What is the time difference between Greenwich Mean Time (GMT) and Indian Standard Time (IST)?

Introduction / Context:
Greenwich Mean Time (GMT) is a reference time based on the prime meridian at 0 degrees longitude in Greenwich, England. Indian Standard Time (IST) is the time observed throughout India and is based on a standard meridian that runs through 82.5 degrees east longitude. Knowing the time difference between GMT and IST is a basic but important part of Indian geography and is frequently tested in competitive exams.

Given Data / Assumptions:

- The question asks specifically for the time difference between GMT and IST.

- Options provide several hour and half hour combinations: 6 hours 30 minutes, 6 hours, 5 hours 30 minutes, 5 hours, and 4 hours 30 minutes.

- It is assumed that you know India uses a single standard time across the country.

- No calculation of longitude difference is required if you remember the standard value of IST offset.

Concept / Approach:
India's standard time is calculated based on the 82.5 degrees east longitude, which is 5.5 hours ahead of Greenwich Mean Time. Each 15 degrees of longitude corresponds to a difference of 1 hour in local solar time. The 82.5 degrees east meridian is therefore 82.5 / 15 = 5.5 hours ahead of Greenwich. This means IST is GMT plus 5 hours 30 minutes. The correct approach is either to recall this well known fact directly or to derive it quickly using the longitude difference.

Step-by-Step Solution:
Step 1: Recall or note that the standard meridian for IST is 82.5 degrees east longitude. Step 2: Remember that Earth rotates 360 degrees in 24 hours, so a 15 degree longitude difference corresponds to 1 hour of time difference. Step 3: Compute the time difference: 82.5 degrees divided by 15 degrees per hour gives 5.5 hours. Step 4: Express 5.5 hours as 5 hours 30 minutes. Step 5: Recognise that IST is ahead of GMT, so the offset is +5 hours 30 minutes. Step 6: Match this with option C, 5 hours 30 minutes.

Verification / Alternative Check:
If GMT is taken as time at 0 degrees longitude and IST is set at 82.5 degrees east, you can quickly confirm that 82.5 / 15 equals 5.5. Also, you may recall that when the clock shows noon in London in winter, it is already evening, about 5:30 PM, in India. This everyday observation matches the calculated offset and confirms that the difference between GMT and IST is 5 hours 30 minutes.

Why Other Options Are Wrong:
6 hours 30 minutes would correspond to a longitude difference of 97.5 degrees, which is greater than India's standard meridian offset. 6 hours would correspond to 90 degrees of longitude, which is also not the standard for IST. 5 hours would correspond to 75 degrees of longitude and does not match the 82.5 degrees east meridian used for IST. 4 hours 30 minutes would correspond to 67.5 degrees of longitude, which again is not the standard meridian chosen for India.

Common Pitfalls:
Some students confuse GMT with Coordinated Universal Time (UTC) or mix up different country offsets. Another common mistake is to remember only the hours and forget the half hour component, leading them to answer 5 hours instead of 5 hours 30 minutes. India is one of the countries that uses a half hour time zone offset, so it is important to remember that IST is GMT plus 5 and a half hours, not a whole number of hours.

Final Answer:
The time difference between Greenwich Mean Time and Indian Standard Time is 5 hours 30 minutes, with IST being ahead of GMT.