Slashdot Mirror

You thought wrong.
The kilogram is the last remaining base unit of the SI that is still defined by a material artefact.
http://www.bipm.fr/en/scientific/mass/prototype.ht ml

Since this is a "world" resource, time should no longer be managed by the UK, but by the UN standards body. Surely this will be a much more equitable and fair solution than hogging all of the world's time by one nation.

The Bureau international des poids et mesures is already responsible for measuring UTC as part of the SI system, by international treaty...

Since this is a "world" resource, time should no longer be managed by the UK, but by the UN standards body. Surely this will be a much more equitable and fair solution than hogging all of the world's time by one nation.

The Bureau international des poids et mesures is already responsible for measuring UTC as part of the SI system, by international treaty...

No no no. A day is the earth's rotational period, and always will be unless you want to change the meaning of the term 'daytime' to 'when it's dark outside' in a few million years...

The problem is, the earth's orbital period (a Year) just happens to contain about 365.25 rotations (see here). So if you want to keep the definition of day what it is now and you also want winter to still happen in winter (and not in summer), you need leap days to make up for it.

Now, leap seconds are needed because in deed the earth's rotational period is not exactly 86,400 seconds long. Now, the day is still defined astronomically. But the second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom (see here). That's close enough to make the rotational period almost exactly 86,400 seconds. It just does not fit perfectly. And since, as we see because of the quake, the rotational period can change easily enough, it's simpler to add leap seconds than to constantly redefine the second and to figure out how to update the clocks. You could also define the day to be, say, 86,400.00017422 SI seconds. No problem. Just remember to change it after the next quake. You go and start a business producing clocks for it...

(Disclaimer: I'm not a physicist or an astronomer, just a guy with some basic knowledge and, hopefully, some common sense. Correct me if I got anything wrong.)

Define "couple hours"

"Couple" literally means two, but in practice it can mean any smallish number.

An hour used to be 1/24th of a day, but a day is not just difficult to define, it's variable, so let's take a bottom up approach. An hour is 60 minutes, a minute is 60 seconds and a second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

Thus a couple [of] hours is 33093474372000 n where n is an arbitrary smallish number.

>And to the folks that corrected my terminology: yes, I know "Z" or "UTC" is the more correct term, but old habits die hard. :-)

The fact that the computer industry improperly defined kilo- as 2^10, mega- as 2^20, and giga- as 2^30 for describing memory sizes doesn't change the proper meaning of the terms.

Please tell me you're joking. If so, it was way too subtle for me.

1. 10^3 = 10 * 10 * 10 = 1,000
2. The prefix "kilo", or "K" for short, is defined to represent 10^3
3. In binary (i.e. base two), the string "11111101000" converted to decimal (i.e. base ten) is actually equal to 2,024

Of course ! Just after ur ancestors inspired our, we made metric system in our country just after our revolution (1789~1799)

they choose an easier system (decimal) than the hundred we had through the french realm.

1m was 1/10 000 000 part of a terrestrial meridian. Calculated by some of our most stupidiests mathetimaticians/geometers of that time (among them Laplace & Lagrange (Not ZZtop tune))

1kg was the weight of 1L of water

Thoses units was shaped in iridium as etalon in every day life.

Some adds in the metric system was also made by the well known idiot "Gauss" about the magnetical unit.

Our deer neighbour of the "perfid albion", contributed to the metric system too.

Brief Historical (eek a link to this hugly-english-bad-speaking frenchman's country ! don't jump on it ... you could maybe goes mad & weird) :P

Freezing water was a bad idea, since the volume of water changes as it freezes, and I'm sure I read that they switched to 20C. The litre is, of course, a cubic decimetre or 1/1000 of a cubic metre, and is thus derived from the standard metre.

Whatever the reasons (practical?), the two standards were separated, but it's still quite easy to get a ballpark figure for the weights of fluids. Ten litres (2.624 gallons) of water weighs about ten kilograms (22.05 pounds). Some fluids will be less (gasoline), some more (beer, oils, mercury). There are other such shortcuts, too, so I ain't goin' back.

PS: If you Yanks are wondering why it's easier to get drunk in the UK, it's because a UK pint is 20% larger than a US pint. Standards are great - that's why we have so many of them...

