GK : THE WORLD

International Date Line:
If we travelled westward to a place X on longitude 180^o W, the time there would be 12 hours behind Greenwich time (180 x 4 minutes = 720 minutes = 12 hours). If we journeyed eastward to a place Y on longitude 180o E, the time there would be 12 hours ahead of Greenwich time. Thus X and Y both on 180o have the same time but differ in date by a day (12 hours + 12 hours = 24 hours). To overcome the confusion that would otherwise arise, the International Date Line has been established. 

It runs along 180o E or W. Westward-bound vessels crossing the Date Line drop a day from the calendar, while those going eastward add a day by giving the same date to two consecutive days. Instead of changing the time exactly according to change in degrees at the rate of 4 minutes per degree, certain time zones have been established. All places in the same area or time zone or time belt, use what is called Standard Time. Thus we have the Greenwich Mean Time (G.M.T) and the Indian Standard Time (I.S.T). There are five time-belts in Canada and four in United States.

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