Latitude, Longitude & Time Zones – NDA Geography Notes

Introduction

IImagine you are lost somewhere on Earth and you want to tell someone exactly where you are. You cannot say “near a big tree” or “close to a river” — those descriptions only work locally. You need a system that works anywhere on Earth, understood by everyone. That system is the grid of latitude and longitude.

This chapter explains how that system works — what latitude and longitude are, what the important named lines are, how time zones are calculated from longitude, and what the International Date Line is. Once you understand the logic behind the system, the calculation questions become very easy. They all follow the same simple rule.

Understand the Topic

The Need for a Grid System

The Earth is a sphere. On a flat surface, we use X and Y coordinates to locate any point. On the Earth’s curved surface, we use latitude and longitude to do the same thing.

Together, latitude and longitude form an imaginary grid over the Earth’s surface. Every point on Earth has a unique combination of latitude and longitude — like an address. To locate a place on a map, you need both latitude and longitude together — latitude alone tells you how far north or south, longitude alone tells you how far east or west. You need both to pinpoint an exact location. [NDA 2022-I]

A geographical grid has lines running east to west around the globe to mark latitude, and lines running north to south from pole to pole to mark longitude. [NDA 2023-II]

Latitude

Latitude is the angular distance of a point on Earth’s surface measured north or south of the Equator. It is measured as an angle from the centre of the Earth — not from the surface. [NDA 2011-I]

Lines of latitude are also called parallels because they run parallel to each other and to the Equator. They are complete circles going all the way around the Earth.

The Equator is at 0° latitude. Latitudes go from 0° at the Equator to 90° at the poles. 90°N is the North Pole and 90°S is the South Pole.

Lines of latitude are concentric circles numbered from 0° to 90°, running parallel to the Equator on both sides. [NDA 2008-II]

The Equator is the longest line of latitude — it is the circle with the largest radius. Every other parallel gets shorter as you move toward the poles. At 90° latitude, the poles are simply a point — not a circle at all. [NDA 2021-I]

One degree of latitude is approximately 111 km — this distance is constant everywhere on Earth because all parallels are equally spaced from each other. [NDA tested via CDS]

The distance between longitudes becomes zero at the poles — because all meridians converge to a single point there. The distance between longitudes is maximum at the equator. [NDA 2023-I]

Important Named Lines of Latitude

Line	Latitude	Significance
Equator	0°	Divides Earth into Northern and Southern Hemispheres
Tropic of Cancer	23.5°N	Sun directly overhead on June 21
Tropic of Capricorn	23.5°S	Sun directly overhead on December 22
Arctic Circle	66.5°N	Northernmost point with 24 hours of daylight in summer
Antarctic Circle	66.5°S	Southernmost point with 24 hours of darkness in winter
North Pole	90°N	Northernmost point on Earth
South Pole	90°S	Southernmost point on Earth

The Tropic of Cancer is at 23.5°N — in the Northern Hemisphere. It is not at 31.5°S. A student who reverses the hemisphere loses the mark. [NDA 2009-I]

The Tropic of Capricorn passes through Chile, Brazil, Paraguay, and several other South American countries — but NOT through Bolivia. [NDA tested via CDS]

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Important Parallels — Political Boundaries

The 49th Parallel separates the USA and Canada for most of their western shared border. [NDA 2007-I]

The 38th Parallel separates North Korea and South Korea.

Great Circles

A great circle is any circle drawn on the Earth’s surface whose centre coincides with the centre of the Earth. A great circle divides the Earth into two exactly equal halves. Great circle routes are the shortest paths between two points on Earth — which is why aeroplanes often fly paths that look curved on a flat map but are actually the shortest routes.

The Equator is a great circle — it divides the Earth into the Northern and Southern Hemispheres.

The Prime Meridian (together with the 180° meridian on the opposite side) forms a great circle. In fact, every pair of opposite meridians forms a great circle.

The Tropics (Cancer and Capricorn) and the Polar Circles (Arctic and Antarctic) are NOT great circles — their centres do not coincide with the Earth’s centre.

Among the Prime Meridian, Tropic of Cancer, and Equator — only the Prime Meridian and Equator are great circles. The Tropic of Cancer is not. [NDA 2025-I]

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Longitude

Longitude is the angular distance of a point east or west of the Prime Meridian.

Lines of longitude are also called meridians. Unlike parallels which are truly parallel to each other, meridians converge at the poles. They are at their maximum distance apart at the equator and get progressively closer together toward the poles — finally meeting at a single point at each pole.

The statement that all meridians are always parallel to each other is incorrect. Meridians converge — they are not parallel. [NDA tested via CDS]

The Prime Meridian

The Prime Meridian is the starting point for measuring longitude. It passes through Greenwich, England, and continues through France, Spain, Algeria, Mali, Burkina Faso, Togo, and Ghana in Africa.

The Prime Meridian does NOT pass through Morocco. Morocco is west of the Prime Meridian. [NDA 2022-II]

The point where the Equator (0° latitude) and the Prime Meridian (0° longitude) meet is located in the South Atlantic Ocean — not on land and not at Greenwich. [NDA tested via CDS]

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Time Zones and the Relationship Between Longitude and Time

This is the most calculation-heavy part of the chapter. But the rule is beautifully simple.

