I’m guilty of asking this question as a kid, but never really understanding the answer as an adult! So here goes.

Fortunately, it’s actually not so tough to understand. And unsurprisingly, **it’s all the sun’s fault.**

Now we all know that a year is determined by how long it takes the earth to complete one complete rotation around the sun. Right? A lap that takes 365 days….

Wrong.

A year doesn’t *exactly* equal 365 days. Funnily enough, the earth hasn’t decided to line up the speed of its rotation around the sun with any exact number of days.

Instead of taking bang on 365 days to complete a lap, our earth takes a smidge longer: about 365.25 days in fact. That’s 365 days and about 6 hours.

Now you might think that those 6 hours are just a niggle of a rounding error. Ignorable in the grand scheme of things. But sadly not. Over time, they add up. Over 350 years it would delay our concept of a year by about 3 months. We’d be eating our Christmas dinner in the balmy spring weather of today’s late March. Nope. I like snow and Christmas. Wouldn’t work. Too controversial. (And it has in the past!)

So, to ensure our calendars line up with the movement of the earth around the sun, we need to make an actual year equal 365 and 1/4 days. Rather than tag a little runt of a day on to the end of a year, (shame. I could imagine a reasonably boozy, 6 hour-long December 32nd) we just make every *fourth* year one whole day longer.

Hence a leap year.

Wooohoo! Problem solved!

Well not quite. Things are a little more complex.

Us humans are a pinnickity bunch. When folks in 1582 looked at the *precise* duration of a year, they calculated that it actually equates to 365.2425 days, not 365.25. As such, we’ve needed to do a little more jiggery pokery with our calendar to make it line up exactly with our journey around the sun.

The result:

While this extra year is added every four years, it’s not added to years that fall on round 100 years. (Reaching an average year through time equalling 365.242 days.)

But it is added again when the year is divisible by 400. (Bringing us to an average year-length of bang on 365.2425 days)

Well done humanity 👏🏼 …

If you’re happy with that answer, it’s probably best to stop reading now.

However, I had scratched an itch. And from this understanding a new, more fundamental, question emerged:

**How did we prove ***exactly*** how long it takes the earth to go around the sun?**

After all, everything is just floating, spinning and rotating in the empty blackness of space. It’s not like we have some kind of intergalactic odometer stuck the North Pole…. Do we?

The answer comes down to the fascinating history of how we have come to calculate the exact duration of year. Find out more here.