Why do we have Leap Years?
Reality Check
- Have you ever be asked why we have the 29th day in February in some years and none in some others?
- Have you ever been confronted by a 6 yr old child about why we have 365 days for years and 366 days for some others?
- Did you never listen in school? 🥱
- Did you randomly click this link (you can always close it 😜)
If your answers are yes to all the questions then you are in the right place.
TL;DR. we have an extra day to our normal 365 days because we are “trying” to match the number of days to the seasons we have in a year. If unmatched we could have winter in may and spring in december.
Quick Analogy
Imagine a restaurant in a remote part of town, with an almost fixed number of customers per day. If every day we have about 10 orders, it is logical to think that at the 10th order we should be done for the day and should close up to save costs.
Now of course while this may be valid for most days, there will be days when this assumption would make the restaurant lose money. For instance, a city boy comes back home and decides to throw a party in that same restaurant or an increase in income of the customers leading to an increase in demand, and a whole lot of economics I would rather not bore you with.
Explanation
The restaurant analogy above is very similar to how our calendar system works. When the earth spins on its axis, we have day and night, that's what we call a day. It does this all year long as it also revolves around the sun.
How we arrive at the year numbers is by counting the number of spins (day and night) for a complete revolution around the sun. keep in mind that these are two disjointed processes and are not exactly connected.
The common number we arrived at was 365 days, and that's what we use today. But most interestingly, with that number every year, we go out of sync with the seasons by 1/4th of a day.
To solve this it was decided to balance the math by adding 1 day every 4 years. Hence the term “leap” year, to jump. The actual term is “intercalation” but then your 6-year-old child won’t be able to ask you 😜.
So that's why we have 366 days in leap years.
Extra Credit
Even with the explanation above we end up getting ahead by 1/100th of a day every year. And so we still need to balance the math as closely as we can.
The solution to that problem was to skip the leap year every 100 years. And so what that means is, going by our normal 4 year — leap year cycle as explained above. Every start of a new century which is supposed to be a leap year would be skipped.
This solution seemed great and everything seemed balanced, but not quite yet. We are behind by 1 / 400th every year even with skipping the 100th year.
To solve this problem it was decided that if the 100th year is divisible by 400 then it’s a leap year. That's why the year 2000 was a leap year.
