Time 
Author Message
 Time

Quote:


>Greenwich observatory is now a museum, but the 0th meridian is deemed to be
>still HERE and very unlikely to be ever moved !

>So it's GMT for ever !

>Martin
>Greenwich, UK

        I agree with Martin that the prime meridian which runs through Greenwich
will never be moved, but GMT (Greenwich Mean Time) and CUT (Co-ordinated
Universal Time) are slightly different.  It's just a matter of style as to which
the E-mail clients display, but GMT is by far the more traditional.

        Both GMT and CUT use the Gregorian calendar with the "day" defined as the
mean sidereal day as though observed at Greenwich.  But GMT defines the "second"
as one 86,400th of a mean sidereal day, while CUT defines the "second" as
####### oscillations (I don't have the exact value at hand) of a Cesium atom in
an atomic clock.  Both of these definitions were meant to yield the same value,
but due to variations in the earth's rotation the CUT second ends up being a
slightly smaller amount of time than the GMT second, so a "leap second" is added
to the CUT calendar for the last minute in June or December every few years.  In
the worst case CUT can be up to half a second different from GMT.

/Cheers, Keith

P.S. I think that in the 19th and early 20th centuries the clock (whose hour
bell is known as Big Ben) at the Houses of Parliament in Westminster was set
because the observatory at Greenwich could be seen by someone looking through a
telescope, and they added or removed a penny every week or so half-way up the
pendulum, which weighs several tons, to keep the clock accurate.



Thu, 24 Jun 2004 07:43:45 GMT  
 Time

Quote:

> will never be moved, but GMT (Greenwich Mean Time) and CUT (Co-ordinated
> Universal Time) are slightly different.  It's just a matter of style as to which

I believe the correct abreviation is UTC.  (Otherwise the French would
never have agreed to it.)

Ted



Wed, 30 Jun 2004 02:47:02 GMT  
 Time


Quote:

>> will never be moved, but GMT (Greenwich Mean Time) and CUT (Co-ordinated
>> Universal Time) are slightly different.  It's just a matter of style as to
>which

>I believe the correct abreviation is UTC.  (Otherwise the French would
>never have agreed to it.)

Maybe the Americans and British (two nations well-known the world over
for their inability to learn foreign languages) should get together and
campaign to get it changed to CUT. :-)

Quote:

>Ted

--
John Sullivan


Wed, 30 Jun 2004 18:45:20 GMT  
 Time
I think it's time for me to add my 5p worth :-)

Historically (i.e. before atomic clocks) standard time measurement was
dependent on the "mean solar day", the interval between two successive
lower meridian passages of the "mean sun". For convenience (or was it to
deal with the British superiority complex) the meridian running through
the Greenwich observatory was chosen as the "universal" standard, and
hence called 0 degrees. This is still the standard, and will probably
remain so (despite the number of French people who think that 0 should
run through Paris).

Note that the "mean sun" is not a real observable body - it moves around
the earth's equator smoothly, which the "apparent sun" (this is the body
that gives us light and makes life possible) doesn't. The importance of
the observatory (w.r.t. the measurement of time) was then to calculate
the difference between these suns (called the equation of time) and
report on the meridian passage of the non-existent one! This does not
have to occur at Greenwich, or in fact on the Greenwich meridian, for
GMT to be a valid concept.

Now we have the ability to measure 290,091,200,500,000,000 oscillations
of a Cesium atom and can do the measurement and prediction of time far
more accurately than astronomers (at Greenwich or elsewhere) ever could,
some seem to think that the concept of GMT should be relegated to the
dustbin of history. However, as has been pointed out, UT needs the
occasional leap second, which GMT doesn't. In fact, while the
measurement may have increased in accuracy, UT is still only an
approximation of GMT, which is based on the only sensible time concept:
the earth's rotation around the  (mean) sun.

Regarding the use of Z rather than GMT: this is used at sea as well.
Originally this replaced 3 chars with 1 for radio and light
communication (abbreviations are always welcome if you are using Morse).
With the advent of voice telecoms, spelling "zulu" rather than "golf
mike tango" is still a worthwhile saving.

Anyway, I think the whole argument is a bit frivolous: I defy anybody to
keep two watches, one set to GMT and the other to UT and to tell the
difference. This rarely exceeds half a second, and there are few
instances where this level of timekeeping accuracy would affect our lives.

For more interesting history: a free plug for the book "Longitude" by
Dava Zobel, ISBN 1-85702-502-4 tells of the importance of accurate
clocks in a previous age.

Quote:



>>Greenwich observatory is now a museum, but the 0th meridian is deemed to be
>>still HERE and very unlikely to be ever moved !

>>So it's GMT for ever !

