If there was a rotting cherry on the top of the poop sauce covered sundae that was 2020, it is that it was an extra day long. 2020 was a leap year. 2021 will at least not subject us to that. Today is the last day of February, and what a February it was. You probably have some idea of why there are leap days and leap years, but it’s probably worth spending a few minutes revisiting the subject, because let’s face it, what else do you have to do today?
First, I’d like to apologize for the unplanned two-month hiatus. The end of 2020 burned me out on politics, and in the aftermath of the Capital riot, I was struggling to find the right words for quite some time, until my day job reclaimed my soul for several weeks. Next thing you, know, it’s almost March, and the Machine hasn’t said a damned thing in 2021. Until now.
There are three fundamental things that define our broader perception of time on Earth: Earth’s orbit around the Sun, Earth’s rotation about its axis, and the tilt of Earth’s axis relative to its orbital plane. Let’s dig into what each of those things means just a little bit more…
First, as known by some prior to the birth of Christ, and finally proclaimed to the world again by Copernicus, Earth revolves around the Sun. Broadly speaking, this is what defines a year: the length of time it takes our world to make a complete loop around our mother star. This is entirely dictated by mathematics, as discovered by Kepler. If Earth was closer to the Sun, it would complete an orbit more quickly – Mercury only takes 88 Earth days to complete an orbit. If Earth was farther from the Sun, it would complete an orbit more slowly. Pluto takes 248 Earth years to complete an orbit. Imagine if Pluto had life, and if that life had to live through a 2020 that was 248 years long. Poor bastards.
Second, Earth is also spinning, and broadly speaking, this is what defines a day: the length of time it takes our world to spin once about its axis.
Third, Earth’s axis is tilted. What does that mean exactly? Picture Earth making one full orbit around the Sun, and tracing that path all the way around. When it closes that “loop”, you get something looking like a disk. If Earth’s axis wasn’t tilted, it would poke through the North and South poles exactly perpendicular to the disk. But it is in fact tilted about 23.5 degrees away from that scenario, and this makes a huge difference for us. If Earth’s axis wasn’t tilted, the Northern and Southern Hemispheres would get the same amount of sunlight every day. But with a tilted axis, one hemisphere gets more than the other, and as Earth revolves around the Sun, that alternates between hemispheres. At what Northerners have arrogantly labeled the winter solstice, the Southern Hemisphere is getting more sunlight, while the opposite happens at the summer solstice. At the spring and autumn equinoxes, everything is even for a fleeting moment. In other words, the tilt of Earth’s axis is what gives us our seasons, and without it we’d care a lot less about what time of year it is.
Ok, so… as we go about our daily business, we generally do so as though there are 24 hours in a day (one Earth rotation) and 365 days in a year (one Earth orbit around the Sun), and that’s the end of it. But of course, nothing is ever that easy. For one thing, the combination of events that led to the amount of time it takes for Earth to orbit the Sun is different from the combination of events that led to the amount of time it takes for Earth to spin around its axis once. In other words, there is no reason for one to be an even multiple of the other – and they are not.
In an absolute sense, it actually takes Earth 23 hours, 56 minutes, and 4 seconds to spin around its axis once. That is what astronomers call a sidereal day. But we don’t perceive the passage of a day by the Earth rotating (we can’t even tell it’s doing that). We perceive the passage of a day by the time it takes the Sun to “cross the sky”. Since Earth is orbiting the Sun at the same time it is spinning on its axis, the movement of the Sun across the sky actually takes a little longer than a sidereal day. So as various ancient civilizations converged on the notion that the day should be divided into 24 equal segments called hours, they did so based on what astronomers call the mean solar day, which by definition set the hour such that 24 of them make up our modern notion of a day.
That’s worked out just fine for us on a daily basis. But it doesn’t take exactly 365 mean solar days to orbit the Sun either – it actually takes 365.242 days, which is known as a sidereal year. If we therefore did nothing, the seasons would shift by nearly a quarter of a day every year. In a little over 700 years, summer and winter would be completely reversed on the calendar. But of course that hasn’t happened, thanks in large part to Julius Caesar, who created a new calendar (the Julian calendar) that added a leap year every four years. That made up for the almost-quarter-day lost each year. When Pope Gregory XIII introduced a new calendar in 1582 (our modern Gregorian calendar), the math got even better – since it’s not quite a quarter day discrepancy each year, not every fourth year should get a leap day. The rules under which we currently operate are that every year divisible by four is a leap year, except for years that are divisible by 100 but not by 400. So 1700, 1800, and 1900 were not leap years, but 2000 was. This seems to make things work fairly well and with minimal intrusion into our daily lives – other than adding a day here and there.
And then there’s the even broader sense – all of this applies to the small snapshot in time that is modern human history. Earth’s rotation is slowing down because the Moon is taking away some of its energy through the tides – causing us to add a leap second every now and then. And Earth is slowing down in its orbit around the Sun. And Earth’s axis precesses around a bit. And by the time you’ve thought through the implications of all that, it’ll be March 1, and you won’t even have noticed we didn’t have a leap day.