Your physics prof is wrong. The BIPM (keepers of the SI) indicate that the definition of the kilogram is still

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

There are numerous current attempts to define the kilogram in terms of physically reproducible experiments, but at present, they are not sufficiently accurately reproducible, or are horrendously complicated to define (the basic problem has to do with counting the number of particles in your sample, which turns out to be a difficult problem).

You might want to check out these URLs, about the kilogram and the current state of the SI in this regard:

Pictures of the kilogram

the BIPM SI brochure

Resolution 7 of the 21st CGPM, 1999 on the future of the SI mass standard

Your physics prof is wrong. The BIPM (keepers of the SI) indicate that the definition of the kilogram is still

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

There are numerous current attempts to define the kilogram in terms of physically reproducible experiments, but at present, they are not sufficiently accurately reproducible, or are horrendously complicated to define (the basic problem has to do with counting the number of particles in your sample, which turns out to be a difficult problem).

You might want to check out these URLs, about the kilogram and the current state of the SI in this regard:

Pictures of the kilogram

the BIPM SI brochure

Resolution 7 of the 21st CGPM, 1999 on the future of the SI mass standard

Your physics prof is wrong. The BIPM (keepers of the SI) indicate that the definition of the kilogram is still

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

There are numerous current attempts to define the kilogram in terms of physically reproducible experiments, but at present, they are not sufficiently accurately reproducible, or are horrendously complicated to define (the basic problem has to do with counting the number of particles in your sample, which turns out to be a difficult problem).

You might want to check out these URLs, about the kilogram and the current state of the SI in this regard:

Pictures of the kilogram

the BIPM SI brochure

Resolution 7 of the 21st CGPM, 1999 on the future of the SI mass standard

Your physics prof is wrong. The BIPM (keepers of the SI) indicate that the definition of the kilogram is still

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

There are numerous current attempts to define the kilogram in terms of physically reproducible experiments, but at present, they are not sufficiently accurately reproducible, or are horrendously complicated to define (the basic problem has to do with counting the number of particles in your sample, which turns out to be a difficult problem).

You might want to check out these URLs, about the kilogram and the current state of the SI in this regard:

Pictures of the kilogram

the BIPM SI brochure

Resolution 7 of the 21st CGPM, 1999 on the future of the SI mass standard

exa-
zetta-
yotta-

Ref: Bureau International des Poids et Mesures

Note that NIST is not SI. The binary multiple prefixes are from an IEEE publication, specifically IEC 60027-2. The standards organization responsible for the SI units is the Bureau International des Poids et Mesures.

Humour aside, I think it's the marketing department again that thought people wouldn't grok units that look like bits per square micron.

Engineers and scientists wouldn't grok it either; the term micron (meaning one millionth of a meter) was abolished in 1968 in favor of micrometer (BIPM's SI brochure, page 28).

As you may know if you've been following recent IEC and IEEE standards (or if you've ever bothered to figure out exactly how large a terabyte is), what disk manufacturers call a terabyte and what this article calls a terabyte differ slightly.

When used in the standard way, the "tera" prefix means 1 * 10^12, so a terabyte would be 1 000 000 000 000 bytes. Unfortunately, computer systems don't use base 10 ("decimal"), they use base 2 ("binary"). When trying to express computer storage capacities, somebody noticed that the SI prefixes kilo, mega, giga, tera, and so on (meaning 10^3, 10^6, 10^9, 10^12, ...) were about the same as 2^10, 2^20, 2^30, 2^40, and so on, so used the terms as multiples of 1024 rather than the usual 1000. On the other hand, many hardware manufacturers (especially hard disk manufacturers) use these prefixes in the standard way to mean exactly multiples of 1000.

This discrepancy causes some confusion. For instance, if you could afford to purchase such a 2 terabyte hard disk, you might well be annoyed when your system tells you your disk is almost 200 gigabytes (2 * (2^40 - 10^12)) smaller than you thought it would be (most systems would report a 2 terabyte disk as a 1.8 terabyte disk).

The moral of the story is one of:

1. don't buy 2 terabyte hard disks (blame the hard disk manufacturers)
2. complain about it then continue the current ambiguity
3. use the standard terminology for binary units
   

Interestingly the Slashdot community seems to think it should be a combination of 1 and 2.

As you may know if you've been following recent IEC and IEEE standards (or if you've ever bothered to figure out exactly how large a terabyte is), what disk manufacturers call a terabyte and what this article calls a terabyte differ slightly.

