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I'm an American science student, and as such have to constantly fight with the metric and English unit systems. One thing that a couple other students and I were wondering is why, of all the different quantities to measure (length, weight, energy), did time become standard between the systems?

Edit: I am asking more about the history of the unit than the act of timekeeping itself. For nearly every other quantity I can think of, with the exception of charge, there is an SI and an English unit for it. The meter and the foot, the pound and the Newton, the BTU and the calorie. I know that SI units now define the English ones, that the French tried to make a decimal time system, and that "standard" is kind of a misnomer, but the real question is why do both modern systems use the second as their base time unit? Why isn't it ke (just for example, I don't know of any other vastly different units) in one and the second in the other? I can't see it being because of international coordination, because the US gets along fine measuring in English units and doing any conversions on other quantities, while the rest of the world uses the much better system. So to summarize, the unit specifically is what I'm wondering about, not timekeeping in general.

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    @SMSvonderTann One could say the same thing about other measurement systems. – Schwern May 23 '16 at 18:46
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    @SMSvonderTann "Time is harder to change in people's minds"... got a citation for that? "Converting it would be a pain"... so is converting an industrial plant from imperial to metric, but it happened. None of these statements are unique to time. – Schwern May 23 '16 at 19:15
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    There's nothing as important, obvious, and periodic in distance as the day/night cycle is in time. – user2357112 supports Monica May 23 '16 at 21:57
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    @user2357112: I think the questioner is asking why everyone uses (very nearly) the same second, rather than why everyone uses the same day. Ask Swatch what happens when you try to introduce a time unit that competes with hour/minute/second. – Steve Jessop May 24 '16 at 1:46
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    If you've ever gotten tripped up by the difference between Julian and Gregorian dates, you'll know it's not always been quite as standard as you think. And if you've ever tried to write software that has to do complex manipulations of dates and times across different time zones, you'd appreciate the simplicity of Metric and English units. The libraries that deal with those calculations have to get into ridiculously complex levels of detail about the history of time. – Zach Lipton May 24 '16 at 3:33

12 Answers 12

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Most likely because it wasn't until the modern era that anyone really needed, or could achieve, much precision with time measurements.

Prior to modern timekeeping, people pretty much lived on daily and seasonal schedule. The main thing you'd need to know was how close between sunup and sundown you were (and which side, but that's typically obvious by looking at the sky). So the world over, it makes sense to base your time off of a day (and a days off a year), and slices thereof.

That being said, time isn't really as "standard" as you imply. Originally the second was just 1/86400 of the time it took the earth to rotate back around to where the sun was roughly in the same position. After a while timkeeping instruments got accurate enough that the seasonal variances were a problem, so it got redefined based on the average "day" . After the development of Atomic (Cesium) clocks in the 1950's, timekeeping got so much better that the variances in the average day got to be an issue too. So now the second is defined based on the period of radiation coming of a cesium atom (the clocks are right, not the Earth!), so periodically extra fudge ("leap") seconds will be added to a day to keep it from drifting from what the rotation of the earth has actually been doing.

And then of course when the railroads came in the 19th century, the differences in time of day at the various stops became a problem. So the railroads started placing their stops in "time zones" for timetable purposes, rather than having to worry about using hundreds of different time offsets along all their routes. Generally these zones are in whole multiples of hours offset from the GMT zone, but not always.

Even with this, you often have to consult a map (or the locals) to know which zone you are in. But wait, we're not done! There's Daylight Savings Time. Not every area of a time zone will use it, and when they do then when they go on or off of it can vary from place to place (and year to year).

As for days, months, and years, until quite recently there were multiple different calendar systems in worldwide use. The Soviets switched Russia off of theirs onto the Gregorian one in 1918, although for a while they got rid of Sundays to try to retard theism. The Chinese also officially switched to Gregorian, but also continue to use their own native calendar for many purposes.

