The Earth is constantly turning on its axis. If people everywhere set their clocks to the same time, midnight would be in the middle of the night on one side of the globe, but the middle of the day on the opposite side. To avoid this problem the Earth is divided into artificial time zones. These generally follow lines of longitude – imaginary lines running from the North to the South Pole. Some large countries, such as the USA, cover several time zones. Mainland USA is divided into Atlantic, Eastern, Central, Mountain, Pacific and Alaska zones, with Hawaii and other islands falling into further zones.
The Earth makes one complete turn in every 24 hours. In that time, each of the 360 degrees of longitude passes the Sun. This means that time progress eastwards by four minutes for every degree of longitude.
The International Date Line is an imaginary line between the North and South Poles. It marks the end of one day and the start of another. Countries to the east of the Date Line are always a day ahead of those to the west. Travelers who cross the line gain or lose a day, depending on which direction they are going.
Most of the Date Line follows the 180 degree Meridian. The line generally passes through sea, but where it would pass through or near certain land areas, it is adjusted. It zigzags around islands, putting them either into the west or the east, and avoids dividing Siberia in north-east Asia into two time zones.
