ImagineclearingleavesonChristmas,orsweeponMemorialDay...

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Imagine clearing leaves on Christmas, or sweep on Memorial Day. What about going for a swim on Thanksgiving? Without Leap Day(閏日), which takes place every four years, it could happen.

Our calendar(日曆)is normally 365 days long. It was created to match the cycles of the seasons. But Roman Dictator Julius Caesar 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, dictates that every year evenly divided by 400 is a leap year. These complicated equations(方程式)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.

So what happens when someone is born on Leap Day? Do these people celebrate their birthday each year, or do they instead have to wait four years to age? How rare is a Leap Day birthday? The chance of someone being born on a Leap Day is 1 out of 1,461. Babies born on February 29 are sometimes known as leapers. How will you celebrate Leap Day?

28. What could happen without Leap Day?

A. Spring would start later than March 20

B. Our calendar is 365 days long every year

C. A day would be left over to the next year

D. Seasons would stay the same every year

29. Which of the following will be a leap year?

A. 2040 B. 2100 C. 2200 D. 2400

30. We still use the Gregorian calendar because _________.

A. it is a perfect calendar

B. it is in balance with the orbit of the Earth

C. it is accurate to show the birthday of people

D. it is Pope Gregory XIII that created the calendar