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    As someone who's had to write (and debug) code that had to work across timezones and DST regimes, I'd really like this to all die in a fire. I'm ready to switch to stardates (but not Trek Stardates, which apparently have similar issues). – T.E.D. May 23 '16 at 20:38
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    You've answered why we use time zones and common calendars, and told some of the advantages of precision time keeping, but I read the question as "why do we use the same units of time, eg, seconds, minutes, hours?" – Peter Diehr May 23 '16 at 23:31
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    @PeterDiehr - I addressed that in the first paragraphs. I mostly concentrated the rest on the history of the second, as that's probably the particular unit most of interest to a science student. But other units were thrown in there both for completeness, and because of the problems they cause modern users. Read the rest as "why standard is a bit of a misnomer" – T.E.D. May 24 '16 at 0:09
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    + 1 Billion Billion Billion (that's a lot) As a software developer I can tell you right now, time is not, at all, anything that even resembles standardized. There are things that help make time more standard, but there is no single definition of a unit of time that works across the board. It's important to remember that time is a period of things happening and not a finite unit. Mass is mass, volume is volume, but time is relative. – coteyr May 24 '16 at 5:48
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    @coteyr Well, nothing's really "standard", particularly not standards. Obligatory XKCD, "standards". – HopelessN00b May 24 '16 at 15:38
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All through history, people needed fair, repeatable measurement of physical quantities. How much grain am I buying? How much land do you have (so it can be taxed)? How much flour and water do I use for my bread? Since most trade was local, they used whatever system they came up with. So long as the people nearby understood it, and it was based on some objective measure, it was good enough. And they built tools around those systems. When the need for globalization came around in the 19th century, it was a mess with competing systems and a large base of existing tools that would have to be scrapped in order to change. This is part of why the US is still not on the metric system, they didn't have their infrastructure wrecked in two world wars.

Unlike precision measurement, the need for precision time of day was introduced late into common use. While units of time existed (for example, the Chinese ), they weren't needed by most people. For most of history, clocks were expensive and inaccurate, most people didn't have them and simply told the time by the Sun. If you needed to be more precise, ring a bell when it's time.

Then two things happened. First, the British Navy kept getting lost. You can figure out your latitude from the Sun, but to figure out longitude you need a very accurate clock. In the mid-18th century, John Harrison introduced a series of accurate clocks for navigation. Later trains became a thing. With trains came a need for accurate train schedules, and that meant accurate time keeping. So every train station needed a clock. The clock spread with British ships and the train.

The need for a universal system of timekeeping came pretty late. It happened after global trade and communications were a thing. The need for it was spread by Western technological inventions which would all be using the system we know today. And, unlike other measurements, there wasn't a large install base of existing time keeping devices.

As to why time didn't go metric along with everything else, they tried, but base 12 time was goo enough. Unlike the dizzying array of nonsense numbers in the Imperial system of measurement (12 inches to a foot; 3 feet to a yard; 1760 yards to a mile... or is it 8 furlongs to a mile... and those are the easy ones), time was already divided into fairly regular base 12 (some would argue superior to base 10). Even France quickly dumped decimal time.

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    So 7 days each week, 2 half days each day, 12 hours each half day, 60 minutes each hour is "fairly regular" – Henry May 23 '16 at 20:17
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    @Henry, in the real world, conversion between time units is almost never done. You pick the unit most convenient for the job, and use multiples and fractions of that -- for which a large variety of small-integer factors is very useful. – Mark May 23 '16 at 20:28
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    @Henry Calendaring is something different entirely and wasn't worked out globally until the early 20th century. – Schwern May 23 '16 at 20:42
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    @Schwern It still isn't. There's still plenty of (incompatible) calendars being used daily all over the world. It's only the influence of the "western world" that makes it appear as if the calendar were standardized already, but it really isn't. – Luaan May 24 '16 at 14:40
  • @Mark It is actually common to try to represent days as a 86400 seconds in software development. Many bugs result due to DST and leap seconds. Another related issue is attempting to use 365 days as a year. – JimmyJames May 24 '16 at 15:07
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Why does everyone use seconds/minutes/hours today? These units were adopted in the west in ancient times, by the Greeks, who borrowed it from their middle eastern neighbors, the Babylonians, who had used it before them.

