Here's the rule: all years that are evenly divisible by four are leap years.
But wait! There's more! (I see too many infomercials.)
Century years must be evenly divisible by 400 to be leap centuries.
I do a lot of pondering about the calendar this time every year. I should also warn you that whenever Jennifer can't sleep, she says, "Tell me about the Gregorian calendar."
First of all, you must accept the fact that all timekeeping is artificial. It's an invention to help us put order in our lives. You can be assured that this is so because there have been and still are many systems to track time.
Here's a thought experiment: what if you lived underground like the Morlocks in H.G. Wells' The Time Machine? How would you measure time?
There would be no day or night
The "weather" would never change
There would be no seasons
You could not see the moon
You could not see the planets
You could not see the stars
You could not see the sun
Studies have shown that people in such underground environments develop their own biological rhythms. Most of them fall into similar patterns of sleep and wakefulness (which are usually different from those of people on the surface).
So about the only regular cycles you could measure would be internal. You might begin counting from one sleep to the next. You might create some sort of little machine to cycle itself along with your own. But it would be a haphazard system at best, what with older people getting up in the middle of the night to...well, getting up a lot.
Underground is a very bad place to try to measure time.
Fortunately for the Rigid Tool Company, we live in an environment rich with truly regular cyclic events, so they can happily make calendars with pretty girls and pipe threading machines every year.
So where did we get our calendar?
Let's start with an aside: one of my good friends knows more about woodlore and such than most people I know. In spite of that, one day when I mentioned something about the appearance of the moon at lunchtime, he said, "I didn't know you can see the moon during the day."
This is not to belittle my good friend, but to point out that even the smartest among us don't pay much attention to the sky. For one thing, we spend almost no time outside at night with the sky. Even if we look at it, it's just a glance to make sure it's still there. With modern light pollution, even that is dicey.
Before television, the internet, cars, movies, Monopoly, indoor plumbing, teepees, and Life magazine, people spent a lot of time under the stars.
They would see all the stars and planets, and over time they saw and pondered on the motion of these things. They saw patterns in the stars and gave them names.
They noticed that five of the brightest stars seemed to move about the background of stars, so they called them "the wanderers." The Greek word planētēs means just that.
The brightest object in the sky is the sun. DUH.
Our most fundamental cycle is the rising (or setting) of the sun. I'm pretty sure that every civilization has used that cycle as the heart of their time keeping scheme.
Even in today's insular world, we are all pretty much aware of the sun's comings and goings.
Obviously, we call that cycle a day.
The brightest object in the night sky was the moon. It was also the most dynamic. It moved across the sky and against the background of stars. It also changed its shape.
The moon goes from completely dark (new moon) to completely lit (full moon) every 29-1/2 days.
A lot of ancient people developed their calendars around this cycle. The main measure was one moonth...er, month. It was easy to keep track of the interval from one new (or full) moon to the next.
Furthermore, there were four distinct points along the way that were easy to visualize. They were when:
exactly half illuminated (first quarter)
totally illuminated (full)
the "other" half illuminated (last quarter)
You can reveal your ignorance of astronomy by calling the half-illuminated moon a "half moon" instead of a quarter moon.
Why is it a quarter moon and not a half moon?
Imagine the moon as an orange with four equal slices. We can't see the two slices on the "back" side, and only one of the two slices on the "front" side is lit up (to us). So of the four slices, we see only one lit up, hence a quarter orange...uh, moon.
But the important point is this: there is a distinct shape (phase) of the moon approximately every seven days.
What else occurs every seven days?
Well, not Poison Prevention Week, but A week.
There's really no proof of this, but it's my determined belief that the week of seven days is based on the four distinct phases of the moon.
So we have days based on the sun's cycle and months based on the moon's cycle.
People watching the sky "back then," and that was practically everyone (unlike today, when it's practically no one), observed that the weather changed on a regular basis. For a while it was warm. Later, for a while, it was cold. The farther you are from the equator, the more noticeable this is. Closer to the equator, it was more about more rain and less rain.
