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NDA Geography · Earth in Space, Maps and Coordinates

Time Zones and the International Date Line

The Earth turns 360 degrees in 24 hours, so every 15 degrees of longitude equals one hour — going east adds time, going west subtracts it, and the date changes by a day at the International Date Line near 180 degrees.

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Why this matters

3 PYQs, including the chapter's signature HARD calculation. The whole subtopic rests on one rule: 15 degrees of longitude = 1 hour, and east is ahead, west is behind. Add the International Date Line facts (it lies near 180 degrees, zig-zagging to avoid land, and crossing it changes the date) and you have every mark here.

Concept 1 of 3: The 15-degrees-per-hour rule and the direction of time

The Earth makes a full 360-degree spin in 24 hours. Divide and you get 15 degrees of longitude for every 1 hour (or 1 degree = 4 minutes). Because the Earth spins west to east, places to the EAST see the Sun first, so their clocks are AHEAD; places to the WEST are BEHIND. To find another place's time, count the longitude difference, convert at 15 degrees per hour, then add if going east or subtract if going west.

Definition

  • The Earth rotates 360 degrees in 24 hours, so 15 degrees of longitude = 1 hour (and 1 degree = 4 minutes).
  • The Earth spins west to east, so a place to the EAST is AHEAD in time and a place to the WEST is BEHIND.
  • Method: find the longitude difference between the two places; divide by 15 to get hours (or multiply degrees by 4 to get minutes); add the result if the target place is to the east, subtract if it is to the west.
  • India keeps a single standard time (IST) based on the 82.5 deg E meridian, which is 5 hours 30 minutes ahead of GMT (the 0 deg Prime Meridian).

Worked example

If it is 12 noon at the Prime Meridian (0 deg), what is the local time at 30 deg E?
Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2021 · Q113Hard

Example 1 · Earth in Space, Maps and Coordinates · Time Zones and International Date Line

If it is 12·00 Noon in India, on which meridian will it be 7·00 am of the same day ?

East adds, West subtracts

The single biggest error is the sign. The Earth spins west to east, so an eastern place is AHEAD (add) and a western place is BEHIND (subtract). Decide east-or-west first, THEN add or subtract the converted hours.

Concept 2 of 3: Finding a place's clock time from GMT

Many questions give you the time in one city and ask the time in another. The fastest route is to go through GMT (the 0-degree Prime Meridian): India is 5.5 hours ahead of GMT, so London (near 0 deg) is 5.5 hours BEHIND India. Subtract 5:30 from Delhi's time to get London's, ignoring summer-time shifts in the basic version.

Definition

  • London/UK sits near the Prime Meridian (0 deg), so it runs on (or near) GMT; India (IST) is 5 hours 30 minutes ahead of GMT.
  • Therefore London is 5 hours 30 minutes BEHIND New Delhi.
  • To convert Delhi time to London time, subtract 5:30 (Delhi is to the east, so London is behind).
  • (In the basic NDA version, ignore British Summer Time; the standard offset is 5:30.)

Worked example

It is 6:00 p.m. in New Delhi. What is the standard time in London (GMT)?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Apr 2023 · Q136Easy

Example 2 · Earth in Space, Maps and Coordinates · Time Zones and International Date Line

If it is 12 noon in New Delhi, what will be the time in London, UK?

Concept 3 of 3: The International Date Line

Travel far enough east or west and the clock alone is not enough — you also need a place to change the calendar date. That is the International Date Line (IDL), running roughly along the 180-degree meridian, zig-zagging east and west to keep island groups on the same date. Crossing it, the date jumps by one day. The catch the NDA tests: the date to the EAST of the line is one day BEHIND (earlier than) the date to the west.

Definition

  • The International Date Line (IDL) runs roughly along the 180 deg meridian, in the middle of the Pacific Ocean (not the Atlantic), deviating east and west to avoid splitting land/island groups.
  • Crossing the IDL changes the calendar date by one day.
  • The date to the EAST of the line is EARLIER (one day behind) the date to the WEST. (Going west-to-east across the line you SUBTRACT a day; east-to-west you ADD a day.)
  • It is a practical, zig-zagging line, NOT a perfectly straight meridian.
180 W90 W0 (GMT)90 E180 EWEST: subtract timeEAST: add time15 deg of longitude = 1 hour180 deg = International Date Line (date changes)

Worked example

A ship crosses the International Date Line travelling from west to east. Does it gain or repeat a calendar day?
Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2024 · Q69Moderate

Example 3 · Earth in Space, Maps and Coordinates · Time Zones and International Date Line

Consider the following statements regarding the International Date Line : 1. It is roughly 180° meridian, which deviates slightly East and West to avoid land areas surrounded by the Atlantic Ocean. 2. The date to the East of this line is (24 hours) earlier than to the West. Which of the statements given above is/are correct ?

The IDL is in the Pacific, not the Atlantic

A statement-trap says the date line deviates 'to avoid land surrounded by the Atlantic Ocean'. Wrong ocean — the IDL runs through the Pacific at ~180 degrees. That statement is treated as incorrect.

East of the line is BEHIND, not ahead

It is easy to assume 'east = ahead' (true for clocks) carries over to the date. Across the IDL it flips: the date to the EAST is one day earlier (behind) the date to the west.
Drill 1 more on the international date line

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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