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Imagine picking up leaves on Christmas, or shoveling (铲起) snow on Memorial Day. What about going for a swim on Thanksgiving? Without Leap Day, which takes place every four years, that could happen. "If we didn't have leap years, our calendar would be totally mixed," says Geoff Chester of the U.S. Naval Observatory in Washington, D.C, the nation's official timekeepers.

Our calendar is normally 365 days long. It was created to match the cycles of the seasons. But Roman Dictator Julius Caesar (See-zer) noticed a problem: The Earth doesn’t circle the sun in exactly 365 days. It actually takes 365 and one-quarter days. He figured out that the extra part of a day would cause the calendar to grow apart from the seasons over time. Over 100 years, the seasons would shift about 24 days. Spring would start on April 13 instead of March 20.

Caesar used math to figure out a way to stop the calendar from shifting. He decided to add an extra day to the month of February every four years. His idea helped keep the seasons and calendar matched up. Even so, it still wasn’t perfect—his calendar was adding too many days.

In 1582, Pope Gregory XIII worked on Caesar’s idea. His calendar, called the Gregorian calendar, shows that every year that is evenly divided by 400 is a leap year. Turn-of-the-century years, or years ending in “00,” would not be leap years unless they could be divided evenly by 400. These complicated equations(方程) help keep the calendar in balance with the orbit of the Earth. Today, we still use the Gregorian calendar. In about 3,000 years, the calendar will be only one day out of step with the seasons. It’s still not perfect, but mathematicians decided it was as close as we could get.

1.What is stressed in the first paragraph?

A The measure of Leap Day.

B The decision of Leap Day.

C The standard of Leap Day.

D The function of Leap Day.

解析：选D。推理判断题。根据首段内容特别是第三句Without Leap Day, which takes place every four years, that could happen.可判断出本段主要强调了Leap Day的作用，故选D。

2.It can be concluded from Caesar's discovery that the Earth________.

A is the planet on which we live

B has a satellite called the moon

C is the land surface on which we move around

D doesn’t move around the sun in exactly 365 days

解析：选D。细节理解题。根据第二段第三句But Roman Dictator Julius Caesar (See-zer) noticed a problem: The Earth doesn’t circle the sun in exactly 365 days.可知罗马独裁者尤利乌斯•凯撒(See-zer)注意到一个问题:地球绕太阳运行一周并不是正好是365天，故选D。

3.What was the shortcoming of Caesar’s idea?

A It sounded too good to be true.

B It put too many days to the calendar.

C It had a hard time celebrating Leap Day.

D It was no difference from other calendars.

解析：选B。细节理解题。根据第三段最后一句it still wasn’t perfect — his calendar was adding too many days.可知它仍然不是完美的——他的日历会增加太多的天数，故选B。

4.What was Pope Gregory’s contribution to calendar?

A Roman calendar.

B Islamic calendar.

C Gregorian calendar.

D Sporting calendar.

解析：选C。细节理解题。根据末段的倒数第三句Today, we still use the Gregorian calendar. 可知今天我们仍然用此历法，故选C。