Standard Time and Its Necessity
Now for each city or a place to keep individual time based on its longitudinal position would complicate matters, especially in cases of medium to big countries. Since for each degree change in longitude, there is a time difference of four minutes, people travelling from one part to other part of a country would continuously have to keep changing and adjusting time. To avoid such confusion and impracticability, a system of standard time is observed by all countries.
Central Meridian and Calculation of Standard Time
Each country has their own central meridian, and the standard time of the country is calculated in relation to the Prime Meridian at Greenwich with the central meridian of the country. In case of India, the meridian of 82.5°E is taken as the Central Meridian. The Indian Standard Time is calculated with respect to the time difference between Greenwich and the Indian Central Meridian. Indian Standard Time (IST) is 5 hours and 30 minutes ahead of Greenwich Mean Time.
Global Standard Time Zones
The whole world is divided into 24 standard time zones, and each of them differs by 1 hour and 15° in longitude. Large countries like the USA, Russia, and Canada, which have a huge east-west stretch, have more than one standard time zone for practical purposes. The USA and Canada both have 5 time zones – Atlantic, Eastern, Central, Mountain, and Pacific Time Zones. The difference between the Atlantic and Pacific time zones is 5 hours.
International Date Line
Concept of the International Date Line
This concept becomes very interesting here. A traveller going eastward from Greenwich gains time till he/she reaches 180°E longitude. At 180°E meridian, he is 12 hours ahead of GMT. Similarly, a traveller going westward from Greenwich, at 180°W meridian, loses 12 hours. Thus, there is a difference of 24 hours or one whole day on two sides of the 180° meridian.
Date Change Across the International Date Line
This is called the International Date Line, where the date changes by exactly one day once it is crossed. The traveller crossing the International Date Line from west to east gains a whole day, and the traveller crossing from east to west loses a day. Suppose, on the Asia side of the International Date Line, it is midnight on Friday, 13th September; on the American side of the International Date Line, it is actually midnight on Thursday, 12th September. A whole day is thus gained by travelling from east to west of the International Date Line.
Adjustments in the International Date Line
The International Date Line in the mid-Pacific curves from the normal 180° meridian at the Bering Strait, Fiji, Tonga, and a few other islands to prevent confusion of date and time, as a few of the islands fall on both sides of the International Date Line.
What is standard time, and why is it necessary?
Standard time is a uniform time adopted by a country or region based on its central meridian. It is necessary to avoid confusion caused by individual cities or places keeping their own time based on longitudinal differences.
How is Indian Standard Time (IST) calculated?
Indian Standard Time (IST) is calculated based on the meridian of 82.5°E. It is 5 hours and 30 minutes ahead of Greenwich Mean Time (GMT).
Why do some countries have multiple time zones?
Large countries like the USA, Russia, and Canada have a vast east-west stretch, which necessitates multiple time zones to maintain practical and uniform time management across different regions.
What are the major time zones in the USA?
The USA has five major time zones: Atlantic, Eastern, Central, Mountain, and Pacific. The difference between the Atlantic and Pacific time zones is 5 hours.
What is the International Date Line (IDL)?
The International Date Line (IDL) is an imaginary line near the 180° meridian where the date changes by one day when crossed. It helps in maintaining the global calendar.
How does the International Date Line affect time and date?
Crossing the International Date Line from west to east adds one day, while crossing from east to west subtracts one day.
Why does the International Date Line deviate from the 180° meridian?
The International Date Line deviates to avoid splitting countries or islands into two different days, ensuring local consistency for those regions.
What is the time difference between two longitudes?
The time difference between two longitudes is 4 minutes for every 1° change in longitude.
How many standard time zones are there globally?
The world is divided into 24 standard time zones, each differing by 1 hour and covering 15° of longitude.
What happens to the time at the poles with respect to longitude?
At the poles, longitudes converge, and the linear distance between them narrows to zero. Hence, time differences become less practical to measure.
How is time determined in relation to Greenwich?
Time at any place is compared to the local time at Greenwich, and adjustments are made by adding or subtracting hours based on the longitude of the location.
What is the significance of the Prime Meridian in standard time calculation?
The Prime Meridian (0° longitude) serves as the reference point for calculating standard time worldwide, dividing the globe into eastern and western hemispheres.
How does Earth’s rotation influence time zones?
The Earth rotates 360° in 24 hours, covering 15° every hour. This rotation determines the division of time zones globally.
Why do places to the east of Greenwich experience sunrise earlier?
As the Earth rotates from west to east, places to the east of Greenwich experience sunrise earlier because they face the Sun first.
What is the role of longitude in determining time zones?
Longitude determines the local time of a place based on its angular distance from the Prime Meridian. Time zones are calculated by dividing the Earth’s 360° rotation into 24 equal zones.