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Speaker 1: Welcome to Brainstuff from house Stuff Works dot Com, where

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Speaker 1: smart happens Him Marshall Brain with today's question, why are

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Speaker 1: leap years so weird? For example, how could the year

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Speaker 1: two thousand be a leap year when nineteen hundred was not.

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Speaker 1: We all know that February is a funny month to

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Speaker 1: begin with. Every four years it has one extra day,

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Speaker 1: February twenty nine instead of the normal twenty eight days.

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Speaker 1: When February has twenty nine days, we call it a

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Speaker 1: leap year. The year two thousand was a leap year,

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Speaker 1: but nineteen hundred was not, and neither eighteen hundred nor

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Speaker 1: seventeen hundred were leap years either. But seventeen hundred, eighteen hundred,

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Speaker 1: nineteen hundred, and two thousand are all divisible by four,

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Speaker 1: So why aren't they all leap years? And why do

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Speaker 1: we have leap years in the first place. Let's start

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Speaker 1: with the concept of a year. We define a year

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Speaker 1: to be the amount of time it takes for the

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Speaker 1: Earth to make one complete orbit around the Sun. The

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Speaker 1: reason we care about our orbital position around the Sun

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Speaker 1: is because of the seasons in the northern hemisphere. We

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Speaker 1: expect summer weather to occur around June, July, and August,

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Speaker 1: and winter weather to occur in December, January, and February.

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Speaker 1: A normal calendar year is defined as three hundred sixty

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Speaker 1: five days. However, if you measure the exact amount of

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Speaker 1: time it takes for the Earth to orbit the Sun,

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Speaker 1: the number is actually three hundred sixty five point two

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Speaker 1: four to two days. By adding one extra day to

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Speaker 1: every fourth year, we get an average of three hundred

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Speaker 1: sixty five point to five days per year, which is

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Speaker 1: pretty close to the actual number. To get even closer

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Speaker 1: to the actual number, every hundred years is not a

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Speaker 1: leap year, but every four hundred years is a leap year.

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Speaker 1: That brings the average length of the year to three

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Speaker 1: hundred six five point two four to five days, which

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Speaker 1: is very close to the actual number of three sixty

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Speaker 1: five point two four to two days. The remaining corrections

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Speaker 1: are added with things like leap seconds. Putting all these

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Speaker 1: rules together, you can see that a year is a

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Speaker 1: leap year not only if it is divisible by four.

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Speaker 1: It also has to be divisible by four hundred if

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Speaker 1: it's a centurial year. So seventeen hundred and nineteen hundred

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Speaker 1: were not leap years, but two thousand was Do you

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Speaker 1: have any ideas or suggestions for this podcast? If so,

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Speaker 1: please send me an email at podcast at how stuff

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Speaker 1: works dot com. For more on this and thousands of

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Speaker 1: other topics, go to how stuff works dot com