The Universe, believe it or not, is not a perfect place. At least not from a human perspective. One obvious example of this is the concept of leap years like this one, 2024.
What is a leap year?
By definition, a leap year occurs every four years, when instead of having 365 days in a year, we have 366. The extra day is tagged onto the end of February, giving it 29 days as opposed to the usual 28.
Why do leap years happen?
The reason for this strange ensemble is the disconnect between the way we define a year – the amount of time it takes the Earth to orbit completely around the Sun – and a day – the time it takes for the Earth to rotate once on its axis.
These two motions might feel connected through our calendar, but they’re no more related than a spinning top on a table in the carriage of a moving train.
"Leap years play a crucial role in aligning our calendar with the Earth's orbit around the Sun,” says Dr Minjae Kim, an astrophysics research fellow at the University of Warwick.
As Kim explains, the amount of time from one spring equinox to the next – known as a “tropical year” – is approximately about 365.24 days, slightly longer than our standard 365-day calendar.
That surplus of 0.24 days might seem insignificant, but let it accumulate for a few centuries, and we’d find ourselves seriously out of sync with the seasons. “Leap years, therefore, are essential to prevent this drift and maintain the alignment of our calendar with the Earth's journey around the Sun,” Kim says.
As 0.24 is approximately a quarter of a day, we add four of them together on 29 February in a leap year to keep in step.
Who invented leap years?
This system of adding one day every four years was introduced by none other than the ancient leader of Rome, Julius Caesar, in the year 46 BC. Prior to this appropriately named Julian calendar, the Roman Empire had operated using a calendar with 355 days in a year.
Instead of adding the occasional extra day, back then it was the job of the high priests in Rome to call a leap month whenever they felt things were getting out of phase. So, did the Julian system solve our calendar woes once and for all? Not exactly.
“The Julian calendar was actually a slight overcorrection to the problem,” explains Dr James McCormac, an assistant professor in the astronomy and astrophysics research group at the University of Warwick.
“As the solar year was not exactly 365.25 days but was in fact slightly less at 365.2422 solar days, the Julian Calendar and the solar year were now drifting apart again, albeit much more slowly, at a rate of 11.2 minutes per year.”
11.2 minutes per year gives the Julian calendar an error of about one day every 128 years.
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How often do leap years happen?
With the error in the Julian system ultimately deemed unacceptable, the calendar was reformed once more by Pope Gregory XIII in the year 1582.
The Gregorian calendar, which we use today, has three simple rules for finding out how often we have a leap year:
- Every year that is exactly divisible by four is a leap year.
- Unless that year is a new century, in which case discount rule one,
- Unless that new century is divisible by 400, then ignore rule two.
These rules explain why the years 1896 and 1904 were leap years, but 1900 wasn’t. Applying rule three is why the year 2000 was still a leap year.
With the Gregorian calendar in place, we now have an error of just one day approximately every 3000 years.
Celebrities with a leap-year birthday
- Jack Lousma, a NASA astronaut who logged over 1,619 hours in space over two missions, including 11 hours of spacewalking, was born on 29 February 1936.
- American early-noughties rap legend Ja Rule was born on 29 February 1976.
- Spanish Prime Minister Pedro Sánchez Pérez-Castejón was born on 29 February 1972.
- American competition swimmer Cullen Jones, who won a gold medal in the 4×100-metre freestyle relay in a world record time of 3:08.24 at the 2008 Beijing Olympics, was born on 29 February 1984.
Can we make a perfect calendar?
Despite efforts by some to improve the Gregorian calendar or even create a new “permanent calendar”, unfortunately, no perfect solution exists. The Universe just isn’t a perfect place.
“Leap years beautifully illustrate how we have harmonised our constructed concept of time with the natural rhythm of the Universe,” Kim states. But this harmony isn’t built to last.
For starters, scientists estimate that our days are getting longer by around 1.8 milliseconds a century, in part as a result of the friction between the oceans and land, which causes our planet to lose some rotational momentum.
A slowing of the rotation of the Earth’s core could also be having a lengthening effect on our days.
Furthermore, the Earth’s orbit around the Sun – the other critical factor when creating any calendar – is spiralling outwards by about 1.5 centimetres per year, mostly as a result of the Sun’s mass loss as it fuses hydrogen into helium and as it spits out particles as solar wind.
As Earth’s orbit gets bigger, our years get ever so slightly longer, too.
These changes will only have a significant effect over timescales much greater than the span of human history, however, so no need for a new calendar just yet.
If our species makes it that far, the concept of a calendar based on how long one planet takes to orbit an average star in a typical galaxy may seem entirely irrelevant anyway.
Read more:
- Something very weird is happening inside Earth’s core. Here's what that means for the planet
- The Earth’s rotation is changing speed: should we be worried?
- How old is Earth? Our world's surprising age, explained
About our experts
Dr Minjae Kim is a research fellow in the Department of Physics at the University of Warwick, focusing primarily on planetary science. Kim’s research looks at traces in the dust that stemmed from debris discs around stars to understand details about the formation and evolution of planetary systems. Additionally, Kim is interested in chemical and dynamical evolution in planetary systems, dust formation and its role in planetary environments and habitability in exoplanetary systems.
Dr James McCormac is an assistant professor in the astronomy and astrophysics group at the University of Warwick. His primary focus is in the field of transiting exoplanets. Having previously worked on the Next Generation Transit Survey, since 2018, McCormac has been involved in the European Space Agency’s PLAnetary Transits and Oscillations (PLATO) mission, due to launch in 2026. PLATO’s goal will be to discover and characterise Earth-like planets around Sun-like stars.