When used in the standard way, the "tera" prefix means 1 * 10^12, so a terabyte would be 1 000 000 000 000 bytes. Unfortunately, computer systems don't use base 10 ("decimal"), they use base 2 ("binary"). When trying to express computer storage capacities, somebody noticed that the SI prefixes kilo, mega, giga, tera, and so on (meaning 10^3, 10^6, 10^9, 10^12, ...) were about the same as 2^10, 2^20, 2^30, 2^40, and so on, so used the terms as multiples of 1024 rather than the usual 1000. On the other hand, many hardware manufacturers (especially hard disk manufacturers) use these prefixes in the standard way to mean exactly multiples of 1000.

This discrepancy causes some confusion. For instance, if you could afford to purchase such a 2 terabyte hard disk, you might well be annoyed when your system tells you your disk is almost 200 gigabytes (2 * (2^40 - 10^12)) smaller than you thought it would be (most systems would report a 2 terabyte disk as a 1.8 terabyte disk).

The moral of the story is one of:

1. don't buy 2 terabyte hard disks (blame the hard disk manufacturers)
2. complain about it then continue the current ambiguity
3. use the standard terminology for binary units
   

Interestingly the Slashdot community seems to think it should be a combination of 1 and 2.

A meter is defined by the General Conference on Weights and Measures to be the distance light travels in a vacuum in a time interval of 1/299,792,458 of a second. The 1889 definition of the meter was based on an international prototype of platinum-iridium.

The original idea of the king's commission (which included such notables as Lavoisier) was to create a unit of mass that would be known as the "grave". By definition it would be the mass of a litre of water at the ice point (i.e. essentially 1 kg). The definition was to be embodied in an artefact mass standard.

The mole is not a constant, it is a unit. In fact, it is one of the 7 SI base units.

You're right, but I think he meant that Avogadro's number is a constant, not a unit. According to your BIPM link>:

The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.

How many atoms there are in 12 grams of C-12 is a constant, which must be measured empirically, and thus cannot form an "idependent" basis of mass. Of course, we could define Avogadro's number to be exactly 6.022 x 10^23, or just 6 x 10^23 (the math is simpler, and isn't that what SI units are all about?) and say that however much that many atoms of C-12 weighs is 12 grams, but who'd want to count them?

The mole is not a constant, it is a unit. In fact, it is one of the 7 SI base units. See here.

1. Still rely on the French to define your unit of mass, eh? (Rusting, other chemical readtions with the block. Bye bye perfect reference!) And the meter being the distance from the North pole to the equator thru Paris divided into 10,000,000 parts? Yah, that's real accurate

1. And why do people state "weight" or "thrust" in kilograms? Why not Newtons?

1. Why do we still ask for a "pint of ale" in the UK?

1. And if base 10 is so l33t, where is metric time? Base 60? Why stick with millenia old numbering from Babylonian times yet praise base 10 everywhere else

Actually, the time lords are rather the Bureau International des Poids et Mesures (International Weights and Measures Bureau) for TAI (atomic time) and the International Earth Rotation Service for UTC (who make the decision of when to add leap seconds for example).

Granted, on last reading, the USNO master clock was only five nanoseconds fast of UTC, as computed by the BIPM (by averaging many different atomic clock's reading of UTC).

Let's have some fun splitting these hairs in real tiny pieces.

There are several standards used for keeping track of time. The most important, by far, is Universal Time Coordinated (UTC), sometime known as GMT (Greenwhich Mean Time). This is the standard time for Earth, and it is with respect to this time that local time is defined (offset by a certain number of seconds, generally a multiple of 3600, i.e. an integer number of hours).

UTC does not flow linearly. That is, the interval between time t1 UTC and time t2 UTC is not always t2-t1. This is because leap seconds get inserted occasionally in UTC, so as to keep it more or less synchronized with the sun. More precisely: there are 86400 SI seconds in an SI day; but the mean solar day is approximately 2 milliseconds longer, because the Earth's rotational period is getting longer (the Earth is slowing down) at an order of magnitude of 1 millisecond per day per century. Terrestrial Time (sometime called Ephemeris Time) is the astronomical time: it is currently 0.184 seconds (roughly) fast of UTC. And UTC will be corrected so as to always keep it to within 0.9 seconds of TT (i.e. the sun should always be overhead on Greenwhich meridian to within 0.9 seconds at noon UTC).

