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What is the exact length of a calendar year?

A year is the time it takes for the Earth to complete one revolution around the sun. Specifically, this period is 365.242199 days (365 days, 5 hours, 48 minutes, 46 seconds). The fact that a year is not comprised of a whole number of days has posed challenges to calendar-makers throughout the millennia. For instance, the ancient Egyptians, Romans, and Greeks used calendars with 365 days. This calendar, however, was off by an entire day approximately every four years.

A big improvement was made with the Julian calendar, crafted by Julius Caesar (100-44 B.C.) in 46 B.C. The Julian calendar, which was 365.25 days long, inserted an extra day, or "leap day," every fourth year. Even with that adjustment, the Julian calendar was off by 11 minutes and 14 seconds per year. By the mid-1500s that error had accumulated so that the Julian calendar was 10 days ahead of the Earth's...

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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?

1. Augustus Caesar