My previous post got me to thinking. . .
There aren't any separate, uniquely 'metric' units of time - folks who use metric units to measure distance, mass, volume, speed, force, pressure, etc, etc, use the same seconds, minutes, hours, days, etc, as the rest of us. And just as well, I'm sure; 24-hour days, and hours divided into 60 minutes of 60 seconds each are pretty well ingrained in us at a pretty fundamental, intuitive level. But even so, it got me to thinking about how we measure time. . .
Just like the meter is marked off from the dimensions of the earth, the fundamental unit of time on all sorts of levels is the day - sunrise, sunset, 'the evening and the morning - one day'. Our bodies are dialed into this daily rhythm of light and darkness on fundamental levels. All things being equal, we're synched-up with the rotation of the earth on its axis, waking with the sunrise and sleeping during the hours of darkness (I once read a fascinating article discussing the 'daily rhythms' of people who get 'decoupled' from the 24-hour cycle - astronauts, crews of nuclear submarines, a few deep miners who spend long periods underground - and, left to themselves, they tended to settle into a 'day' of roughly 26 hours, give-or-take; so even when you take us out of the direct rhythm of light and darkness, our bodies don't want to vary all that much from what sunrise and sunset would 'impose' on us anyway).
I suppose there's nothing sacred about dividing a day into 24 hours, hours into 60 minutes, and minutes into 60 seconds (a direct 'analog' of how we divide angles, which seems somehow appropriate). I mean, if we say that there are 86,400 seconds in a day, we could as easily divide a day into 100,000 small parts that wouldn't be very different from seconds, then collect 100 of these 'metric seconds' into 'metric minutes', 100 'metric minutes' into a 'metric hour', and then a day would be 10 'metric hours', and we'd have all the nice multiples of 10 that metric freaks are so fond of. A work day could be 3.5 'metric hours', and so on. But. . . why???
When we go bigger than days, we get into some fascinating stuff. At the level above days, there are two more-or-less 'fundamental' measures of time - the year, marking off a complete revolution of the earth on its orbit around the sun, and the month, marking off the cycle of the phases of the moon. Neither is as 'fundamental' to our lives as the day, but both have rhythms of their own. Especially in 'temperate' climates like ours, the year manifests itself in the passage of the seasons, cold winters and warm summers, longer and shorter periods of daylight and darkness (solstices and equinoxes), the cycles of growth and dormancy of plant life (most especially crops), etc, etc. So the year has an intuitive rhythm to it, to the point that we count our own life spans in terms of it. (I suppose, if I were 'King of the World', that I'd align the months with the solstices and equinoxes, so that the solstices and equinoxes were always the first of the month, and the winter solstice would be New Year's Day; but I'm not, and aren't you glad?)
The month seems less 'fundamental' than the year, but there is no denying the simple visual progression of the phases of the moon, And those of us who live near the ocean are at least somewhat aware of the cycle of the tides. Even something as 'bodily' as women's fertility cycles seems at least coincidentally (if not causally, and I can't imagine how it could be; but what do I know?) 'synched-up' with the phases of the moon.
The thing is, neither years nor months match up very cleanly with days. A year is about 365-1/4 days, so as far back as the ancient Romans, folks would add an extra day to the calendar every four years, to get things 'lined back up' (the Gregorian calendar we use today removes three leap years every 400 from the Julian calendar, since the actual number of days in a year is closer to 365.2425).
Things are similarly 'messy' when we take a look at months. The lunar phases complete a cycle in about 29.5 days. Which means that there are more than 12 lunar cycles in a year (but less than 13, so whatchagonnado?). On a 'solar' calendar like the one we're familiar with, the year is regarded as 'fundamental', and months are more-or-less 'arbitrary' - we divide the year into 12 months of 30-31 days ("excepting February, which alone has 28"), without regard to where the phases of the moon fall, so the full moon might fall (as it does this year) on the 29th of August, then the 28th of September (30 days later), and then the 27th of October (29 days later). The phases of the moon have no necessary relationship to the day of the month, so they will 'drift' from month to month.
Some cultures have used 'lunar' calendars, in which the month is taken as 'fundamental' - the new moon, say, is always the 1st of the month - and years are adjusted around the progression of months. For example, in the Hebrew calendar, there is a 19-year cycle of twelve 12-month years, and seven 13-month years (months alternate between 29 and 30 days), since 19 years contain very close to exactly 235 lunar cycles. So, rather than adding a 'leap day' once every four years, the ancient Jews would add a 'leap month' seven out of every 19 years. Different, but it still works out reasonably cleanly.
"But," I hear some of you saying, "what about weeks?" To which I reply - I have no freakin' idea. The Bible roots the seven-day cycle in Creation itself. But virtually all ancient civilizations have some manner of the seven-day cycle built into themselves, whether they ever met a Jew or Christian, or not. Which I find fascinating. . .
So there you have it. Not quite as grandiose as Stephen Hawking's Brief History of Time, but I hope you've gotten at least some meager (or meagre, if you're the Brighton Pensioner) bit of enjoyment from it. . .