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Why are the lengths of the seasons not equal?

The lengths of the seasons are not exactly equal because the shape of the Earth's orbit around the sun is elliptical (oval) rather than circular. And the sun is not at the center (lengthwise) of the ellipse—to the contrary, the sun is closer to one end of the ellipse than to the other.

When the Earth is closest to the sun in January, gravitational forces cause the Earth to move faster than it does in the summer months, when it is farther away from the sun. As a result, fall and winter seasons in the Northern Hemisphere (the half of the Earth north of the equator) are slightly shorter than spring and summer. Spring and summer are shorter than fall and winter in the Southern Hemisphere. The duration of the Northern Hemisphere seasons are as follows:

spring	92.76 days
summer	93.65...

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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?

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