# Why is the second the SI base unit for time?

Specifically, during the moves towards Le Système international d'unités in the 18th and 19th centuries, why didn't anyone attempt to move us away from the definition of there being 24 hours in a day?

Wikipedia informed me that the name second comes from the second division of an hour. It also claims that the concept of having 24 hours in a day dates back at least 6000 years to the earliest Egyptian civilization. It also says the word second (as a measurement of time) didn't make it into English until the 1500's and couldn't be measured with any accuracy for 100 years. So by one way of looking at things, the base unit of time (or perhaps the grand-pappy unit of time) is the hour.

Is there some convenient property of 24 hours = 1 day that caused it to hold on over other units? I understand the significance of a degree being 1/360th of a circle (360 = 60 * 60 = (3 * 4 * 5)^2), and 24 is closely related to those factors, but that seems a flimsy reason. The math for converting earth's rotation to elapsed time doesn't even work out very conveniently (IMO) with these units.

I expected to find a reason in time being one of the basic properties of the universe, being immutable (in 18th century eyes) and therefore used as a pivot point for other unit definitions. But the dependency graph of SI units mostly dispelled me of that: both temperature and mass were found to be as independent as time, and both units were redefined during this period (to Kelvins and kilograms, respectively).

During the same period, other units were attempted to be re-measured from new standards:

• Length was adopted to use the meter at one ten-millionth from the North Pole to the Equator in 1793
• Mass was adopted to use the gram at the cube of the hundredth part of the metre (at STP) in 1795
• Temperature was adopted to use degrees Celsius, one hundredth of the separation between boiling and freezing of water.

And yet, efforts to define the second seem to center on more perfectly measuring the existing definition of 1/86,400th of the Earth's rotation. Given the predilection for base 10 (and the gradual move away from units depending on our earthly frame of reference), why was the second left at such a cumbersome, ancient interval? Why didn't we redefine a new measure of time to be (say) 1/100,000th of a day, put 10 hours in a day, etc. etc.?

Did no one think to challenge the convention of 24 hours in a day? Why not?

That came out a little more jumbled than it was in my head, so let me sum up: I'm mainly interested in answers addressing attempts to redefine units of time and why they didn't achieve wide adoption in the scientific community.

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Seconds are the right time scale for bench-top mechanics experiments and for human movements or motion over laboratory scale distances. –  dmckee Jul 2 at 22:19
The meter: there is some discussion over the fact that $g$ is so close to $\pi^2$ in SI units. Some speculate that the French fudged the surveyors results a bit to match commonly used length standard of the length of the "seconds pendulum" (half-period = 1 second). –  garyp Jul 3 at 0:44
I have studied this a bit. The key is that the SI metric was developed by and for a small class of "eggheads" who knew Greek and Latin and were involved in the sciences. Their communications had almost no effect on the average person. Changing the units of time would be everybody's business. I have not seen any documents about Church opposition, but I can imagine the difficulty of finding new ways to determine when to ring the bells. –  C. Towne Springer Jul 3 at 3:58
@C.TowneSpringer: I don't think that's right. The metric system was developed in the wake of the French Revolution largely in response to populist anger over France's mess of differing and unstandardised units, as these were seen as being used by merchants to rip-off their customers. Rather the opposite of being used by elite scientists, the whole point was that they replaced the mish-mash of units used in everyday life by a single, rational, system. –  simplicio Jul 3 at 23:33
@simplicio I can understand a demand for a sane system, but I have a hard time with every shop customer knowing their Latin and Greek prefixes so they could differentiate between amounts bigger or smaller than 1. I have not seen any demands like that. Also, the syllable rich SI metric flies in the face of any practical use. Other systems use single syllable words for units that are very difficult to mistake when shouted across a work place. SI units sound very similar. I'll stand by the egghead source. –  C. Towne Springer Jul 4 at 1:15

It's my understanding that the invention of the metric system during the turbulence following the French Revolution also included a switch to decimal time, with ten hours per day, etc., but that it didn't take. There's a certain amount of cultural inertia that has to be overcome; as you're probably aware, those of us in the United States still have many miles to go before we can fully adopt the metric system.

As you say, you have to give anonymous inventors of the 24-hour day credit: while the metric approach of powers-of-ten relationships between units is dreadfully easy to handle when you're using base 10 arithmetic, it's quite difficult to divide ten things into three equal-size sets. Remember that base ten is essentially an arbitrary choice made because most people have ten fingers and spend their childhood grouping things into fives and tens to count them. Twenty-four has boatloads of divisors: you can separate into a dozen pairs, three groups of eight, or six quartets. Sixty would make a pretty nice base, since it's the first number divisible by two, three, four, and five; but sixty is too many things for most people to count in their heads.

The second is actually historically based not on the length of a year, not of a day: until the adoption of the cesium clock standard in 1960, the definition of the second was actually the appropriate fraction "of the tropical year 1900." It took roughly half a century for the standards committee to realize that we can't go back and re-run the year 1900 to see whether we're still producing correct seconds.

There are several things that the SI system does that don't quite make as much sense as you might like. Why on earth does the base unit for mass, the kilogram, have a prefix? Why is the base unit for electricity the ampere, when we've known for a century that charge occurs naturally in standard-sized lumps? I put the SI endorsement of the historical relationship between the second, the minute, the hour, the day, and the year in the same category. It's a convenient unit with strong historical and popular support. I don't see a need to decimalize the day.

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Upvote for "many miles to go before we can fully adopt the metric system". –  Jordan Jul 2 at 23:23
I jokingly present the fact that we have ten fingers as proof that there is no god. If there were a god, he would have given us 12 fingers. (In case you missed it: that's a joke.) –  garyp Jul 3 at 0:40
@garyp: I'd counter that. Surely the obvious number of fingers to equip His creatures with is 42, so obvious indeed that it would be a dead giveaway had He actually chosen it, thus conclusively proving Himself a puff of logic. Since we do not in fact have 42 fingers⁽ⁱᵗ⁴ᵗⁱ⁰ⁿ ⁿ³³⁹³⁹, God must therefore exist. ⬛ –  leftaroundabout Jul 3 at 1:14
@garyp Of the folks I know without ten fingers, the most common number is nine; nine has more factors than eleven does. There's some lesson buried in there about graceful failure. (What a bizarre tangent …) –  rob Jul 3 at 1:39
That ephemeris time stuff is very interesting. I'll give it a more thorough read tomorrow, cause it's not making much sense tonight. –  Patrick M Jul 3 at 5:32

The French Revolutionary Gov't did try to move towards a decimalized system of time measurement, with a second defined as one-one hundred thousandths of a day (along with decimal hours, minutes and a new calander), around the same time as it introduced the proto-metric system. But unlike the rest of the metric system, the new time keeping system and calender failed to catch on.

I suspect the reason is because while the rest of the metric system met a real need for standardized units in a France that had dozens of differing unit-systems for lengths, weights, volumes, etc., the sexigesmal system of time, along with the twelve month calendar, was already standardized enough by 1795 that the advantages to switching didn't outweigh the costs.

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I will search for some info on Decimal Time post revolution. Do you have a link for anything about it? –  Patrick M Jul 3 at 5:33
@Patrick: There's a wiki-page on "decimal time" which mentions several post-Revolution attempts to establish it in France. Not much detail there, but maybe you can follow the references there for more info if your interested. The same page mentions a Chinese decimal system that was apparently in-place for several millenia until the adoption of the European system. So not only does there not seem to be many places that have adopted a decimal system, at least one major civilization abandoned it. –  simplicio Jul 3 at 16:08