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What is meant by Universal Time?

Universal Time (UT) is the precise, internationally accepted measure of time, used for all civil timekeeping and scientific work. Universal Time is measured by an atomic clock. Its basic unit is the atomic second—the length of time in which 9,192,631,770 oscillations (vibrations) are emitted by a radioactive cesium atom.

On January 1, 1972, UT went into effect, replacing Greenwich Mean Time (GMT). In GMT, a day was measured as the time between successive passages of the sun over Greenwich, England. Greenwich is the location of the prime meridian, the meridian of 0 degrees longitude, used as a point of reference for positions east and west on the globe.

One advantage of UT over GMT is that it is a more accurate time-keeping system. Another advantage of UT is that time can be readily determined without making astronomical observations.

Sources: Illingworth,...

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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?