Question

Why is there a time difference of about 5 hours between India and London?

Answer 1

HINT:

The Earth rotates from west to east, and because of this, different parts of the planet face the Sun at different times. When a place rotates toward the Sun, it experiences sunrise, and when it rotates away, it has sunset. Since every location lies at a different longitude, they enter daylight and darkness at different moments. This is why sunrise and sunset do not occur simultaneously across the globe.

COMPLETE ANSWER:

There is a time difference of 5 hours and 30 minutes between India and London because both locations fall in different time zones determined by their longitudes. India follows Indian Standard Time (IST = UTC+5:30) based on 82.5° East longitude, while London follows Greenwich Mean Time (GMT = UTC+0) based on 0° longitude.

Since Earth rotates 15° every hour, the 82.5° longitudinal difference creates a 5.5-hour difference.

1. Earth’s Rotation Creates Time Differences

• Earth rotates 360° in 24 hours

• Therefore, in 1 hour, Earth rotates 15° (360 ÷ 24).

• To maintain consistent clocks worldwide, the globe is divided into 24 time zones, each covering roughly 15° of longitude.

This means every 15° you move east, the time becomes 1 hour ahead, and every 15° west, it becomes 1 hour behind.

2. Why London Is Taken as the Starting Point?

• The world’s central reference for time is 0° longitude, called the Prime Meridian, which passes through Greenwich, London.

• This creates GMT/UTC (Coordinated Universal Time).

All countries measure their local time as an offset from GMT.

3. India’s Time Zone: Why IST = UTC + 5:30?

India’s official time is calculated from 82.5° East longitude, which runs through places like Mirzapur (near Allahabad).
Now calculate the time difference:

• Longitude difference between India’s time meridian and Greenwich = 82.5°

• Time gained per 15° = 1 hour

So:
82.5° ÷ 15° = 5.5 hours
= 5 hours 30 minutes

This is why IST is 5 hours 30 minutes ahead of London’s GMT.

4. Why India Has a Half-Hour Time Zone?

Not all countries use whole-hour offsets.
India uses UTC+5:30 because:

1. The 82.5° longitude does not divide into a whole hour.

2. The mid-point was chosen to provide a uniform time for the entire country.

3. India wanted a central meridian that represents both East and West India fairly.

So India uses a single time zone with a half-hour offset, which is scientifically accurate.

5. Why the Difference Sometimes Becomes 4 Hours 30 Minutes?

The UK uses Daylight Saving Time (DST) in summer.

• Winter: London = GMT (UTC+0) → India ahead by 5 hours 30 minutes

• Summer: London = BST (UTC+1) → India ahead by 4 hours 30 minutes

India does not use DST, so London's clock changes while India's does not.

NOTE: Longitudinal Difference

Location	Standard Longitude	Time Zone	Offset
London	0°	GMT/UTC	+0
India (IST)	82.5°E	IST	+5:30

Time Calculation:

82.5° ÷ 15° per hour = 5.5 hours
→ Hence, 5 hours 30 minutes difference.

• London is at 0° → UTC+0.

• The longitude gap creates 5 hours 30 minutes difference.

• DST in the UK temporarily changes this to 4 hours 30 minutes.