Why do we have leap years and what date would it be without them?
Not February, that's for sure.
Every four years, we indulge in an extra day and stick it at the end of February.
Everyone knows this happens, and even know this date brings about a number of traditions too, but do they know why?
As we prepare to mark a leap year in 2024, we asked the experts for the story behind the day.
Why do we have leap years?
Well, it’s all to do with how our little rocky planet spins around the Sun. You might think, given the calendar, this takes a cool 365 days.
Not quite.
It’s just a smidge under, at around 364.25 days. You can see where this is going.
In order to even things out, every four years – because 0.25 x 4 = 1 – we enjoy that extra day.
Except it still isn’t quite that simple.
Astrophysicist Dr Minjae Kim, from the University of Warwick, picks up the story.
‘The orbit, otherwise known as a tropical year, takes about 364.24 days to complete,’ he says. ‘This is slightly longer than our standard calendar year of 365 days.
‘This extra quarter of a day each year may seem insignificant, but over time, it adds up, leading to a noticeable shift in our calendar.
Get ready for more leaplings
A leapling is someone born on February 29 – unlucky, only getting one birthday every four years…
‘Without adjusting for this extra time, our calendar would gradually fall out of sync with the astronomical seasons, causing a significant drift over the years. Leap years, therefore, are essential to prevent this drift and maintain the alignment of our calendar with the Earth’s journey around the Sun.’
Eagle-eyed readers may have spotted Dr Kim’s figure of 364.24 days, not 364.25. It may seem like a tiny difference, but over time it all adds up, and without correction, would mean us getting ahead of ourselves again.
This is why, drumroll please, there isn’t actually a leap year every four years. Just mostly.
But three out of every four centuries have to discard one leap year to keep things on track. Most recently, the years 1700, 1800 and 1900 all had the regular 365 days.
When will the next missing leap year be?
You might have guessed, it will be the year 2100 – soon enough that some people reading this now will get to enjoy the special event.
The correction was originally built into the Gregorian calendar when it was created by Pope Gregory XIII in 1582.
Where would we be without leap years?
Datawrapper’s Ivan Lokhov did the maths by creating a ‘no-leap calendar’.
In a leap year-free world, working from the introduction of the Julian calendar, right now it would be July 8, 2025.
Now that’s a miserable summer.
This calendar is now adopted by large parts of the world, but before than many stuck to the Julian calendar, proposed by none other than Julius Caesar in 46 BCE.
This replaced the Roman calendar, which followed the movements of the Sun and Moon and required more regular leap days, weeks and even months to keep everything in line.
These leap months, called Mercedonius or Intercalaris, were added every two to three years.
Man would that mess with your annual leave.
Time is a funny thing.