Why does February have 29 days in a leap year?
We were taught in primary school that a year — the amount of time it takes for the Earth to complete a revolution around the sun — is 365 days.
Well, it's in fact approximately 365 ¼days. Take note of “approximately”.
If we were to ignore the quarter day and count every year as 365 days, our 12-month calendar would gradually get out of sync with the earth’s revolution till the seasons can no longer be matched with calendar months. This means Harmattan could start in April this year and start in August a couple hundred years down the line.
Imagine waiting an extra quarter day every year to celebrate New Year, or your birthday (it might be 6 am one year and 2 pm some years down the line) instead of just getting the party started at midnight like we do now. Not very convenient, right?
To keep things in sync, Julius Caesar and a team of smarty-pants added one extra day every four years — basically piling up all the quarter days and adding them to the Calendar when they summed up to a day. So 365 days every other year and 366 days on years divisible by 4 (like 2024!).
Problem solved, right? Well, not quite.
To be very precise, it takes the earth 365.2422 days — a tiny-winnie-little-bit less than 365-and-a-quarter days completely revolve around the sun. So while Caesar’s solution was pretty dope, it still wasn’t perfect — his years were 0.0078 days too long.
Over time this seemingly insignificant difference would cause a slight but noticeable discrepancy between the calendar year and the actual time it takes for the Earth to complete its orbit around the sun.
In a bid to improve the precision of the approximation, when the Gregorian calendar was introduced, it was decided that every hundred years, we would skip a leap year. However, that would mean the average number of days per calendar year would be around 365.24 days, but this was still 0.0022 days shorter.
So to make things even more precise, once every four hundred years, we add back a leap year that would have otherwise been skipped, making the length of a calendar year 365.2425 — close enough to 365.2422 to not cause any major discrepancies in the next few thousand years.