Why are there leap years

It takes our Planet about 365.2422 days to turn once around the sun, but the “normal” calendar just has 365 days. The decimal part that is put aside is successively added to the following year. After the Earth turning 4 times, 1460.9688 days will have passed. But 4 years correspond to 1460 days and so the remaining 0.9688 almost correspond to a whole day. That's why it was decided to add 1 extra day every 4 years. The month of February was chosen because it is the Shortest and it has 29 days every 4 years and this year is called the leap year. As far as Maths is concerned, it was stated that if the year could be divided by 4, it would be a leap year.

For thousands and thousands of years, people have used this kind of calendar which was called the Julian calendar and it was imposed by Julio Cesar in 46 B.C. But 0.9688 does not really correspond to a whole day, and so little by little the Julian calendar started to be out of step with reality. In 1582 the Pope Gregory solved this problem when he ordered people to follow a new set of rules according to the Gregorian calendar, which is nowadays into force and whose rules are the following:

• Every 4 years we have a leap year. So, 2004, 2008, 2012, 2016, ... are leap years;
• However, every 100 years it is not a leap year. So, 1900, 2100, ... are not leap years;
• But, every 400 years, there will be a leap year. So, 2000, 2400, ... are leap years.

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