What is the International Date Line?

            The earth is divided into 24 one-hour zones 15 degrees longitude for each hour, which make one full day on earth. Since earth rotates eastward, the time on the clock progresses westward round the world. Thus 12 o’ clock noon occurs in London (0 degree longitude) five hours before it doe sin Washington D.C (75 degree west of London), and eight hours before at San Francisco 120 degrees west of London. So, when it is noon I London, it is midnight at the International Date Line.

The International Date Line extends from North Pole to South Pole

            The International Date Line is an imaginary line extending from the North Pole to the South Pole. It passes thorough the Pacific Ocean. As the earth rotates, each day begins and ends on this line. Whenever it passes over land or divides countries, it is diverted to pass over the Pacific Ocean.

            The line deviates east of the 180 longitude to pass through the Bering Strait to include eastern Siberia and then westward to include the Aleutian islands with Alaska. South of the equator, it bulges eastwards again to allow various island groups to have the same day as that of New Zealand.

            The earth is divided vertically into 360 longitudes (180 degrees east and 180 degrees west), zero degrees longitude passes through Greenwich and 180 degrees longitude is called the International Date Line. On either side of the International Date Line, the time is the same but there is a difference of 24 hours.

            It is for this reason that a traveler moving westward across the line sets his calendar back by one day, and a person crossing form west to east will have to advance his calendar by a day. In short, it implies that if you cross the date line going eastward, you gain a day while those traveling westward lose a day.