The reason behind having a leap day after every 4 years is because our Earth completes one complete revolution around the sun in around 365 days and 6 hours.
And Over a period of 4 years, the extra 6 hours from each year gets added up to become an entire day(6*4=24 hours). Hence we have a leap day/ leap year after every 4 years.
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The month of February is a bit different from other months in the sense that every year, out of nowhere, a 29th day is added to it, and the month of
February becomes one day longer than usual.
Put simply, these additional 24 hours are built into the calendar to ensure that it stays in line with the Earth’s movement around the Sun.
While the modern calendar contains 365 days, the actual time it takes for Earth to orbit its star is slightly longer—roughly 365.2421 days.
The difference might seem negligible, but over decades and centuries that missing quarter of a day per year can add up.
To ensure consistency with the true astronomical year, it is necessary to periodically add in an extra day to make up the lost time and get the calendar back in synch with the heavens.
Leap day History
The people from Egypt were the first to calculate the need for a leap year, but the practice didn’t arrive in Europe until the reign of the Roman dictator Julius Caesar.
Before then, the Roman calendar had operated on a muddled lunar model that regularly required adding an extra month to maintain celestial consistency.
Finally, in 46 B.C., Caesar and the astronomer Sosigenes revamped the Roman calendar to include 12 months and 365 days.
This “Julian Calendar” also accounted for the slightly longer solar year by adding a leap day every four years.
Caesar’s model helped realign the Roman calendar, but it had one small problem. Since the solar year is only .242 days longer than the calendar year and not an even .25, adding a leap year every four years actually leaves an annual surplus of roughly 11 minutes.
This minute discrepancy meant that the Julian Calendar drifted off course by one day every 128 years, and by the 14th century it had strayed 10 days off the solar year. To fix the glitch, Pope Gregory XIII instituted a revised “Gregorian Calendar” in 1582.
In this model, leap years occur every four years except for years evenly divisible by 100 and not by 400. For example, the year 1900 was not a leap year because it was divisible by 100, but not 400.
The Pope’s updated calendar remains in use to this day, but it’s still not perfect—experts note that the remaining discrepancies will need to be addressed in around 10,000 years.
What makes a Leap year?
A leap year consists of 366 days which is one day more than regular year which has 365 days.
Every 4 years, a year comes along that has 1 more day to it, that means 366 days in total. Those years with 366 days are called leap years.
More clearly, the year in which the month of February has 29 days(instaed of 28 days) is called a Leap year. Example 2016 was a Leap year and then 2020 will be a leap year.
How often is a Leap day?
A leap year comes after every 4 years and so as the leap day with it. A normal year has 365 days and a Leap year has 366 days i.e 1 day more from a normal year.
Why a Century is not a Leap year?
Century years are not leap years because by adding 1 day to the year every 4 years, we end up adding too much over a period of 100 years. And that's why ever 100 years, we have to remove a day from the year.
In other words, a century year cannot be a leap year unless it is divisible by 400. Thus 1700, 1800, and 1900 were not leap years, but 1600, 2000, and 2400 are leap years.
What are the Odds of being born on a Leap day?
The probability of being born on the leap day is around 0.00068. In other words, your chances of being born o February are 1 in 1461.
To conclude, adding a day to February every 4 years is a practical requirement, not some majestic joke from an ancient Greek ruler who thought February was so far Romantic to have 28 days!
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