Adding a leap second can take place after December 31 or June 31 (or possibly also March 31 or September 31, but that has never occurred), in the following form: after 23:59:59UTC comes 23:59:60UTC and after that comes 00:00:00UTC. The last leap second happened after December 31, 1998, and there will be no leap second after December 31, 1999 (today). It is the International Earth Rotation Service that is in charge of deciding when a leap second should be inserted. (Theoretically, a second can also be substracted, but that has never happened and presumably never will.)

The other important time standard is the Temps Atomique International (this is in French because the Bureau International des Poids et Mesures is in Sèvres, France), TAI for short. Contrary to UTC, TAI is a linear time scale (to the best of the precision we can achieve, that is, i.e. to within a few dozens of nanoseconds per year). TAI ticks one second every SI second, and it is maintained by averaging over about 50 atomic clocks around the world (there is no Master clock for TAI); the calculated offsets of the atomic clocks wrt TAI can be found in this FTP directory.

The Temps Atomique International and the Universal Time Coordinated are offset one to the other by an integer number of SI seconds (since 1972). This offset increases by one every time a leap second is inserted in UTC. Currently (since January 1, 1999 and at least to June 31, 2000) TAI is 32 seconds fast of UTC (so by the time UTC reaches January 1, 2000, 00:00:00, TAI will read January 1, 2000, 00:00:32).

So TAI will say Y2k precisely 32 seconds before UTC says so. (There is also GPS time, which is exactly 19 seconds back of TAI, but never mind that one. And, of course, there is Terrestrial Time, which nearly coincides with UTC, but not by a round number.)

Now, which of these times should be used on computers? Well, if you look in the /usr/share/zoneinfo/ directory of a GNU system, you will notice that there is a right/ subdirectory which contains nearly identical zone info files. The difference is this: the zone info files in the right/ directory account for leap seconds, whereas the ones outside this directory do not. Thus, if your /etc/localtime points to a right/ time zone, exactly 32 seconds will be substracted from your system clock before it is corrected by the time zone offset.

System time should be a linear time. If clocks were precise enough, it would be inadmissible to skew the clock by as much as one second (even by diluting the effect over a certain period). Thus, system time should be put to TAI (and not to UTC, let alone local time). This is why the right/ time zones are there: if you set your system clock to TAI and set your /etc/localtime to point to a right/ time zone, then your local time (as returned by the localtime() library function call) will be offset to UTC, as it should.

On the other hand, the POSIX standard (see POSIX.1, Annex B, 2.2.2) specifies that the time() system call should measure the difference between the current UTC time and the UTC time of the Epoch (January 1, 1970 at 00:00:00UTC). This is most unfortunate, because a difference of UTC times is not a number of seconds elapsed. And it is especially unfortunate since the rules for computing UTC from TAI were rather complicated before January 1, 1972 (at which time UTC was resynchronized to TAI-10s). Thus, the Unix Epoch, though it is January 1, 1970 at 00:00:00UTC, is actually January 1, 1970 at 00:00:08.000082TAI, and although on January 1, 2000 at 00:00:00UTC (January 1, 2000 at 00:00:32TAI) exactly 946684823.999918 seconds (as measured with respect to TAI) will have elapsed since the Unix Epoch, the time() function will return 946684800.

This being so, either the POSIX standard is mad, or the right/ timezones are wrong. I would tend to say that POSIX is crazy, and that system clocks should measure TAI and leave out the leap seconds. But since system clocks are synchronized by NTP, and since NTP gives UTC (while skewing the system clock to somehow jam in the leap seconds), the POSIX standard is followed de facto. (As a compromise, I would suggest moving the Epoch back in time by 82 microseconds to avoid these funky non-integer figures.)

If I recall correctly, VMS measures time using the Modified Julian Date. This is also synchronized with UTC. January 1, 2000 will be julian day 2451544.5, so MJD 51544.

To summarize, I say that Y2k is when the Unix time() function returns 946684800, which is exactly 946684823.999918 second of atomic time after the Unix Epoch.

Another stupid bit of trivia: according to ISO (the ISO8601:1988 standard), Y2k doesn't start until the first monday of the year, i.e. January 3, 2000. As for January 1, 2000, it is still ``day 6 of week 52 of 1999''. See your local emacs for information on what this day is in various other calendars.

oz, gallon, doublequart.....

WHEN will you guys learn how to use SI, just as the rest of the world (Europe!)

FWIW, I drink 2 litres of coffee a day.