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Here is some good news for those who need an extra（额外）day to catch up on sleep — 2016 is a leap year. This important thing that occurs every four years means that instead of the 365 days， the year will have 366 days. Since the extra day is added to the shortest month of the year， February will have 29 days！
The extra day is not because the earth is suddenly taking longer to go around the sun， but to balance out （平衡） the little time difference between our calendar and the solar calendar （阳历）. As you may or may not know， our typical 365-day year is based on the time it takes our planet to get around the sun. However， that number is a close one. In reality， it takes the earth 365 days， 5 hours， 49 minutes and 16 seconds to complete the orbit （轨道）.
Adding the unexpected number of hours to our year would make for a strange day. But paying attention to the extra time is not a good idea either. That’s because the extra quarter day each year adds up to a full 25 days every century. This means that while our calendar year Summer would begin in June， the solar year Summer would come a month later！ If left unchecked， our calendar would be completely out of the way with the solar calendar.
So， in 46 B.C. Roman Emperor Julius Caesar and his team of astronomers （天文学家） came up with an idea. They rounded up the extra time to six hours and then added a day to the calendar every four years.
While this was a good idea， it brought another problem. Rounding the 5 hours 49 mins and 16 seconds to six hours added extra time to the day every four years. Though it was just a few minutes， over time it added up. To deal with the problem， Pope Gregory XIII’s astronomers invented another clever but confusing （令人费解的） rule.
Leap year would occur every year whose number is perfectly divisible （可被整除的） by four —except for years that are both divisible by 100 and not divisible by 400！ This means that 1700， 1800 and 1900 were not leap years but 1600 and 2000 were！
Happy leap day to all of you and a very very Happy Birthday to anyone that is born on this extra special day. Be sure to let us know， if you are among them.
Questions：
1. What is leap year？
2. How often does it occur？
3. Why was it necessary to introduce leap year to the calendar？
4. Why were 1600 and 2000 leap years？
The extra day is not because the earth is suddenly taking longer to go around the sun， but to balance out （平衡） the little time difference between our calendar and the solar calendar （阳历）. As you may or may not know， our typical 365-day year is based on the time it takes our planet to get around the sun. However， that number is a close one. In reality， it takes the earth 365 days， 5 hours， 49 minutes and 16 seconds to complete the orbit （轨道）.
Adding the unexpected number of hours to our year would make for a strange day. But paying attention to the extra time is not a good idea either. That’s because the extra quarter day each year adds up to a full 25 days every century. This means that while our calendar year Summer would begin in June， the solar year Summer would come a month later！ If left unchecked， our calendar would be completely out of the way with the solar calendar.
So， in 46 B.C. Roman Emperor Julius Caesar and his team of astronomers （天文学家） came up with an idea. They rounded up the extra time to six hours and then added a day to the calendar every four years.
While this was a good idea， it brought another problem. Rounding the 5 hours 49 mins and 16 seconds to six hours added extra time to the day every four years. Though it was just a few minutes， over time it added up. To deal with the problem， Pope Gregory XIII’s astronomers invented another clever but confusing （令人费解的） rule.
Leap year would occur every year whose number is perfectly divisible （可被整除的） by four —except for years that are both divisible by 100 and not divisible by 400！ This means that 1700， 1800 and 1900 were not leap years but 1600 and 2000 were！
Happy leap day to all of you and a very very Happy Birthday to anyone that is born on this extra special day. Be sure to let us know， if you are among them.
Questions：
1. What is leap year？
2. How often does it occur？
3. Why was it necessary to introduce leap year to the calendar？
4. Why were 1600 and 2000 leap years？