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Time zones came about due to the earth's rotation that causes day and night. As one can see in the animated illustration above, not all the earth can be facing the sun at once. Thus while one part of the earth is experiencing night, the other side is experiencing day. When it is eight o'clock in the morning in London, it is still very dark in the United States - the earth has not yet rotated so that the portion of the earth where the United States is located faces the sun.
To help deal with the differences in night and day throughout the earth, scientists have designated time zones. There are 24 time zones corresponding to the twenty-four hours in a day and night. The table below shows the time in the various time zones in the world. Notice that at the bottom of the chart, Wellington, New Zealand is a day ahead of New York City.
The time zones are based on various degrees of longitude which is the measure of distance from the Prime Meridian which goes through Greenwich England, the starting point for such measures.
There are six time zones in the United States - the Eastern Time Zone, Central Time Zone, Mountain Time Zone, Pacific Time Zone and two more time zones that affect Alaska and Hawaii further west than the Pacific zone. If it is noon in the Eastern Time Zone in New York City, it is three hours earlier in San Francisco or nine o'clock in the morning.
Due to the great amount of global traveling, it is difficult for our bodies to readjust when traveling to different time zones. This is referred to as "jet lag".
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