Background: Why GMT may be a history?

• Greenwich Mean Time (GMT) was established in 1675 when the Royal Observatory was built as an aid to (English) mariners to determine longitude at sea, providing a reference time at a point in history when each city in England kept a different local time.
• Greenwich Mean Time (GMT) is the time measured on the Earth’s zero degree line of longitude, or meridian. This runs from the North Pole to the South Pole, passing through the Old Royal Observatory in the London suburb of Greenwich. The line has been called the Greenwich Meridian since 1884, and it is from here that all terrestrial longitudes are measured and the world’s time zones are calculated.
• When we are in a country east of the Greenwich Meridian, our local time is ahead of GMT, such as India, local time is GMT+5.30 hours. When we are in the west of the Greenwich Meridian, local time is behind GMT such as local time in New York is GMT -5 hours in winter and GMT -4 hours in summer.

GMT and International Date Line

• The dividing line between East (GMT+) and West (GMT-) on the opposite side of the world to the Greenwich Meridian is the International Date Line. This is a modification of the 180° meridian running north to south through the Pacific Ocean.

What is difference between GMT and BST?

• GMT remains constant throughout the year. In the winter months, local time in the UK is the same as GMT, but in March, local time is moved forward one hour to British Summer Time (BST) until the end of October. A number of other countries around the world also use this daylight saving measure and change their local times to take advantage of earlier sunrises.

What is difference between GMT and UTC?

• Some broadcasters show times in UTC (Co-ordinated Universal Time). This is essentially the same as GMT, but UTC is measured by an atomic clock and is thus more accurate – by split seconds. It is used primarily for scientific purposes.

Why GMT could soon be history?

• Scientists from around the world recently met in Britain to consider a proposal that could eventually see Greenwich Mean Time (GMT) relegated to a footnote in history. For more than 120 years GMT has been the international standard for timekeeping, but it is now under threat from a new definition of time itself based not on the rotation of the Earth, but on atomic clocks. In January 2012, the International Telecommunication Union will meet in Geneva to vote on whether to adopt the new measure.

What is International Atomic Time ?

• International Atomic Time (TAI), a time standard calculated using a weighted average of signals from atomic clocks located in nearly 70 national laboratories around the world.

How UTC is different from TAI?

• Coordinated Universal Time is based on TAI but the only difference between the two is that UTC is occasionally adjusted by adding a leap second in order to keep it within one second of UT1, which is defined by the Earth’s rotation. In the 50 years up to and including 2011, a total of 34 leap seconds have been added. Thus even UTC is not without defects. As the Earth’s rotation continues to slow, positive leap seconds will be required more frequently. The long-term rate of change of LOD is approximately +1.7 ms per century. At the end of the 21st century LOD will be roughly 86,400.004 s, requiring leap seconds every 250 days. Over several centuries, the frequency of leap seconds will become problematic.