In the coming years, leap years might get a bit strange. Let me explain why in a simple way.
Surprisingly, the Universe is not perfect, at least not from our human point of view. An easy example of this imperfection is the idea of leap years, like the current one, 2024.
What is a leap year?
Let me break it down for you: A leap year happens every four years. Instead of the usual 365 days, that year has 366. We add an extra day to the end of February, making it 29 days instead of the usual 28.
Why do leap years happen?
The reason for this odd arrangement is because we define a year as the time it takes Earth to go all the way around the Sun, while a day is the time it takes Earth to spin once on its axis. These two cycles don’t perfectly match up.
Even though our calendar makes it seem like these two motions are linked, they are as unrelated as a spinning top on a table inside a moving train.
“Leap years are essential to make sure our calendar matches up with Earth’s orbit around the Sun,” explains Dr. Minjae Kim, a research fellow in astrophysics at the University of Warwick.
Kim clarifies that the time between one spring equinox and the next, called a “tropical year,” is roughly 365.24 days, a bit longer than our regular 365-day calendar.
You might think that the extra 0.24 days isn’t a big deal, but over a few centuries, it adds up. Without leap years, our calendar would fall out of step with the seasons.
Dr. Kim emphasizes that leap years are crucial to prevent this drift and keep our calendar aligned with Earth’s orbit around the Sun.
Since 0.24 is close to a quarter of a day, we add four of these increments together on February 29 in a leap year to stay in sync with the seasons.
Who invented leap years?
The idea of adding an extra day every four years was brought in by the ancient Roman leader Julius Caesar in 46 BC. Before this, the Roman Empire used a calendar with 355 days in a year, which was known as the Julian calendar, named after Julius Caesar himself.
Instead of occasionally adding an extra day, the high priests in Rome used to decide when to insert a leap month if they sensed a misalignment. So, did the Julian system completely fix our calendar issues? Not quite.
“The Julian calendar was, in fact, a bit of an overcorrection to the issue,” clarifies Dr. James McCormac, an assistant professor in the astronomy and astrophysics research group at the University of Warwick.
“The solar year wasn’t precisely 365.25 days but a bit less, around 365.2422 solar days. Consequently, with the Julian Calendar, the solar year and the calendar began to drift apart again, albeit at a much slower pace—around 11.2 minutes per year,” details Dr. James McCormac.
With a deviation of 11.2 minutes per year, the Julian calendar accumulates an error of approximately one day every 128 years.
How often do leap years happen?
Due to the deemed unacceptability of the error in the Julian system, Pope Gregory XIII reformed the calendar in 1582. The resulting Gregorian calendar, which we use today, follows three simple rules to determine leap years:
- Every year divisible by four is a leap year.
- However, if it’s a new century, skip rule one, unless…
- The new century is divisible by 400, in which case, ignore rule two.
These rules explain why 1896 and 1904 were leap years, but 1900 wasn’t. The application of rule three is why the year 2000 was still a leap year. With the Gregorian calendar, the error is now just one day approximately every 3000 years.
Famous Individuals Born on Leap Day
Here are some notable individuals born on February 29:
- Jack Lousma, a NASA astronaut with over 1,619 hours in space, born on February 29, 1936.
- Ja Rule, an American rap legend from the early 2000s, born on February 29, 1976.
- Spanish Prime Minister Pedro Sánchez Pérez-Castejón, born on February 29, 1972.
- Cullen Jones, an American competition swimmer and Olympic gold medalist, born on February 29, 1984. He set a world record in the 4×100-metre freestyle relay at the 2008 Beijing Olympics with a time of 3:08.24.
Is It Possible to Create the Perfect Calendar?
Even though some have tried to enhance the Gregorian calendar or devise a new “permanent calendar,” there’s no flawless solution. The Universe simply isn’t perfect.
“Leap years beautifully show how we’ve synchronized our constructed idea of time with the natural rhythm of the Universe,” says Kim. However, this harmony isn’t meant to endure.
To begin with, scientists reckon that our days are gradually getting longer by approximately 1.8 milliseconds per century. This is partly due to the friction between the oceans and land, causing our planet to lose some rotational momentum.
The slowing rotation of the Earth’s core could also be contributing to the lengthening of our days. Additionally, the Earth’s orbit around the Sun, a crucial factor in creating any calendar, is gradually expanding by approximately 1.5 centimeters per year.
This is mainly due to the Sun losing mass as it converts hydrogen into helium and releases particles as solar wind.
As Earth’s orbit enlarges, our years become slightly longer. However, these changes will only significantly impact timeframes much larger than human history spans. So, there’s no urgent need for a new calendar just yet.
If humanity endures for such extended periods, the idea of a calendar based on the time it takes one planet to orbit an average star in a typical galaxy might eventually seem irrelevant.