Latitude, Longitude & Time Zones – NDA Geography Notes

Imagine 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.

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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. 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]

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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, as 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 ≈ 111 km. This distance is constant everywhere on Earth because all parallels are equally spaced from each other.

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

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Important Named Lines of Latitude

Line	Latitude	Significance
Equator	0°	Divides Earth into Northern and Southern Hemispheres; longest latitude; Great Circle
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. The pairing “Tropic of Cancer: 31.5°S” is completely wrong. [NDA 2009-I]

The Tropic of Capricorn passes through Chile, Brazil, Paraguay, and Argentina in South America, but NOT through Bolivia. Bolivia lies entirely north of the Tropic of Capricorn.

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

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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.

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

Every pair of opposite meridians forms a great circle, including the Prime Meridian together with the 180° meridian.

The Tropics (Cancer and Capricorn) and the Polar Circles (Arctic and Antarctic) are NOT great circles, as 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 and the Prime Meridian

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 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.

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.

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Time Zones: The Core Rule

The Earth is a sphere of 360°. It completes one full rotation in 24 hours. This gives us a simple rule that underlies every time zone calculation:

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]

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Time Calculation: Step by Step

Every time zone question in NDA follows the same three-step 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 of the first, add the difference. If west, subtract.

Worked Examples

Example 1: Basic subtraction [NDA 2010-I]

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 → it is behind. Answer = 12:00 − 1:00 = 11:00 hours.

Example 2: Latitude does not matter [NDA 2010-II]

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. City B is east → 80 minutes ahead.

Example 3: India to London [NDA 2023-I]

If it is 12 noon in New Delhi, what is the time in London? IST = GMT + 5h 30m. London = New Delhi time − 5h 30m = 6:30 AM.

Example 4: Crossing midnight [NDA 2013-II]

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.

Example 5: London to Dhaka [NDA 2014-II]

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.

Example 6: Allahabad to Wellington [NDA 2007-II]

When it is 12 noon in Allahabad, what time in Wellington, New Zealand? Wellington (~174°E). Allahabad (~82.5°E). Difference ≈ 91.5°. 91.5 × 4 ≈ 366 minutes ≈ 6 hours 6 minutes ahead. 12:00 + 6:06 ≈ 6:06 PM same day: closest to 6:30 PM. Answer = (c).

Example 7: Opposite sides of globe [NDA 2009-I]

Time difference between 165°E and 165°W? Total angular difference = 165 + 165 = 330°. 330 ÷ 15 = 22 hours.

Example	Key Calculation	Answer	PYQs
90°W to 105°W	15° × 4 min = 60 min	11:00 hrs	[NDA 2010-I]
60°E to 80°E	20° × 4 min = 80 min	80 min ahead	[NDA 2010-II]
New Delhi to London	IST − 5h 30m	6:30 AM	[NDA 2023-I]
88.5°E from 0° at 19:00 (28 Feb)	88.5° × 4 = 354 min = 5h 54m	00:54, 1 March	[NDA 2013-II]
0° to 90°E at 13:45	90° ÷ 15 = 6h	19:45 Monday	[NDA 2014-II]
165°E to 165°W	(165+165) ÷ 15 = 22h	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]

Country	Time Zone
India	UTC +5:30
Nepal	UTC +5:45
Sri Lanka	UTC +5:30
Bhutan	UTC +6:00 (maximum)

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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. The International Date Line (IDL) solves it.

The IDL sits at approximately 180° longitude, which is simultaneously 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. [NDA 2007-I]

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

The IDL passes mainly through the Pacific Ocean, not the Atlantic Ocean. [NDA 2024-I]

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]

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

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Key Parallels as 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.

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