Leap Year: A Day Out of Sync and the Journey to Fix It
Hiya! Yesterday was the 28th of February. Ordinarily the end of this short month, but not this year. Because 2024 is a number divisible by four, February gets to last an extra day than it usually would. Celebrating February's extended stay begs the question: what exactly is a leap year, and why does it exist?
A leap year is a special kind of year. A year, roughly speaking, is the length of time it takes the planet Earth to complete a trip around its middle-aged star, the Sun. It's a rough measure as calculating the amount of time it takes the earth to do so, from the earth, in the era before the invention of the stopwatch is kinda tricky. So one way to arrive at that rough answer is to count the number of days it takes for the seasons marked by change in weather, the hours of daylight, and, consequently, vegetation and soil fertility, to complete a full cycle. Thus we have 365 days, the basis of our calendar today. But as it is only a rough estimate, it kinda has a bit of a problem.
Over time, the seasons drift apart from the dates derived from such a calendar as described above. Why is this? Well, it has to do with the difference between the time taken to complete a revolution around the sun and the time such a calendar predicts it would take for such a revolution to be complete. By our Calendar, we have 365 days X 24 hours = 8, 760 hours. But the actual timing of the earth's rotation puts it as 8,765 hours, 48 minutes and 46 secs. Almost 6 hours longer than the time predicted by our calendar. That amount of time our rough calendar ignores quickly accumulates. In six days, our Calendar ignores more than a full day. In a month, the seasons already lag behind our calendar by a week. And in a year, we are looking at almost 100 days.
On closer inspection, the reason for this disparity is obvious: the day and night cycle, from which we get the days in our calendar, has as much to do with the year as the spin of a coin does. You see, the day/night cycle is determined by the rotation of the earth about its axis. The side of the earth that faces the sun due to that rotation has day and the side that faces away from the sun has night. Being conscious of this, it becomes clear that there’s no causal link between the day and the year and as such it would be absurd for the former to be a perfect subdivision of the latter.
So, while 365 days are too few, 366 days are too many, causing the seasons to drift out of sync oppositely: the calendar becomes slower than the earth's trip. Hence the leap year— the idea to add an extra day every four years to bring the Calendar in alignment with the seasons as each year, the calendar is a quarter of a day faster than the earth's trip. However, this is only a partial solution as one day every four years is too many as well, over the scale of a hundred years that is. The calendar eventually drifts out of sync with the seasons by a day every hundred years. Good enough for you and me I guess. But not for Pope Gregory XIII.
He went into holy meditation, from which he emerged with the inspired idea that every year that marks the turn of a century will not be a leap year. So 1900, divisible by 4, lost its leap year status. This worked... Up to an extent. The Calendar, having lost quite a few leap years, now drifts from the seasons by a day every 400 years. But Greg was unsatisfied, so to fix this, if the century year is divisible by 400, it becomes a leap year. Hence 2000 was a leap year. This works even better to keep the calendar in sync with the seasons, only drifting out of sync by a day every 8000 years.
Finally good enough, for our calendar-obsessed priest. After all, 8000 years ago, we just discovered agriculture, and 8000 years from now…meh.
But you may ask, why are we merely patching up what is fundamentally an inaccurate calendar anyway? Why not make a more perfect one? Well, our bodies operate on the circadian rhythm—biological processes that repeat roughly every 24 hours, so the day is not just a measure we can readily drop. Even the day as represented by the rotation of the earth is not exactly 24 hours, because our planet’s orbit is not a perfect circle, some days are longer than 24 hours by a few minutes, and some are shorter. Only four days a year would a day equal 24 hours. This not accounting for the slowing of the speed of the earth’s spin; every hundred years, a day increases by 1.7 milliseconds. So in tens of billions of years, the day would eventually be as long as the current month. But no need to worry, the oceans would have boiled over long before then.
