World Clocks and International Time Zones

The Earth is a globe. It rotates around the Sun and revolves around its own axis. This creates days and nights on the various places on its surface. At any given point of time, a part of the globe experiences daytime, while another part experiences nighttime.

Therefore, when the sun rises in one part of the world, it sets in another part. This has led to the creation of what is known as 'Time zones' across the globe. This helps maintain the local time and helps co-ordinate that time with other places in the world.

So what is the basis for deciding the time around the world?

The longitude of 0 Degrees is set at Greenwich, in the United Kingdom. Greenwich was chosen simply because the 0 Degree Longitude is measured from the Greenwich Meridian Line, at the Royal Observatory in Greenwich.

The time at that place is known as the Greenwich Mean Time or GMT. This is the standard time against which the various international time zones are referenced. This remains the same all throughout the year and is not affected by Daylight Saving Time or Summer Time.

The simple formula for the world clocks states: 'Spring Forward and Fall Back, But Greenwich Mean Time stays the same.'

The system of Greenwich Mean Time was put into practice since the discovery of America, in the 15th Century. The purpose was to aid naval navigation, as the globe had started to open up for trade purposes. In 1840, when railroads began to make their foray into land routes for trade, the system was also adopted across the globe.

The world clock time zones are set up from -12 to +12. Here, 0 = GMT. Each hour zone is of 15 Degrees, and is measured from the East to the West of Greenwich. Some countries have adopted a non-standard version, which has a 30 minute offset. Examples are: Kuwait, Riyadh, and Moscow, which are GMT +3:30, as compared to Lima and New York, which are GMT -5.

The places that are East of Greenwich are designated with a '+' sign, whereas the places West of Greenwich are designated with a '-' sign. This means that the places with a '+' sign are those many hours ahead of GMT, while the places with a '-' sign are those many hours behind GMT. This is the standard method of referencing the various International Time Zones across the world.

All these time zones are based on the definition of a second. This is again based on the fact that a day consists of 24 hours. However, in reality, only 4 days of the year are precisely 24 hours in length. They are: 25th of December, 15th of April, 14th of June, and 31st of August.

So in the rest of the year, the overhead sun at noon is anywhere between 16 minutes and 18 seconds early, to 14 minutes and 28 seconds late, compared to the overhead sun at noon on these 4 days. Based on this fact, the calculation of the time period of one second has varied from time to time.

In 1884, the International Meridian Conference held at Washington D.C., concluded that the calculation of one second was based on the length of the mean solar day at Greenwich, England. In 1956, it was decided to base the calculation on the Ephemeris Time, which is based on lunar observations.

However, in 1958, Atomic Time was introduced. This was based on the radiation patterns of the element Caesium.

Co-ordinated Universal Time, or UTC, replaced GMT, in 1986. This calculation was based on atomic measurements as compared to the time of Earth's rotation. However, GMT is still preferred by many people, as it has been in use for over 300 years.

The Use of Time Zones has primarily been to facilitate trade and commerce. Nowadays, this is also used to quote the time in e-mail messages, setting up of cookies on web browsers, synchronization of time schedule on space shuttles, weather mapping, and for posting news stories around the world.

They are also an integral part of international communications, because people in different parts of the world can schedule their telephone calls, conference calls, or even chat times, and radio conferences, based on these time zones.