Time traveling to get to leap year
Math plus history equals extra day for February in 2024
NEW YORK — Leap year. It’s a delight for the calendar and math nerds among us. So how did it all begin and why?
Have a look at some of the numbers, history and lore behind the (not quite) every-four-years phenom that gives February a 29th day.
The math comes down to fractions of days and minutes. There’s even a leap second occasionally, but there’s no hullabaloo when that happens.
The thing to know is that leap year exists, 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.
It’s a correction to counter the fact that Earth’s orbit isn’t precisely 365 days a year. The trip takes about six hours longer than that, NASA says.
Contrary to what some might believe, however, not every four years is a leaper. Adding a leap day every four years would make the calendar longer by more than 44 minutes, according to the National Air & Space Museum.
Later, on a calendar yet to come (we’ll get to it), it was decreed that years divisible by 100 not follow the fouryear leap-day rule unless they are also divisible by 400, the JPL says. 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.
The next leap years are 2028, 2032 and 2036.
So who came up with leap year? The short answer: It evolved.
Ancient civilizations used the cosmos to plan their lives, and some calendars date to the Bronze Age. They were based on the phases of either 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 an array of calendars that started in a vast array of ways in the vast Roman Empire.
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 a 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. The solar year isn’t precisely 365.25 days, it’s 365.242 days, said Nick Eakes, an astronomy educator at the Morehead Planetarium and Science Center at the University of North Carolina in Chapel Hill.
Thomas Palaima, a classics professor at the University of Texas at Austin, said adding time to a year to reflect variations in the lunar and solar cycles was done by the ancients. The Athenian calendar 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, he said.
The Julian calendar was 0.0078 days (11 minutes and 14 seconds) longer than the tropical year, so errors in timekeeping gradually accumulated, according to NASA. But stability increased, Palaima said.
The Julian calendar was the model used by the Western world for hundreds of years. Enter Pope Gregory XIII, who calibrated further. His Gregorian calendar took effect in the late 16th century. It remains in use and, clearly, isn’t perfect. But it was a big improvement, reducing drift to mere seconds.
Why did he step in? Well, Easter. It was coming later in the year over time, and he fretted that events related to Easter, such as Pentecost, might bump up against pagan festivals. The pope wanted Easter to remain in the spring.
He eliminated extra days accumulated on the Julian calendar and tweaked the rules on leap day. It’s Pope Gregory and his advisers who devised the gnarly math on when there should or shouldn’t be a leap year.
Leap year also has a marriage angle. Leap day comes with lore about women popping the marriage question to men. It was mostly benign fun, but it came with a bite that reinforced gender roles.
One story places the idea of women proposing in fifth-century Ireland, with St. Bridget appealing to St. Patrick to offer women the chance to ask men to marry them, historian Katherine Parkin wrote in a 2012 paper in the Journal of Family History.
Nobody really knows where it all began.
In 1904, syndicated columnist Elizabeth Meriwether Gilmer, aka Dorothy Dix, summed up the tradition this way: “Of course people will say ... that a woman’s leap year prerogative, like most of her liberties, is merely a glittering mockery.”
Advertising perpetuated the leap-year marriage game. A 1916 ad by the
American Industrial Bank and Trust Co. read: “This being Leap Year day, we suggest to every girl that she propose to her father to open a savings account in her name in our own bank.”
Being born on a leap day certainly is a talking point.
But it can be kind of a pain from a paperwork perspective. Some governments and others requiring forms to be filled out and birthdays to be stated stepped in to declare what date was used by leaplings for such things as driver’s licenses, whether Feb. 28 or March 1.
Technology has made it far easier for leap babies to jot down their Feb. 29 milestones, although there can be glitches in terms of health systems, insurance policies and with other businesses and organization that don’t have that date built in.
About 5 million people worldwide share the leapday birthday out of about 8 billion people on the planet.
Shelley Dean, 23, in Seattle chooses a rosy attitude about being a leapling. Growing up, she had normal birthday parties each year, but an extra special one when leap years rolled around.
This year is different. “It will be the first birthday that I’m going to celebrate with my family in eight years, which is super exciting, because the last leap day I was on the other side of the country in New York for college,” she said. “It’s a very big year.”
So, what would happen without a leap day?
Eventually, nothing good in terms of when major events fall, when farmers plant and how seasons align with the sun and the moon.
“Without the leap years, after a few hundred years we will have summer in November,” said Younas Khan, a physics instructor at the University of Alabama at Birmingham. “Christmas will be in summer. There will be no snow. There will be no feeling of Christmas.”