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How is time measured?

Time is a system for describing the continuous passage of events from past to present to future. Time can be measured in large increments such as years, months, and days, as well as in small increments such as hours and seconds. The passage of time is measured in three principle ways: rotational time, dynamic time, and atomic time.

Rotational time has as its basic unit the mean solar day, which is the average time it takes the Earth to complete one rotation about its axis.

Dynamic time relies not only on the orbital movements of the Earth, but on the movements of the moon and/or planets as well. The first dynamic time scale was Ephemeris Time, proposed in 1896 and modified in 1960. Ephemeris Time is based on the orbit of the moon around the Earth. The motion of the moon is determined by observing its position with respect to the stars. Given that the Earth's rotation exhibits slight variations in...

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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?