When Is the Next Leap Year?
When is the next leap year? Why do we need (or not need ) to leap? And how much do we leap at a time? Keep on reading The Math Dude to find out – and to learn how to calculate if a year requires a leap.
Jason Marshall, PhD
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When Is the Next Leap Year?
Is this year a leap year? The answer to that question depends upon when you’re reading this.
If it’s sometime in 2015, the answer is “no.” If it’s sometime in 2016, the answer is “yes.” And if it’s sometime after 2016, the answer is “2020, 2024, 2028, or some other year that’s a multiple of four…for the most part.”
But why is this “for the most part?” What’s the reasoning behind all of these leap years in the first place? And how can you figure out when the next leap year is going to be? Those are exactly the questions we’ll be answering today.
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How Many Days In a Year?
Thirty days have September,
April, June, and November;
February has 28 alone,
All the rest have 31;
Except leap year, that’s the time,
When February’s days are 29.
As made quite clear by Mother Goose here, most years contain (4 x 30) + (1 x 28) + (7 x 31) = 365 days. Except, of course, those mysterious leap years, which contain an extra day.
Why are there 365 days in most years? Because that’s the number of days it takes the Earth to orbit the Sun for one complete revolution. But it that exactly 365 days? No!
If you think about it, you’ll see that it would be phenomenally remarkable if it was. After all, the length of a day is determined by how long it takes Earth to spin once on its axis. And surely this has nothing to do with the time it takes Earth to go around the Sun. So it should come as no big surprise that the number of days in a year isn’t quite 365 – it’s actually a little bit more.
Why Are There Leap Years?
The amount of time it takes Earth to go around the Sun—the length of a year—is known to be approximately 365.242375 days. This is actually a phenomenally precise approximation.
Broken down into hours, minutes, and seconds, it says that it takes 365 days + 5 hours + 49 minutes + 1.2 seconds for Earth to complete its annual lap around its 584 million mile jogging track. (By the way, that makes for an average speed of over 66,500 miles per hour!)
The length of a year is not an integer number of days.
So, as we’ve seen, the length of a year is not an integer number of days. Which means that if we used a calendar that had 365 days each and every year from now until forevermore, we’d eventually be celebrating Christmas in the northern hemisphere’s summer!
That’s because we’d be short one day roughly every four years, and therefore short 180 days (or roughly half a year) after 720 years. Sure, that’s a long time to wait for things to get fully out of sync, but a good calendar should be able to deal with that sort of thing. Which is precisely why leap years have been added into the mix over the centuries.
When Do Leap Years Occur?
Since there are approximately 365.25 days each year, all we have to do to keep things relatively in sync is add one extra day every four years—so we have 3 normal 365 days years followed by a 366 day leap year (with the extra day tacked onto the already oddball-in-length month of February.)
But that’s not quite right. Because the Earth doesn’t actually take 365.25 days to travel around the Sun, it takes 365.242375 days – a little less. So by adding a leap day every four years, we’re actually overcompensating a bit.
How much? About 0.01 days per year—which is 1 day every 100 years too many. Which is why every century on the century mark (meaning years divisible by 100), we skip the leap year and enjoy a normal 365 day year.
Once we do that, we end up with 365.24 days per year averaged over a century. This is pretty good, but still not perfect, since an actual year is 365.242375 days long. That extra 0.002375 days that we’re not accounting for equates to about 3 minutes 25 seconds of lost time every year.
What can we do about this? We can add an extra leap day at some regular interval to make up for this lost time. And over the centuries, this regular interval has become every 400 years. Why? Because one extra day every 400 years is equivalent to 0.0025 extra days per year…which is pretty close to the 0.002375 days we were after.
Is This a Leap Year?
We can put all of this together to come up with a rule that you can use to figure out if this year, or next year, or any other year is a leap year.
- If the year is evenly divisible by 4, then it’s a leap year…
- Unless it’s also evenly divisible by 100, and then it’s not.
- Unless it happens to be evenly divisible by 400, and then it is again.
So 2012 was a leap year, and 2016, 2020, 2024, and so on – all the way up through 2096 – will be, too. But 2100 will not be, since it’s evenly divisible by 100. And even though the year 2000 was evenly divisible by 100, it was also evenly divisible by 400 – which is why it actually was a leap year.
And that’s all there is to it!
Wrap Up
OK, that’s all the math we have time for today.
For more fun with math, please check out my book, The Math Dude’s Quick and Dirty Guide to Algebra. And remember to become a fan of The Math Dude on opens in a new windowFacebook and on opens in a new windowTwitter.
Until next time, this is Jason Marshall with The Math Dude’s Quick and Dirty Tips to Make Math Easier. Thanks for reading, math fans!
opens in a new windowCalendar image courtesy of Shutterstock.