It's all because time-keeping, with seconds, minutes, and hours, originates with astronomical observation, and the art and science are ancient: the 60 seconds dates to at least the late Babylonian period, and was taken up by the Greeks.

Tracing back further, the Babylonian's defined the circle as 360 degrees, as did the Akkadians, and the Sumerians before them: The 360-degree circle is 4400 years old. The usual theory is that 360 is close to the number of days in a year, and as a multiple of 60, it has many exact divisors: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24, 30, 36, 40, 45, 60, 72, 90, 120, 180, and 360. This makes 24 exact divisors!

As the stars are the great clock, astronomers lead the way in time-keeping, and their systems were useful also for navigation. In the West, the continuity of history almost guaranteed the primacy of a single system; the march of technology and armies carried it to the rest of the world during the last few hundred years. Today it is enshrined in international standards. For example, China converted to the western system in 1645.

See Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day?, in Scientific American.

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    Because the Babylonians did it that way, obviously! – Peter Diehr May 23 '16 at 18:50
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    Why was the Babylonian system adopted instead of someone else's system? And why was it adopted universally? If we did because they're the Babylonians, why didn't we adopt their base-60 number system and their system of writing numbers? Why wasn't it supplanted with metric time like the rest of the old measurement systems? – Schwern May 23 '16 at 18:53
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    I thought this was common knowledge; the Greeks used the Babylonian data, and their astronomical data used the base 60 system - but this wasn't used for all purposes. – Peter Diehr May 23 '16 at 19:24
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    Ok, so the Greeks used the Babylonian system. Now replace "Babylonians" with "Greeks" in my last comment. Why was the Greek system adopted? Why universally? If your reason is "because they're the Greeks" then why didn't we adopt the rest of the Greek measurement system? You're answering who came up with the existing system, but that's not the question. It's why was it universally adopted? – Schwern May 23 '16 at 19:27
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    @schwern We did in fact replace the Babylonian number system with the Indian/Arabic decimal system in all measurements except cyclic measurements. The Babylonians knew how to divide a circle into six equal angles. For smaller angles they divided by 60 to get 360° in a circle, 60 minutes in a degree, 60 seconds in a minute, etc. As time is measured cyclically we kept the Babylonian system. Before base 10, it was the most accurate. Today we say the sine of 30° is 0.5 but before base 10 they said sine of 30° was 30 seconds. – John Wayland Bales May 25 '16 at 15:11
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In their original zeal for things decimal, when the French originally adopted the metric system after the Revolution, they also adopted a new system of time keeping called French Revolutionary Time to go along with the French Republican calendar.

French Revolutionary time divided the period of the day from midnight to midnight into 10 hours, each hour being divided into 100 decimal minutes each containing 100 decimal seconds. There are still extant some clocks made to keep time in this system.

Decimal Time

The description of the French Republican calendar is best left to those interested in such historical minutiae. The Republican calendar was abolished in 1805 after the French themselves found it too complicated to follow.

Republican Calendar

Decimal time had an even briefer run, being in mandatory use only between September 1794 and April 1795, although some decimal municipal clocks and personal watches were made to keep decimal time.

The mathematician Laplace had a decimal watch made for him which kept time in fractions of a day, and he kept all his time in these units, which is reportedly why astronomers presently reckon time using fractions of a day.

