Difference Between GMT and IST

By : Neha Dhyani

Updated : Apr 13, 2022, 5:59

All countries in the world do not live at the same time of day. Different countries fall under different time zones and calculate their time accordingly. GMT and IST are two such time zones.

GMT, or Greenwich Mean Time, is the mean solar time at the Royal Observatory in Greenwich, London. Similarly, IST is the Indian Standard Time observed throughout India. There is a five and a half-hour Difference Between GMT and IST.

GMT - Meaning

Greenwich Mean Time was established in the year 1675. The Royal Observatory built it to assist the mariners in determining longitude at sea. It also helped them keep a standard reference time with respect to other cities of England that used to keep different local times.

The Shepherd Gate Clock displays GMT at the Royal Observatory in Greenwich, London. When the sun is at its highest point exactly above the Prime Meridian, it is 12:00 noon at Greenwich. It is the reference point for every time zone in the world. Some countries that use GMT include the UK, Iceland, Greenland, and Ghana.

IST - Meaning

The Indian Standard Time is used in all of India and Sri Lanka. It was established in India in 1947 after India got its independence. However, Kolkata and Mumbai adopted it only in 1948 and 1955, respectively.

The IST is calculated from a clock tower in Mirzapur with directions 25.15 degrees North Latitude and 82.58 degrees East Longitude.

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Difference Between GMT and IST

Basis of Difference

Greenwich Mean Time (GMT)

Indian Standard Time (IST)

History

GMT was established in 1675 by the Royal Observatory to help navigators at sea.

IST was established in India in 1947 after India gained independence from British Rule.

Time Calculation

GMT is considered equivalent to UTC (Universal Coordinated Time) for UK Civil purposes, but it is deemed equal to UT1 for navigation. UT1 is the modern form of mean solar time at 0 degrees longitude. UTC and UT1 differ by up to 0.9 seconds.

The Clock tower in Mirzapur is the basis of the calculation of IST. The tower coordinates are- 25.15 degrees North Latitude and 82.58 degrees East longitude.

Accuracy

GMT calculates time-based on Sun. However, because of the earth's uneven angular velocity and tilt in its axis, noon GMT may not be the exact time when the sun reaches the highest point in the sky.

The Time and Frequency Standards Laboratory at the National Physical Laboratory in New Delhi generates the official time signal for IST.

The signals are based on atomic clocks that are synchronized with UTC.

Replacement and Sufficiency

GMT was replaced by UTC as the international time standard in 1972. UTC is based on atomic measurements and hence is more accurate.

Demand for separating India into different time zones has been raised time and again. A time zone differentiated by one hour has been requested for Northeastern regions.

Countries that follow the time zone

GMT is used in the UK and other commonwealth nations such as Australia, New Zealand, and the countries of the Eastern Hemisphere.

IST is used in India and Sri Lanka.

 

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FAQs on Difference Between GMT and IST

Q.1. What is the historical Difference Between GMT and IST?

The historical Difference Between GMT and IST is that GMT was established in 1675 by the Royal Observatory to help navigators at sea. On the other hand, IST was established in India in 1947 after India gained independence from British Rule.

Q.2. What is the Difference Between GMT and IST in the calculation?

The Difference Between GMT and IST in the calculation is that GMT is considered equivalent to UTC (Universal Coordinated Time) for UK Civil purposes. Still, it is deemed equal to UT1 for navigation. The Clock tower in Mirzapur is the basis of time calculation for IST.

Q.3. What is the Difference Between GMT and IST with respect to accuracy?

The Difference Between GMT and IST with respect to accuracy is that GMT calculates time based on the sun, which may not always be exact. On the other hand, IST time signals are based on atomic clocks synchronized with UTC.

Q.4. What is the Difference Between GMT and IST with respect to countries?

The Difference Between GMT and IST with respect to countries is that GMT is used in the UK and other Commonwealth nations such as Australia, New Zealand, and countries of the Eastern Hemisphere. IST is used in India and Sri Lanka.