Leap year. It’s a delight for the calendar and maths nerds among us. So how did it all begin and why?
We break down the science, and history, behind this (not quite) every four year phenomenon that adds a 29th day to February.
What is a leap year?
It’s a year with 366 days instead of the usual 365 days.
And that extra day is added to the end of February.
Usually, February only has 28 days, but this year February has 29 days.
Why do we have leap years?
The thing to know is that leap years exist, in large part, to keep the months in sync with annual events, including equinoxes and solstices, according to the Jet Propulsion Laboratory at the California Institute of Technology (JPL).
It’s a correction to counter the fact that Earth’s orbit isn’t precisely 365 days a year.
Leap year birthday
An 80-year-old great-grandmother is celebrating her 20th official birthday this leap year.
How often do we have leap years?
Roughly every four years but, contrary to what some might believe, not every four years is a leap year.
That’s because adding a leap day every four years would make the calendar longer by more than 44 minutes, according to the National Air and Space Museum.
It was decreed that years divisible by 100 not follow the four-year leap day rule unless they are also divisible by 400, the JPL notes.
In the past 500 years, there was no leap day in 1700, 1800 and 1900, but 2000 had one.
In the next 500 years, if the practice is followed, there will be no leap day in 2100, 2200, 2300 and 2500.
When is the next leap year?
The next leap years are:
2028
2032
2036
The last leap year we had was in 2020.
What would happen if we didn’t have a leap day?
Eventually, the seasons would be out of sync with the months — but that would take a bit of time.
“Without the leap years, after a few hundred years, we [in the Northern Hemisphere] will have summer in November,” Younas Khan, a physics instructor at the University of Alabama, said.
Meanwhile, for Australians and the rest of us living in the Southern Hemisphere, it would mean we would have winter in November.
Ancient civilisations used the cosmos to plan their lives, and there are calendars dating back to the Bronze Age.
They were based on either the phases of the Moon or the Sun, as various calendars are today. Usually they were “lunisolar,” using both.
Now hop on over to the Roman Empire and Julius Caesar.
He was dealing with major seasonal drift on calendars used in his neck of the woods. They dealt badly with drift by adding months.
He was also navigating a vast array of calendars starting in a vast array of ways in the vast Roman Empire.
Why do we have seasons?
Only a few parts of the world experience the classic four seasons of spring, summer, autumn and winter.
He introduced his Julian calendar in 46 BCE. It was purely solar and counted a year at 365.25 days, so once every four years an extra day was added. Before that, the Romans counted a year at 355 days, at least for a time.
But still, under Julius, there was drift — there were too many leap years.
That’s because a true solar year isn’t precisely 365.25 days.
It’s actually 365.242 days, Nick Eakes, an astronomy educator at the University of North Carolina, pointed out.
And while the difference between 0.25 and 0.242 might not seem like much, it all adds up over time.
“If the solar year was a perfect 365.25 then we wouldn’t have to worry about the tricky math involved,” Mr Eakes said.
Thomas Palaima, a classics professor at the University of Texas at Austin, said adding periods of time to a year to reflect variations in the lunar and solar cycles was done by the ancients.
The Athenian calendar, he said, was used in the fourth, fifth and sixth centuries with 12 lunar months.
That didn’t work for seasonal religious rites. The drift problem led to “intercalating” an extra month periodically to realign with lunar and solar cycles, Professor Palaima said.
The Julian calendar was 0.0078 days (11 minutes and 14 seconds) longer than the tropical year, so errors in timekeeping still gradually accumulated, according to NASA. But Professor Palaima said stability increased.
