UNDERSTANDING TIME & DATE MERIDIAN

A time zone is a region of the globe that observes a uniform standard time for legal, commercial, and social purposes.
The International Date Line (IDL) is an imaginary line of demarcation on the surface of the Earth that runs from the North Pole to the South Pole and demarcates the change of one calendar day to the next.

1. UNDERSTANDING TIME & DATE MERIDIAN HIMADRI SAMAL FOURTH SEMESTER 04S16AG04

2. LONGITUDE: It is a geographic coordinate & an Imaginary lines running N-S that specifies the east-west position of a point on the Earth's surface. Planes which contain the rotation axis intersect the surface at the meridians; and the angle between any one meridian plane and that through Greenwich (the Prime Meridian) defines the longitude. Longitude is given as an angular measurement ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. The Greek letter λ (lambda ),is used to denote the location of a place on Earth east or west of the Prime Meridian

3. Latitude: latitude is a geographic coordinate that specifies the north–south position of a point on the Earth's surface & Lines of constant latitude, or parallels, run east–west as circles parallel to the equator The latitude of an arbitrary point is the angle between the equatorial plane and the normal to the surface at that point:

4. TIME ZONE A time zone is a region of the globe that observes a uniform standard time for legal, commercial, and social purposes . Before 1972, all time zones were specified as an offset from Greenwich Mean Time ( GMT,established in 1965), which was the mean solar time at the meridian passing through the Royal Observatory in Greenwich, London. In January 1972, however, the length of the second in both Greenwich Mean Time and atomic time was equalized . Before the invention of clocks, people marked the time of day with apparent solar time. For Example, the time on a sundial – which was typically different for every settlement. The concept of standard time was adopted in the late 19th century . Sir Sandford Fleming outlined a plan for worldwide standard time in the late 1870s.  He recommended that the world be divided into twenty-four time zones, each spaced 15 degrees of longitude apart. All of China (which should span five time zones) uses a single time zone.

5. PRIME MERIDIAN A prime meridian, based at the Royal Observatory, Greenwich, in London, England,was established by Sir George Airy in 1851. The zero degree longitude line or the Prime Meridian passes through GreenwichGreenwich was chosen as a lot of the world’s sea charts were being drawn up in England at that time.

6. COORDINATED UNIVERSAL TIME Coordinated Universal Time (abbreviated to UTC) is the primary time standard by which the world regulates clocks and time The first Coordinated Universal Time was informally adopted on 1 January 1960,but the officially in 1967. The westernmost time zone uses UTC−12, being twelve hours behind UTC; the easternmost time zone, theoretically, uses UTC+12, being twelve hours ahead of UTC.

7. Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London. GMT was formerly used as the international civil time standard, now superseded in that function by Coordinated Universal Time (UTC ) The term "GMT" is especially used by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy. Greenwich Mean Time

8. INDIAN STANDARD TIME Indian Standard Time (IST) is the time observed throughout India, with a time offset of UTC+05:30. India does not observe daylight saving time ( DSTu ) or other seasonal adjustments . Indian Standard Time is calculated on the basis of 82.30' E longitude, in Allahabad which is nearly on the corresponding longitude reference line. History After independence in 1947, the Indian government established IST as the official time for the whole country, although Kolkata and Mumbai retained their own local time until 1948 and 1955, respectively. The Central observatory was moved from Chennai to a location at Shankargarh Fort in Allahabad district, so that it would be as close to UTC +5:30 as possible approved by Himanshu Sharma . Criticism and proposals

9. WHY DAYLIGHT SAVING TIME? Also known as summer time in some countries, is the practice of advancing clocks during summer months so that evening daylight lasts longer. Typically , regions that use daylight saving time adjust clocks forward one hour close to the start of spring and adjust them backward in the autumn to standard time . George Hudson proposed the idea of daylight saving in 1895 . The main  purpose of Daylight Saving Time  (called " Summer Time " in many places in the world) is to make better use of  daylight .

10. INTERNATIONAL DATE LINE The International Date Line (IDL) is an imaginary line of demarcation on the surface of the Earth that runs from the North Pole to the South Pole and demarcates the change of one calendar day to the next. The IDL is roughly based on the meridian of 180° longitude, roughly down the middle of the Pacific Ocean, and halfway around the world from the Greenwich meridian.

11. Proceeding from north to south , first deviation of the IDL from 180° is to pass to the east of Wrangel Island and the Chukchi Peninsula, It then passes through the Bering Strait between the Diomede Islands . The IDL crosses between the U.S. Aleutian Islands and the Commander Islands, which belong to Russia. It then bends southeast again to return to 180°. The IDL remains on the 180° meridian until passing the equator. Two US-owned uninhabited atolls, Howland Island and Baker Island The IDL circumscribes Kiribati by swinging far to the east, almost reaching the 150°W meridian. Kiribati's easternmost islands, the southern Line Islands south of Hawaii, have the most advanced time on Earth, UTC+14 hours. South of Kiribati, the IDL returns westwards but remains east of 180°, passing between Samoa and American Samoa . In much of this area, the IDL follows the 165°W meridian . The IDL then bends southwest to return to 180°. It follows that meridian until reaching Antarctica,

12. CIRCUMNAVIGATING THE GLOBE A person who goes around the world from east to west would gain or set their clock back one hour for every 15° of longitude crossed, and would gain 24 hours for one circuit of the globe from east to west if they did not compensate by setting their clock forward one day when they crossed the IDL. In contrast, a west-to-east circumnavigation of the globe loses an hour for every 15° of longitude crossed but gains back a day when crossing the IDL.

13. FACTS DEPENDENT ON THE IDL For the two hours between 10:00 and 11:59 UTC each day, three different calendar dates are observed at the same time in different places on Earth. For example, at 10:15 UTC Thursday, it is 23:15 Wednesday in American Samoa (UTC−11), Thursday in most of the world, and 00:15 Friday in Kiritimati (UTC+14 ). According to the clock, the first areas to experience a new day and a New Year are islands that use UTC+14. These include portions of the Republic of Kiribati, including Millennium Island in the Line Islands, as well as Samoa during the southern summer. The first major cities to experience a new day are Auckland and Wellington, New Zealand

14. Over the years, the position of the International Date Line has changed several times. Until 1845, the Philippines were on the eastern side of it. When the United States bought Alaska in 1867 the line was moved to the west of it . The most recent change in the line was in 1995 when  Kiribati  moved a large segment of it to the east, so that the entire nation would be on the same side of the International Date Line.

15. How IDL Works? Let's say you fly from the United States to Japan. Let's suppose you leave the United States on Tuesday morning. Since you're traveling west the time advances slowly thanks to time zones and the speed at which your airplane flies, but once you cross the International Date Line, it's suddenly Wednesday. On the reverse trip home you fly from Japan to the United States. You leave Japan on Monday morning but as you cross the Pacific Ocean, the day gets later quickly as you cross time zones moving eastward in an airplace . However, once you cross the International Date Line, the day changes to Sunday.

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