By Shannon Horning
Can you answer this riddle?
Jamie is having her third birthday party, but she’s been alive for 12 years. How is this possible?
If you said that it’s because her birthday is on a leap day, you would be correct! While people born on a leap day still age as a normal human does, their birthday technically only occurs once every four years. And if you didn’t notice before spring break, this year happens to be a leap year. In other words, instead of having 28 days, February had 29. But why do we have leap years and why are they important?
A calendar year is typically 365 days long – the time it takes for the Earth to orbit once around the sun. However, 365 is just a rounded number. More precisely, it takes the Earth 365.25 days to orbit the sun. This leaves five hours, 48 minutes, and 56 seconds that are unaccounted for at the end of a normal 365-day calendar year.
As tempting as it might seem, we can’t simply disregard this additional quarter of a day and expect it to have no consequences. If we did, the start of our seasons would gradually shift later and later. After 60 years, they’d be 15 days off. After 200 years, they’d be 50 days off. And the next thing we know, Halloween is in the spring and the Fourth of July in the early winter.
The extra day every four years keeps our seasons and solstices consistent and in check. However, a leap year doesn’t actually occur every four years.
There is still a discrepancy with the number of 365.25 days — it’s rounded, not exact. It actually takes closer to 365.242190 days for the Earth to orbit the sun. Therefore, adding an extra day every four years isn’t the most precise solution.
When Julius Caesar introduced the 365-day calendar, which included the leap year, the discrepancy seemed insignificant. The calendar only deviated by one day every 128 years. However, this slight difference between the solar and calendar year accumulated over time and was eventually observed in the 1500s when Easter had shifted noticeably from its traditional Sunday date.
To amend this, Pope Gregory XIII introduced a revised “Gregorian Calendar” which we still use today. Furthermore, to fix the 10-day shift caused by the Julian Calendar that year, Oct. 4, 1582 was followed by Oct. 15, 1582.
The Gregorian Calendar fixes the issue of the Julian Calendar by eliminating the usage of leap years on years that end with two zeroes (such as 1900 and 200), but are not divisible by 400. In other words, every year divisible by four is considered a leap year, except for centurial years, which must also be divisible by 400 to qualify as leap years. Therefore, 1200 and 1600 are leap years, but not 1300 and 1700. What a complicated rule, right?
However, this system remains imperfect. Compared to the solar year, it is still about 26 seconds off. Yet, when compared to the Julian Calendar — where a day shift accrued every 129 years — a day shift only occurs every 3,333 years under our modern-day calendar. So, eventually, the calendar will experience a significant shift once again, but that’s a concern for a generation far ahead of us and a problem left for our distant descendants.