  • "the French ... also adopted a new system of time keeping called French Revolutionary Time " ... OMG. Of course they did. For those that are interested, there are multiple Pebble watchfaces that keep time based on non-standard schemes. I wouldn't be at all surprised if there was a French Revolutionary Time watchface available too. – T.E.D. May 24 '16 at 18:43
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    @T.E.D. Some Swatch watches had "beats" equivalent to French Revolutionary time (though a Central European meridian rather than Paris), such as this and that, though they did not have French Revolutionary dates – Henry May 24 '16 at 20:41
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Time is not standard at all. Historically, there have been many different systems of measure.

The year has been measured in lunar and solar months. For instance the Muslim world uses a lunar calendar starting in the 600s for religious purposes. In Roman times, the calendar was also revised several times.

The initial date is now widely accepted as the supposed birth of Christ, but before Christianity popularized this fashion, many nations had their own reckonings based on things such as important local events or reigns of monarchs.

The subdivisions of the day evolved by culture and time also. Before modern clocks were commonplace, there were several systems for dividing the day, sometimes with named hours. I believe in some cases the day would be divided into the same number of hours regardless of season, leading to an unstable measurement system ("short hours" in winter).

Even with our modern system, there have been alternatives proposed. Famously Revolutionary France attempted to use 10-hour clocks (some survive to this day).

The spread of unit systems is closely tied to the spread of measurement technology. Typically, devices for measuring time are more complex and difficult to manufacture than length or weight. Consider for instance that the ruler remains the tool of choice to measure length even today, the balance scale is an acceptable method to ascertain weight, but most people would laugh at the idea of substituting an hourglass or sundial for their digital watch (so much so that of all these quantities, we chose to carry on our person a device for measuring time only). Thus it was harder for alternative systems of timekeeping to emerge after the spread of modern clocks, because most societies simply adopted someone else's technology (and unit system along with it) as opposed to inventing their own.

In contrast, making a ruler is not exactly a great intellectual feat. As such, by the time more advanced weight measuring technology became common place, everybody and their brother had their own "traditional" ruler system. An example that remains today is the United States - which to this day uses an arguably inferior system, despite some notorious accidents occurring as a result, because the entrenchment in society and industry is greater than the pressure of adopting foreign technology.

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One of the design principals of the Metric System is that the units should be based on universal natural phenomena, not random human artifacts (or average grains of corn, etc.) -- so the kilogram was originally the weight of a cube of water at a certain temperature sized at a certain fraction of a meter, the meter was a certain fraction of the earth's circumference, and temperature was calibrated to observable state changes in water.

As time went on, some of the units were redefined to match greater technological precision in measurements and a realization that some natural phenomena aren't really universal, or change over time.

I don't know for a fact, but I presume that seconds were considered to be a reasonable natural-phenomena based measurement back in the early 1800s as it was based on the rotation of the earth. As with other units, the definition of a second has changed over the years to match increased precision and the knowledge that the earth's rotation period is variable.

See Wikipedia's article on the Metric System: https://en.wikipedia.org/wiki/Metric_system

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There is a simple connection between litres, metres and grammes. A litre is 1/1000 of a metre cubed. A gramme is 1/1000 of the mass of a litre of water.

Originally it would have been possible to define a metre as being equal to a yard. The corresponding litre (one yard cubed / 1000) would not be a decimal multiple of the pint or the gallon. The corresponding gramme would not be an decimal multiple of the ounce or pound. If we want to maintain the nice relationship between the length, mass and volume at least two of the old units needs to be replaced. The French decided to replace all three. The metre was defined as being 1/10,000,000 of the distance from the North Pole to the equator.

At the time that the metric system was introduced there was no simple way to connect time to mass/volume/distance, so time measurements were dealt with separately - the old second was retained. If we were to reinvent the metric system today we could possibly define the newsecond as being the time that it takes light to travel 100,000,000 metres.

Notes)

1) In the Imperial system there is actually a decimal relationship between mass and volume - a gallon is the volume of 10 pounds of water. This does not work in the American system.

2) the standard litre is actually 28 parts per million larger than it should be due to an error in the manufacture of the standard litre container.