As societies slowly switched from chasing rabbits and wooly mammoths, and foraging for beetle nuts and beetles for food, to planting, nurturing, and harvesting crops for food, more and more awareness of these seasons developed. If you could schedule your planting to coincide with rain and avoid being frozen, you could improve your chances of getting that 4H award at the county fair.
What really got their attention was not the sun, nor the moon, but the stars.
From some high point, they would perhaps watch for the earliest appearance of the very bright star Sirius to mark the beginning of their calendar year.
The thing to remember is that the stars, like the moon, have two movements in the sky. They all seem to spin across the sky each night. And they all "shift" a little bit in the sky every night.
Every 365 nights, the stars have shifted all the way back to where they started. And that's what we call a year.
In the old movie serials, somewhere around chapter 10, the good characters would gather in the laboratory, the conference room, the bat cave, or police headquarters and synopsize the action so far.
One cycle of the sun is a day
One cycle of the moon is a month
One cycle of the stars is a year
Here's the big problem: Months (based on the moon) can't be evenly divided by days. Years can't be evenly divided by months (lunar) nor by days.
There are ABOUT 29-1/2 days in a lunar month.
There are ABOUT 365 days in an astronomical year.
The Hebrews used calendars mostly to keep track of festivals, so they were keen observers of the new moon. For the longest time, they simply used twelve "moonths" to mark a year and cover all their ceremonial bases.
The Egyptians simply used 365 days for a year and closely watched for the earliest arrival of Sirius so they'd know when the Nile would flood and they could plant.
It turns out there are closer to 365-1/4 days in a year. So every four years, the Egyptian calendar would slip by a day.
"Big deal," as Bugs Bunny would say.
But along came the Romans. Like most ancient societies, the early Romans used a month-oriented calendar. They had to make all sorts of crazy adjustments to keep it in some sort of rhythm with the seasons.
One of those Romans was Julius Caesar, and he enjoyed a lot of success taking over the known world.
Along the way, he spent some time in Egypt and met a cute girl there.
In addition to dating Cleopatra, he learned about the Egyptian's solar-based annual 365 day calendar. He liked a calendar based on the sun and stars instead of the moon, but hated the adjustments they were making to allow for the missing 1/4 day, so he put his best thinkers to work on it and they came up with the idea of the leap year.
By now you've noticed a molecular bond between calendars and pretty girls.
It was an elegantly simple solution. Just add one extra day every four years.
It was called the Julian Calendar and it was almost perfect.
It worked very, very well for about 1600 years. Not many things work well, if at all, for that long.
But it turns out there are not exactly 365-1/4 days in a year. It's about eleven minutes short of that. So even the Julian Calendar was shifting a little bit every year. By the late 1500's, it had "gained" over ten days when compared to the astronomical year.
The church folk were concerned that in a few more centuries, Easter would be celebrated in the middle of winter.
So in 1582, Pope Gregory had his smartest people tackle the problem and they came up with another elegantly simple solution.
Every century year is a leap year only if divisible by 400. That meant that 1600 was a leap year, but 1700 was not. There wasn't another "leap century" until 2000.
That still wasn't perfect, but it is accurate to one day in 7,700 years. Not shabby.
When the Gregorian Calendar was adopted, they had to make up for the extra days that had accumulated, so the calendar jumped from Thursday, October 4, 1582 to Friday, October 15, 1582.
The people were not happy.
(Those folks above are actually protesting a new prayer book. Imagine the protest to the loss of eleven days!)
I'm not certain if such an adjustment could even be possible in our modern world.
Finally, the new year in ancient times began at the Spring (Vernal) Equinox when the night and day are exactly equal. That occurs in late March.
If you number the months starting with March, you get:
I had often wondered why those months were "numbered" thusly. Makes sense, now.
So why do we begin our new year on January 1? Because that's when the Roman consuls started taking office in 153 B.C., and they called it their Consular Year. Our calendar has its roots in the old Roman calendar.
Blame it all on the Romans, I say, and have a Happy New Year!
