Welcome to Brainstuff from house Stuff Works dot Com, where smart happens Him Marshall Brain with today's question, why are leap years so weird? For example, how could the year two thousand be a leap year when nineteen hundred was not. We all know that February is a funny month to begin with. Every four years it has one extra day, February twenty nine instead of the normal twenty eight days. When February has twenty nine days, we call it a
leap year. The year two thousand was a leap year, but nineteen hundred was not, and neither eighteen hundred nor seventeen hundred were leap years either. But seventeen hundred, eighteen hundred, nineteen hundred, and two thousand are all divisible by four, So why aren't they all leap years? And why do we have leap years in the first place. Let's start
with the concept of a year. We define a year to be the amount of time it takes for the Earth to make one complete orbit around the Sun. The reason we care about our orbital position around the Sun is because of the seasons in the northern hemisphere. We expect summer weather to occur around June, July, and August, and winter weather to occur in December, January, and February. A normal calendar year is defined as three hundred sixty
five days. However, if you measure the exact amount of time it takes for the Earth to orbit the Sun, the number is actually three hundred sixty five point two four to two days. By adding one extra day to every fourth year, we get an average of three hundred sixty five point to five days per year, which is pretty close to the actual number. To get even closer to the actual number, every hundred years is not a leap year, but every four hundred years is a leap year.
That brings the average length of the year to three hundred six five point two four to five days, which is very close to the actual number of three sixty five point two four to two days. The remaining corrections are added with things like leap seconds. Putting all these rules together, you can see that a year is a leap year not only if it is divisible by four. It also has to be divisible by four hundred if
it's a centurial year. So seventeen hundred and nineteen hundred were not leap years, but two thousand was Do you have any ideas or suggestions for this podcast? If so, please send me an email at podcast at how stuff works dot com. For more on this and thousands of other topics, go to how stuff works dot com