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How many time zones are there in the world?

The Earth's surface is divided into 24 standard time zones—one for each hour in the day. Each zone is roughly 15 degrees of longitude wide (longitude is a measure of how far east or west a given place is on the globe; each degree of longitude is spaced 69 miles [111 kilometers] apart at the equator and narrows as it reaches the poles). The boundaries of some time zones follow national borders and thus are irregular in shape. The time within each zone is constant; each time you enter a new zone the time changes by one hour. The establishment of international time zones was accomplished at the Washington Meridian Conference of 1884.

Sources: Considine, Glenn D., ed. Van Nostrand's Scientific Encyclopedia, 8th ed., vol. 2, p. 3101; Engelbert, Phillis. Astronomy and Space: From the Big Bang to the Big Crunch, vol. 3, p. 644.

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1. Time
   1. How is time measured?
   2. What is meant by Universal Time?
   3. How many time zones are there in the world?
   4. Who establishes the correct time in the United States?
   5. What is military time?
   6. What do the initials a.m. and p.m. mean?
   7. When is Daylight Savings Time observed in the United States?
   8. What is a leap second?
   9. What is the exact length of a calendar year?
   10. Where did the term "grandfather clock" come from?
   11. What were the longest and shortest years on record?
   12. Besides the Julian and Gregorian calendars, what other calendars have been used?
   13. When does leap year occur?
   14. When did January 1 become the first day of the new year?
   15. How were the months of the year named?
   16. Where do the names of the days of the week come from?
   17. Why are the lengths of the seasons not equal?
2. Weights And Measures
   1. What is the metric system of measurement?
   2. What are the U.S. and metric units of measurement for area?
   3. How did the yard as a unit of measurement originate?
   4. What is a bench mark?
   5. How was the length of a meter originally determined?
   6. How are U.S. customary measures converted to metric measures and vice versa?
   7. What is the difference between a short ton, a long ton, and a metric ton?
   8. Why is a nautical mile different from a statute mile?
   9. What are the equivalents for dry and liquid measures?
   10. What are U.S. and British measures for volume and weight of water?