Time at any location in the world is calculated on the basis of its distance from the Prime Meridian. The local time at Greenwich, is considered as the accurate time for the Prime Meridian, and is called Greenwich Mean Time or GMT.
Since the earth rotates from west to east, the places east of Greenwich are ahead of Greenwich Mean Time and those to the west are behind it. All the places located on a particular meridian or a particular longitude have the same local time.
How would you calculate the time at a specific longitude?
- The earth rotates 360° in about 24 hours, means it rotates: 15° an hour or 1° in four minutes
Areas on the east of the Prime Meridian are ahead of areas on the west. Each place crosses the sun at different times. So, the time of sunrise also differs from place to place. Sunrise in places east of the Prime Meridian occurs earlier than that in places to the west of the Prime Meridian.
Japan is called the Land of Rising Sun because that is where the Sun rises first.
Uniformity in time needs to be maintained throughout the country and is done by determining the standard time for the country. Most countries adopt the local time of some central meridian as the standard time for the country.
The Indian Standard Time is based on the time on longitude 82.5 degrees east that passes through Mirzapur. This means that the difference between the standard meridian of India and that of Greenwich is 82 ½ degrees. India lies on the east of the Prime Meridian so IST is 5 hours 30 minutes ahead of GMT.
As you move east from the Prime Meridian, you lose time, and as you move west, you gain time.
There are countries like the USA and Russia that have multiple time zones. Multiple time zones exist in these countries because they are spread across a large number of longitudes. Russia has 11 time zones hence it is difficult to choose a single longitude as the standard time.
Canada spreads across about 90 degrees of longitude and has six time zones and each zone has its own standard time. The earth has a total of 24 time zones of one hour each.
To compete for a round-the-world speed record, a flight must cover a distance equal to at least the length of the Tropic of Cancer. It should also cross all meridians and end at the same place from where it took off.