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If you have a look at this year's calendar, what date is the last day of February ? It is the 29th of February. While the month of February normally has 28 days, this year's February has 29 days. This extra day is called a leap day and this year, 2020, is called a leap year. 

The insertion of this extra day is known as “intercalation”. To simplify the term, scholars use “leap year” because each date on the calendar jumps ahead two days of the week instead of one. For example, Christmas 2018 fell on Tuesday, and last Christmas(2019)fell on Wednesday. This year, Christmas will “leap” over Thursday and fall on Friday. 

In astronomy, it takes the earth approximately 365 days and six hours to complete a full orbit around the sun. This is a solar year. That means we need a leap year every four years to synchronise the calendar year with the solar year. 

In 1582, Pope Gregory XIII(教宗額我略十三世)set a new formula where a century year is a leap year only if it is divisible by 400. Thus, in the Gregorian calendar,1900 was not a leap year, but 2000 was. 

A person born on the 29th February may be called a leap-year baby. It is up to them whether they want to celebrate their birthdays on either the 28th of February or the 1st of March. In legal terms, most countries define that a leap-year baby comes of age on the 1st of March. 


Glossary 
Leap year 
閏年
Insertion 
插入
Intercalation 
置閏
Astronomy 
天文學
Orbit 
軌道
Solar year 
太陽年
Synchronise 
同步
Legal 
法律