The Earth is a sphere — 360°. It completes one full rotation in 24 hours. This means:

Every 15° of longitude = 1 hour of time difference [NDA 2008-II]

Every 1° of longitude = 4 minutes of time difference

As you move east, time moves ahead.
As you move west, time falls behind.

All places on the same meridian have exactly the same local time — regardless of how far apart they are in distance north to south. [NDA 2008-II]

Time Calculation — Step by Step

Every time zone question follows the same method:

Step 1 — Find the difference in degrees between the two longitudes.
Step 2 — Multiply by 4 to get time difference in minutes. Or divide by 15 to get hours.
Step 3 — If the second place is east, add the difference. If west, subtract.

Example 1 — Basic subtraction:
If it is noon (12:00) at 90°W, what is the time at 105°W?
Difference = 105 − 90 = 15°.
Time difference = 15 × 4 = 60 minutes = 1 hour.
105°W is west of 90°W, so it is behind.
Answer = 12:00 − 1:00 = 11:00 hours. [NDA 2010-I]

Example 2 — Latitude does not matter:
Time difference between City A (30°N, 60°E) and City B (30°N, 80°E)?
Only longitude matters. Difference = 80 − 60 = 20°.
Time difference = 20 × 4 = 80 minutes. [NDA 2010-II]

Example 3 — India to London:
If it is 12 noon in New Delhi, what is the time in London?
India Standard Time = GMT + 5 hours 30 minutes.
London = New Delhi time − 5 hours 30 minutes = 6:30 AM. [NDA 2023-I]

Example 4 — Crossing midnight:
Local time at 88°30’E when 0° longitude shows 19:00 on 28 February?
88.5° × 4 minutes = 354 minutes = 5 hours 54 minutes.
19:00 + 5:54 = 24:54 = 00:54 on 1st March. [NDA 2013-II]

Example 5 — Simple addition:
London broadcasts at 13:45 Monday. What time in Dhaka (90°E)?
90° × 4 = 360 minutes = 6 hours.
13:45 + 6:00 = 19:45 = 7:45 PM Monday. [NDA 2014-II]

Example 6 — New Zealand:
When it is 12 noon in Allahabad, what time in Wellington, New Zealand?
Wellington is approximately 91.5° east of Allahabad.
91.5 × 4 ≈ 366 minutes ≈ 6 hours 6 minutes ahead.
12:00 + 6:06 ≈ 6:30 PM same day. [NDA 2007-II]

xample 7 — Opposite sides of globe:
Time difference between 165°E and 165°W?
Total difference = 165 + 165 = 330°.
330 ÷ 15 = 22 hours. [NDA 2009-I]

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India Standard Time

India Standard Time (IST) = GMT + 5 hours 30 minutes.

The Standard Meridian of India is 82°30’E — passing through Mirzapur in Uttar Pradesh. This single time zone is used across the entire country to avoid confusion.

Among India, Nepal, Sri Lanka, and Bhutan — Bhutan has the maximum time difference from GMT at UTC+6:00. [NDA 2021-II]

The International Date Line

As you travel eastward around the Earth, you add one hour for every 15°. By the time you have gone all 360° around, you have added 24 hours — a full day. But you are back where you started. This creates a problem — you would be one full day ahead of where you began.

The International Date Line (IDL) solves this problem. It is where the date changes.

The IDL sits at approximately 180° longitude — which is both 180°E and 180°W (they are the same meridian). [NDA 2009-II]

The IDL is NOT at the Greenwich Meridian. Greenwich is at 0°. The IDL is at 180° — on the opposite side of the Earth from Greenwich. [NDA 2007-I]

The IDL is drawn in a zigzag path — not a straight line at exactly 180°. This is done to allow certain land areas and groups of islands to have the same calendar day. If the IDL were a straight line, it would split countries like Russia and island groups like Fiji between two different calendar dates. [NDA 2010-II]

The IDL passes mainly through the Pacific Ocean.

The date to the east of the IDL is one full day (24 hours) behind the date to the west of the IDL. [NDA 2024-I]

A ship sailing westward and crossing the IDL puts the date back by one day. [NDA 2009-II]

The date of Alaska is NOT ahead of the date of Siberia. Alaska is east of the IDL and therefore one day behind Siberia — not ahead of it. [NDA 2007-I]

The difference in date on either side of the IDL is caused by the Earth’s rotation and the 24-hour time zone system — not by the inclined axis of the Earth. [NDA tested via CDS]

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Which Countries Does the Equator Pass Through in Indonesia?

The Equator passes through Sumatra, Borneo (Kalimantan), and Sulawesi in Indonesia. It does NOT pass through Java. Java is south of the Equator. [NDA 2019-II]

Key Summary Rules

• Same longitude = same local time
• 15° = 1 hour time difference
• 1° = 4 minutes time difference
• East = ahead; West = behind
• IDL is at approximately 180° — NOT at Greenwich
• IDL is zigzag to avoid splitting land areas
• East of IDL = one day behind west of IDL
• Great circles = Equator and Prime Meridian (NOT Tropics or Polar Circles)
• 0° latitude + 0° longitude = South Atlantic Ocean
• 49th Parallel = USA-Canada; 38th Parallel = North Korea-South Korea

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