Once (almost) every four years, February has a 29th day. Today’s IFOD is about why the calendar needs this extra day, as well as a few other little-known interesting facts about Leap Day.
1. Why Leap Days are Necessary
To get the obvious fact out of the way, leap days are needed to keep the calendar and seasons aligned because it takes longer than 365 days for the Earth to orbit the Sun — 365.24219 days, to be precise, or a bit less than 365¼ days. (But see the end of this IFOD for a slight technical correction to the prior sentence.) That (almost) extra 1/4th of a day means that (almost) every four years, the calendar needs an extra day to avoid season slippage.
Without a leap day, the solstice and equinox dates would move a quarter day a year. As the below chart from Axios shows, without leap days, the Spring equinox would slip to July within 425 years and the seasons would flip about every 750 years.
2. Leap Days Don’t Occur EVERY Four Years
If it took exactly 365¼ days for the Earth to orbit the Sun, leap days occurring every four years would keep the calendar and seasons nicely aligned. But because it takes a smidge less than 365¼ days for the Earth’s annual orbiting of the Sun (365.2419 days), leap days add a bit too much time — about an extra 44 minutes. For this reason, every so often leap days need to be skipped. The rule is that if the year is divisible by 100 but not by 400, then the leap year is skipped. So, 1700, 1800, and 1900 were not leap years, but 2000 was. Likewise, 2100, 2200, and 2300 will not be leap years, but 2400 will be.
3. What Happens For People Born on February 29th?
People born on February 29th are “Leaplings,” and out of the 8 billion people on the planet, there are only about 5 million of them. At first blush, that seems low, but if you do the math, the odds of a February 29th birthday are 1 in 1,461 vs. the 1 in about 365 for the other calendar days.
Leaplings usually choose to celebrate their birthdays in non-leap years on February 28th (because they are February babies and not March babies). But they can obviously choose March 1st as their birthday celebration, and I guess they could switch it up based on which day of the week works best each year. Fun!. In the U.S., their driver’s licenses and other official documents have February 29th as their date of birth.
4. Why is Leap Day in February?
Why is February 29th Leap Day? Why not have an April 31st or August 32nd?
Duncan Steel, the author of Marking Time: The Epic Quest to Invent the Perfect Calendar, says that “the Romans chose to add the extra day in February for two reasons: They considered it to be an unlucky month, and it was the month that came before Easter.” Source. Huh. I guess that’s a good a reason as any.
Speaking of Romans, Leap Day was first recognized by Julius Caesar in 45 BCE. Under the so-called “Julian Calendar,” Leap Day occurred every four years. It wasn’t until Pope Gregory XIII established the Gregorian calendar system in 1582 that the aforementioned rule about leap years being skipped in century years not divisible by 400.
5. Why is it Called Leap Day?
According to the Smithsonian, “a common year is 52 weeks and 1 day long. That means that if your birthday were to occur on a Monday one year, the next year it should occur on a Tuesday. However, the addition of an extra day during a leap year means that your birthday now “leaps” over a day. Instead of your birthday occurring on a Tuesday as it would following a common year, during a leap year, your birthday ‘leaps’ over Tuesday and will now occur on a Wednesday.”
Final note
Our measurement of a year isn’t how long it takes the Earth to orbit the Sun (which is a “sidereal year” or “astronomical year”). Rather, a year is the amount of time the Sun takes to return to the same position in the sky (as viewed from a celestial body of the Solar System, such as the Earth), thus completing a full cycle of seasons (this definition is known as a “tropical year” or “solar year”) and is about 20 minutes shorter than the astronomical year. Source.
Here’s a great article by an astrophysicist about the physics of Leap Day and an explanation of why we use a tropical year instead of an astronomical year.
Thank you for providing these facts, and even more thought provoking and knowledge building info, (like SIDEREAL, folks look it up, learning is a sign of growing) while I was trying to read financial information, I think helps my ADHD distractions. Now back to…