  • WhIle I have no evidence I would suspect the French decision to go with a metre defined by the planet was similar to the post-raj Indian decision to keep English as the language of government: any other decision would have favored one region over the others (and France had a lot of region measurement systems) and caused even more trouble. – dmckee May 25 '16 at 2:28
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Of course there have been many systems of time, as elaborated in some of the great other answers.

There are several differences between time and systems like weight or length:

  • Weight and length are required by everyone, so everyone has to come up with a system. Can I carry this? How big is this? Is this animal in range of my spear? How valuable is this in trade? These are questions humans had tens of thousands of years ago. With the rising importance of trade a couple thousand years ago, there is soon a requirement for very large and very small measurements. Trade distances were short in the beginning, and converting units was easy, so a lot of different systems existed in parallel.

  • Time is required by everyone, up to a certain precision. A day, many days, a season, morning, noon, evening, these are enough for most people. For a long time measuring time on a smaller scale was much harder than measuring length or weight, so most people simply didn't bother. It's easier to standardize a system if fewer people use it.

  • Converting weights and lengths requires one multiplication. Converting calendars is near rocket science (e.g. leap years in one calendar but not the other), which makes different calendars an impediment to international trade.

So standardizing time was simpler (less competing systems, less people using time), and the benefit of standardizing time was greater, so it happened first. Specifically about the American units: Time was already standardized in Europe by the time the Americas were colonized, while weight and length were not.

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China had its own time measurement, defined by time it takes to burn a stick of incense, boil a pot of water, cook a bowl of rice, etc. It was abandoned because of its inaccuracy.

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As someone else pointed out, railroads were the impetus to get away from local time based on local noon. Lack of a standard time was responsible for many accidents. Harrison is also mentioned -- the connection between time and geographical location made the desire for precise time to have long preceded the ability to obtain it reliably.

Interestingly, the very concept of an abstract and unvarying unit of time has not always existed (perhaps) -- an inventor in ancient Greece devised a clock that was used to measure time in court (things like how long someone could speak) and he spent a lot of effort on making the time vary depending upon the season since they saw hours not as a fixed length but rather a division of the daylight hours into 12, so much shorter period in winter. I do not know if we can conclude from this that the ancient Greeks did not have the concept of a fixed hour but it seems possible and I would wonder when an hour that was the same in summer and winter came about. I would guess that by the 1600s (if not well before) an abstract time not tied to the sun existed since, for example, people were interested in the speed of sound and light and it would be silly to talk about the speed in "winter seconds" or whatever.

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From a scientific perspective, all units ARE standard, under the International System of Units. There are seven base units, including metre for length, kilogram for mass and second for time. The use of yards, or pounds, are a local eccentricity.

  • But few people measure their age in megaseconds (between 11 and 12 days) or gigaseconds (between 31 and 32 years) – Henry May 23 '16 at 20:36
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    This ignores the clear intent of the question. – Schwern May 23 '16 at 20:42
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    Does it though? The question asks why 'time became standard between all the systems', as though there are competing standard systems. There aren't; there are SI units and then there are local approximations. Of course there are historical reasons why certain units became 'standard', but the point seemed worth making that yards, pounds etc. are not 'standard.' – marchanti May 24 '16 at 0:34
  • The international standard yard is 0.9144 metres so the international standard inch is 25.4 mm exactly – Henry May 24 '16 at 10:36
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What I have gathered the best explaination is a combination of the many great answers given, bulleted below

-Time accuracy was not neccesary until fairly recent times

-When it became neccesary, globalization was beginning in earnest and calandar conversion is much more difficult and trade impeding than other units.

-Finally, rather than overhaul the system entirely to say stardate, the second became the base unit of time and was defined by very precise measurement, along with the other base units. (Minus the kilogram as of my current knowledge)

For better explainations, check all of the other answers, as none alone (personally) answered satisfactorily, but together are good.

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