing time).
- The global reference today is Coordinated Universal Time (UTC), maintained with atomic clocks; for most practical purposes it matches Greenwich Mean Time (GMT).
The International Date Line (IDL)
If you travel east around the world, adding an hour every 15°, you gain 24 hours by the time you return. Some line must “absorb” that day. That line is the International Date Line:
- It follows the 180° meridian for most of its length but zig-zags to avoid splitting countries or island groups (for example, it bends around eastern Russia, the Aleutian Islands and several Pacific island groups).
- Crossing it westward (from the Americas towards Asia) — the date advances by one day.
- Crossing it eastward (from Asia towards the Americas) — the date goes back by one day.
- The IDL is a convention, not set by any treaty, which is why countries such as Kiribati (1995) and Samoa (2011) have shifted their side of it.
Memory Anchors
“East is Ahead, West is Behind” — East Gains (add time), West Loses (subtract time). Remember GEWL: Gain East, West Lose.
“15–1–4” — 15° is 1 hour, 1° is 4 minutes. Every time question reduces to these two numbers.
Date line, opposite rule: crossing the IDL, the date does the opposite of the clock on a normal eastward trip. Going west across the IDL, add a day; going east, subtract a day.
IST = 82.5 × 4 = 330 minutes = 5:30. If you can do this multiplication, you never forget IST.
Worked examples
Greenwich is at 12:00 noon. What is the local time at 45° E? 45 × 4 = 180 minutes = 3 hours. East is ahead → 3:00 p.m.
It is 10:00 a.m. IST. What is the time at Greenwich? IST is 5:30 ahead → 4:30 a.m. GMT.
Place A is at 30° W and Place B at 60° E. When it is noon at A, what is the time at B? Difference = 30 + 60 = 90° (add when the places are on opposite sides of Greenwich) → 90 × 4 = 360 minutes = 6 hours. B is east → 6:00 p.m.
Two places have a local-time difference of 2 hours 40 minutes. What is their longitudinal difference? 160 minutes ÷ 4 = 40°.
For Prelims
- The Equator is the only parallel that is a great circle; every meridian is half of a great circle.
- All meridians are of equal length; parallels shrink towards the poles.
- 1° of latitude is about 111 km everywhere; 1° of longitude is about 111 km only at the Equator.
- The Prime Meridian at Greenwich was adopted at the International Meridian Conference, Washington, 1884.
- IST is based on 82°30′ E, passing near Mirzapur (Uttar Pradesh); IST = UTC + 5:30.
- The Tropic of Cancer passes through eight Indian states, from Gujarat in the west to Mizoram in the east.
- Crossing the IDL westward advances the date; crossing eastward sets it back.
For Mains
- Dimensions: physical basis (rotation, 15° per hour); administrative choice (standard time, number of zones); economic (working hours, energy use, trade synchronisation — the Samoa example); social (daylight mismatch, health and productivity in the North-East).
- Keywords: local mean time, standard meridian, time zone, daylight utilisation, circadian rhythm, synchronisation, International Date Line.
- Value-addition: India’s ~30° east–west extent gives a two-hour solar-time spread; China’s single time zone and Russia’s eleven zones are the two extremes of national choice; Samoa (2011) shows how economic links can drive a time-zone decision.
Common Traps
In the News
- December 2011 — Samoa moved to the western side of the International Date Line, skipping 30 December 2011.
Practice
Prelims
Consider the following statements:
- The Equator is the only parallel of latitude that is a great circle.
- The length of one degree of longitude is the same at all latitudes.
- All meridians of longitude are of equal length.
Which of the statements given above is/are correct?
Solution
Statement 1 is correct — every other parallel is a smaller circle. Statement 2 is wrong — meridians converge at the poles, so 1° of longitude is about 111 km at the Equator and shrinks to zero at the poles. Statement 3 is correct — every meridian runs from pole to pole.
Answer: C
When it is 1:00 p.m. Indian Standard Time, what is the local time at a place on the 45° E meridian?
Solution
IST (82°30′ E) is 5:30 ahead of Greenwich, so 1:00 p.m. IST = 7:30 a.m. at Greenwich. A place at 45° E is 45 × 4 = 180 minutes = 3 hours ahead of Greenwich, so it is 10:30 a.m. there.
Shortcut: the difference between 82°30′ E and 45° E is 37°30′ = 150 minutes = 2 hours 30 minutes; the place is west of the IST meridian, so subtract: 1:00 p.m. − 2:30 = 10:30 a.m. Option A (4:30 a.m.) is the trap for anyone who treats 45° E as 45° W; option B is Greenwich time.
Answer: D
How many of the following Indian states does the Tropic of Cancer pass through?
- Odisha
- Tripura
- Jharkhand
- Uttar Pradesh
Solution
The Tropic of Cancer passes through Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram. Of the four listed, only Tripura and Jharkhand lie on it. Odisha lies entirely south of the Tropic; Uttar Pradesh lies entirely north of it.
Answer: B
Mains
India’s large east–west extent creates a gap between solar time and clock time. Discuss the case for and against a second time zone for India.
Solution
Introduction: Mainland India spans nearly 30° of longitude (about 68°7′ E to 97°25′ E), a solar-time difference of about two hours. A single IST, based on 82°30′ E, is used nationwide.
Body:
- For a second zone: in the North-East the Sun rises and sets well before clock time, so daylight is lost in the evening; more daylight working hours could save electricity and suit natural sleep cycles.
- Against: risk of confusion in railways, aviation, banking and broadcasting; a single time reinforces national integration; costs of changing systems; India’s spread is smaller than that of countries such as China, which also uses one time zone.
- Middle paths: adjusted office and school hours in the North-East (the informal “tea-garden time” in Assam is an example of local adaptation).
Conclusion: The choice is administrative, not physical. Flexible working hours for eastern states may capture most of the benefits without the costs of a formal second zone.
Quick Revision
| Point | One-liner |
|---|---|
| Latitude | Angular distance N/S of Equator; 0°–90° |
| Longitude | Angular distance E/W of Greenwich; 0°–180° |
| Great circles | Equator and every meridian pair |
| Time rule | 15° = 1 hour; 1° = 4 minutes |
| Direction | East ahead, west behind |
| Prime Meridian | Greenwich; adopted 1884 (Washington conference) |
| IST | 82°30′ E, near Mirzapur; UTC + 5:30 |
| India’s extent | ~68°7′ E–97°25′ E; ~8°4′ N–37°6′ N (mainland) |
| Tropic of Cancer in India | 8 states, Gujarat to Mizoram |
| IDL | ~180°, zig-zags; westward +1 day, eastward −1 day |
Key Takeaways
- Latitude governs climate and day length; longitude governs time.
- Every time question reduces to 15° = 1 hour and 1° = 4 minutes, with east ahead of west.
- Standard time and the date line are human conventions laid over a physical fact — and countries do change them.
Related Topics
- Earth’s Revolution and the Seasons
- The Universe and the Earth — quick revision
- The Universe and the Earth — practice questions
- Physiography of India
- Climatology