Here in the United States, most of us are preparing to “fall back” — setting our clocks back one hour on Sunday, as daylight saving time comes to an end. But not all of us: Hawaii doesn’t observe DST, nor does Arizona, except the Navajo Nation within Arizona does. Indiana does now, but when I lived there, only part of the state did . . . which got even more interesting when you factored in the way that most of the state is in the Eastern time zone, but the northwestern corner near Chicago is on Central.
Why is time so complicated?
The way we handle time now is the current end-product of centuries, even millennia, of accreted systems. Daylight saving time is a twentieth-century invention, stapled onto the late nineteenth-century invention of time zones, grafted atop the mid-nineteenth-century invention of standard time, built on the early nineteenth-century notion of mean solar time, dependent on the creation of reliable clocks and watches, which all rests atop the concept of “apparent solar time” — the kind of thing you measure with a sundial.
But even that isn’t straightforward. Before you can measure time with a sundial or anything else, you have to settle on some kind of unit to measure it with. Your base unit is the day: the length of time from one sunrise to the next, or one sunset to the next, or one noon to the next. Easy enough to eyeball, but not exactly fine-grained or useful in (literal) daily life. So how do you divide it up?
You divide it into hours, is how. The Egyptians had twelve hours in the night and twelve in the day — but theirs were what we now call seasonal, temporal, or unequal hours, meaning they weren’t of set length. An hour was simply one-twelfth of the night or the day, so they varied with the yearly round. Not a whole lot for people near the equator, but the further north or south you go, the more those variations will grow, and the less useful the concept of “an hour” becomes.
What we use now are instead called equal or equinoctial hours, of set length. (Mostly. Once you get into timekeeping on the level of leap seconds, life gets complicated again.) We have twenty-four of these in the day, whether written as 1-12 a.m. and p.m. or the 24-hour clock, and so instead of 6 a.m. being pegged to sunrise, the clock-measured time of dawn and twilight varies through the year.
Why twenty-four? Why twelve hours of night and twelve of day? The usual answer for this is “blame the Babylonians,” but that doesn’t really answer the question. One theory is that it’s based on the 360-day approximation of the solar year (which is five days off, but close enough for ancient government work); another says it’s based on the angle of an equilateral triangle, six of which will go around a circle. Since the Babylonian counting system was sexagesimal, i.e. base 60, they divided that angle into 60 units, and 6 x 60 gets you 360.
The nice thing about 360 is that it’s divisible by a whole bunch of numbers, meaning you can break it up in all kinds of ways — including 12 and 24. The interesting thing is that East Asia hit upon more or less the same answer, without the Babylonians getting involved: the Chinese shi was equivalent to two of our modern hours, dividing the day into twelve parts instead of twenty-four — again based on the stars. Some credit this choice of number to the twelve moons in a lunar year, or to the twelve-year Jovian orbital cycle.
And you can take more or less the same starting number and group it differently. Beginning in medieval Europe the canonical hours divided the daytime up into units of three modern hours, ringing for prayers at 6 a.m., 9 a.m., noon, and so forth. At night, though, they become more irregular. The six-hour clock of Southeast Asia breaks the day into four quarters of six hours each, with the sixth hour of each block getting its own name — sort of like the a.m./p.m. divide, but with four suffixes instead of two. Ancient Hindu timekeeping was complex and used more than one system, but one of them measured the day as consisting of 60 units of 24 minutes each — those familiar factors of 360 coming back again.
Lest you start to think those numbers are inevitable, though, we do have examples to the contrary. Other Hindu systems take very different approaches. Before the shi came into use in China and also after its introduction, the day was divided into 100 kè, and the night into ten gēng (literally meaning “watch,” as the watchmen beat drums or gongs to signal the divisions). There have been other attempts to institute fully decimal time, most notably for a short period during the French Revolution.
None of those latter attempts have stuck, however rational the idea might be. But going back to the bases of our timekeeping: if you’re writing about another world, whether it’s a fantasy setting or an alien planet, it might make very good sense to divide time differently. If the solar year is nothing like 360 days long, or the lunar year doesn’t have twelve months in it, or there’s some kind of astronomical phenomenon that lends itself to a different division, you could have a twenty-hour day, or a thirty-hour one, or nice well-organized decimal time.
Tracing down to smaller levels — minutes within an hour; seconds within a minute; or whatever the subdivisions are under other systems of measurement — the story is much the same. Our modern minutes and seconds, sixty of each in the next-higher unit, bear the sexagesimal fingerprints of the Babylonians. But for most people’s purposes for most of human existence, those finer-grained units haven’t really been relevant . . . because we haven’t had reliable ways of measuring them.
Which brings us around to clocks. And for those, tune in next week!