Unlike the equator which is centrally placed between the poles, any meridian could have been taken to begin the numbering of longitude. However, in 1984, by an international agreement it was decided to choose the zero meridian one which passes through the Royal Astronomical Observatory at Greenwich near London. This is the first meridian from which all other meridians radiate eastwards and westwards towards 180°.
A traveller going eastwards gains time from Greenwich until he reaches the meridian 180 degree east, correspondingly, in going westwards he loses 12 hours when he reaches 180 degree west. There is thus a total difference of 24 hours between two sides of the 180 meridian. This is the International Date Line where the date changes by exactly one day when it is crossed.
The International Date Line in the mid-Pacific curves from the normal 180 degrees meridian at the Bering Strait, Tonga and other islands to prevent confusion of day and date in some of the island groups that are cut through by the meridian. Some of them keep Asiatic or New Zealand standard time, others follow the American date and time. To find local time in two places on opposite sides of the International Date Line means crossing the ‘line’ going eastwards a whole day is gained. Crossing the ‘line’ going westwards a whole day is lost.
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