WEBVTT - How Leap Years Work

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<v Speaker 1>Welcome to Brainstuff from house Stuff Works dot Com, where

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<v Speaker 1>smart happens Him Marshall Brain with today's question, why are

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<v Speaker 1>leap years so weird? For example, how could the year

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<v Speaker 1>two thousand be a leap year when nineteen hundred was not.

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<v Speaker 1>We all know that February is a funny month to

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<v Speaker 1>begin with. Every four years it has one extra day,

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<v Speaker 1>February twenty nine instead of the normal twenty eight days.

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<v Speaker 1>When February has twenty nine days, we call it a

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<v Speaker 1>leap year. The year two thousand was a leap year,

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<v Speaker 1>but nineteen hundred was not, and neither eighteen hundred nor

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<v Speaker 1>seventeen hundred were leap years either. But seventeen hundred, eighteen hundred,

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<v Speaker 1>nineteen hundred, and two thousand are all divisible by four,

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<v Speaker 1>So why aren't they all leap years? And why do

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<v Speaker 1>we have leap years in the first place. Let's start

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<v Speaker 1>with the concept of a year. We define a year

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<v Speaker 1>to be the amount of time it takes for the

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<v Speaker 1>Earth to make one complete orbit around the Sun. The

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<v Speaker 1>reason we care about our orbital position around the Sun

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<v Speaker 1>is because of the seasons in the northern hemisphere. We

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<v Speaker 1>expect summer weather to occur around June, July, and August,

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<v Speaker 1>and winter weather to occur in December, January, and February.

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<v Speaker 1>A normal calendar year is defined as three hundred sixty

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<v Speaker 1>five days. However, if you measure the exact amount of

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<v Speaker 1>time it takes for the Earth to orbit the Sun,

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<v Speaker 1>the number is actually three hundred sixty five point two

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<v Speaker 1>four to two days. By adding one extra day to

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<v Speaker 1>every fourth year, we get an average of three hundred

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<v Speaker 1>sixty five point to five days per year, which is

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<v Speaker 1>pretty close to the actual number. To get even closer

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<v Speaker 1>to the actual number, every hundred years is not a

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<v Speaker 1>leap year, but every four hundred years is a leap year.

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<v Speaker 1>That brings the average length of the year to three

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<v Speaker 1>hundred six five point two four to five days, which

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<v Speaker 1>is very close to the actual number of three sixty

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<v Speaker 1>five point two four to two days. The remaining corrections

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<v Speaker 1>are added with things like leap seconds. Putting all these

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<v Speaker 1>rules together, you can see that a year is a

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<v Speaker 1>leap year not only if it is divisible by four.

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<v Speaker 1>It also has to be divisible by four hundred if

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<v Speaker 1>it's a centurial year. So seventeen hundred and nineteen hundred

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<v Speaker 1>were not leap years, but two thousand was Do you

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<v Speaker 1>have any ideas or suggestions for this podcast? If so,

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<v Speaker 1>please send me an email at podcast at how stuff

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<v Speaker 1>works dot com. For more on this and thousands of

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<v Speaker 1>other topics, go to how stuff works dot com