Happy Leap Day! Today is the 29th of February, which is known as Leap Day. We know that Leap Days only happen once every 4 years, but that is not the sole event that makes this day special; today is also a Thursday, which is post day for Onward American. Next Leap Day will land on a Tuesday, and the one after that will be on a Sunday. It turns out that the next time a Leap Day will land on a Thursday is in 2052, which is 28 years from today!
We will have to see what the topic of my post will be on February 29, 2052, if Onward American still exists at that time, which would make this blog 30 years old!
I guess it shouldn’t be surprising that it takes 28 years for Leap Day to return to a specific day of the week. Every time you move forward a year, any given day shifts forward one day in the week, for example, February 1 landed on a Wednesday last year, but it landed on a Thursday this year. Another thing that shifts every day one day forward in the week is a Leap Day. These two effects combined cause each Leap Day to move 5 days forward in the week from the last one (4 years + 1 Leap Day = 5 days forward). With these effects in mind, we can figure that it takes 7 Leap Years to cycle through the week and return to a Thursday, but because a Leap Year transpires once every 4 years, it will take 28 years (7 x 4 = 28).
This means that today is the first time a Leap Day landed on a Thursday since 1996, and it won’t happen again until 2052, and then the time after that will be 2080, and then 2120 (yes, the 2120 Leap Day is 40 years after the 2080 Leap Year, I will explain this phenomenon in a moment). Leap Days that land on Thursdays are so far and few between that this is the first one in my lifetime.
What is a Leap Day?
Leap Days are not actually extra days, they are days that we use to catch up to the actual time. We often consider a year to be 365 days long, but in reality, a year is nearly 365 and 1/4 (365.25) days long. Rather than adding a 6 hour long day to each year, we simply wait 4 years and then combine those 4 partial days into one whole day (4 x 1/4 = 1), however, we don’t actually have a Leap Year every fourth year, that’s the way the faulty Julian calendar works, instead, we use the Gregorian calendar, which is also technically imperfect.
The Julian Calendar vs. the Gregorian Calendar
The Julian calendar, which was named after Julius Caesar, was made to replace the Roman calendar. The Julian calendar took effect on January 1, 45 BC, and it lasted for more than 1,600 years. This calendar assigns 365 days to each year, except it designated every fourth year as a 366 day long Leap Year. There were no exceptions to this simple rule, as the calendar assumed that a year was exactly 365.25 days long.
The problem is that an actual year is closer to 365.2422 days long, meaning the Julian calendar gains too many days and it therefore strays from the Earth’s actual position in its orbit around the sun, which is what defines a year.
This is where the Gregorian calendar comes in, the one that is currently used in most of the world. The Gregorian calendar, named after Pope Gregory XIII, went into effect in October, 1582, as a modification of, and replacement for, the Julian calendar.
The rules of the Gregorian calendar are: a Leap Year occurs every year that is divisible by 4, except for the years that are exactly divisible by 100, but if a centurial year is exactly divisible by 400, then it is a Leap Year.
This means that each Leap Year will be divisible by 4, such as 2016, 2020, and 2024, but the years that begin a century are not Leap Years unless they are divisible by 400; meaning that 1700, 1800, and 1900, are not Leap Years, but 1600, 2000, and 2400 are Leap Years.
It’s because of these rules that there is a 40 year gap between the Leap Day that lands on a Thursday in 2080 and the next one that lands on a Thursday. According to the 28 year pattern that I previously explained, we would expect to see that the next Leap Day on a Thursday after 2080 would land on 2108, however, the year 2100 is not a Leap Year because it is not divisible by 400, which alters the pattern, shifting the next Leap Day on a Thursday from 2108 to 2120.
The Inaccuracies of the Julian and Gregorian Calendars
Technically, the rules of the Gregorian calendar would suggest that a year is 365.2425 days long, meaning that the Gregorian calendar is also inaccurate, but the average Gregorian year is only 0.0003 days too long; to put this into perspective, the average year should have about 31,556,926 seconds, and the average Gregorian year only has 26 more seconds than this ideal number, making the Gregorian year 100.00008% the length of an actual year, which is more accurate than using 3.14 to represent π (pi), you would have to write π as 3.14159 to get the same accuracy level as the Gregorian calendar.
The Julian calendar gains one day more than it should every 128 years, but the Gregorian calendar gains one day more than it should every 3,333 years. This means that the Julian calendar would have fallen 16 days behind the Earth’s actual position around the sun in the more than 2,000 years since it was introduced, whereas the Gregorian calendar would have only fallen behind half a day in the same amount of time. That should mean that the Gregorian calendar has only fallen behind about 1/8 of a day since it was introduced in the year 1582, assuming no corrections have been made; in other words, it has fallen behind 3 hours in 442 years, which is not bad at all.
The Legacy of the Switch from Julian to Gregorian
If you look at the calendar on your computer or phone, you should still be able see the effects of the change from the Julian calendar to the Gregorian! All you need to do is scroll back to October of 1582, and you will notice that the month starts with the 1st and ends with the 31st, just like any other October, however, the month is only 21 days long, because the 4th is immediately followed by the 15th. It would appear that 10 days just vanished, but they didn’t, this forward jump was merely an adjustment. The 10 “missing” days still existed, but in the form of Leap Days from much earlier years. The rules of the Julian calendar caused this discrepancy between the calendar day and the Earth’s position around the sun, because the Julian calendar has 100 Leap Days every 400 years, but the Gregorian only has 97 every 400 years, to correct for the Julian calendar’s roughly 3 day per 400 year inaccuracy.
If this hadn’t been corrected, we would have been off by an entire season after about 11,700 years, meaning true summer would begin when the calendar said spring was supposed to begin, and autumn would begin when the calendar said summer should have, and so on.
I now have an appreciation for the millennia that it took to develop and refine Leap Days.
On what day of the week was the first Leap Day that you experienced? For me, it was a Sunday.
Onward American 
Thursday
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