>>Martin
>>Greenwich, UK

>         I agree with Martin that the prime meridian which runs through Greenwich
> will never be moved, but GMT (Greenwich Mean Time) and CUT (Co-ordinated
> Universal Time) are slightly different.  It's just a matter of style as to which
> the E-mail clients display, but GMT is by far the more traditional.

>         Both GMT and CUT use the Gregorian calendar with the "day" defined as the
> mean sidereal day as though observed at Greenwich.  But GMT defines the "second"
> as one 86,400th of a mean sidereal day, while CUT defines the "second" as
> ####### oscillations (I don't have the exact value at hand) of a Cesium atom in
> an atomic clock.  Both of these definitions were meant to yield the same value,
> but due to variations in the earth's rotation the CUT second ends up being a
> slightly smaller amount of time than the GMT second, so a "leap second" is added
> to the CUT calendar for the last minute in June or December every few years.  In
> the worst case CUT can be up to half a second different from GMT.

> /Cheers, Keith

> P.S. I think that in the 19th and early 20th centuries the clock (whose hour
> bell is known as Big Ben) at the Houses of Parliament in Westminster was set
> because the observatory at Greenwich could be seen by someone looking through a
> telescope, and they added or removed a penny every week or so half-way up the
> pendulum, which weighs several tons, to keep the clock accurate.

--
-----BEGIN GEEK CODE BLOCK-----
Version: 3.1
GO/! d- s++:+ a+ C++(++++) US+++$ P+++ L++ E--- W++ N++ w--- O- V- PS+
PE+ Y+ PGP- t+ 5++ X R* tv+ b+ DI++ D G e(*) h++/-- r+++ y?
------END GEEK CODE BLOCK------

-----------------------------------------------------
Bob Hoekstra:   APL & Unix Consultant
Tele:           +44 (0)1483 771028
                 http://www.HoekstraSystems.com

-----------------------------------------------------



Wed, 30 Jun 2004 19:08:25 GMT  
 Time

Quote:

> For more interesting history: a free plug for the book "Longitude" by
> Dava Zobel, ISBN 1-85702-502-4 tells of the importance of accurate
> clocks in a previous age.

Oh, it still matters but I just set my watch from my GPS. :-)

Ted



Thu, 01 Jul 2004 03:24:59 GMT  
 Time
"Ted Edwards" wrote ...

Quote:

> > ...the importance of accurate clocks in a previous age.

> Oh, it still matters but I just set my watch from my GPS. :-)

And if you set it by hand, it probably differs from both UTC and GMT by
more than they differ from each other.


Thu, 01 Jul 2004 20:47:30 GMT  
 Time
All this concern about time,,,
I look out the window and if it's light, it's day; it it's dark, it's night.
And without a watch, it's much easier to miss those pesky meetings.

David Liebtag



Fri, 02 Jul 2004 01:09:11 GMT  
 Time
Life is not that simple. I look out the window, it is dark BUT it is day. In
winter. Then, at midsummer, i look out and see sunshine but it is midnight.
:-)
/ Tomas



Quote:
> All this concern about time,,,
> I look out the window and if it's light, it's day; it it's dark, it's
night.
> And without a watch, it's much easier to miss those pesky meetings.

> David Liebtag



Fri, 02 Jul 2004 03:22:04 GMT  
 Time

Quote:
> "David Liebtag" wrote...
> > All this concern about time,,,
> > I look out the window and if it's light, it's day;
> > if it's dark, it's night.

"Tomas Gustafsson" replied...

Quote:
> Life is not that simple. I look out the window, it is dark
> BUT it is day. In winter. Then, at midsummer, i look out
> and see sunshine but it is midnight.
> :-)

Ah, Tomas, you beat me to the punchline.  My version is that in Finland if
it's light, it's summer, and if it's dark, it's winter.

            ;-)            /Jim



Sat, 03 Jul 2004 01:57:36 GMT  
 Time
In San Jose if it's raining it's winter, otherwise it's 75 degrees F and
sunny.
Ahh, the harsh California life style.

Couldn't resist one more off topic post,,,



Sat, 03 Jul 2004 11:29:25 GMT  
 Time
"David Liebtag" wrote ...

Quote:
> In San Jose if it's raining it's winter,
> otherwise it's 75 degrees F and sunny.

In Denmark we say we have two seasons,

    The white winter...

        And the green winter.

(Actually, the white winter this year has been whiter than usual.)

                /Jim



Sun, 04 Jul 2004 06:42:21 GMT  
 
 [ 11 post ] 

 Relevant Pages 

1. Bug in time.c (was Time.times problems)

2. time and scheduling (was: bug report: [ #447945 ] time.time() is not non-decreasing)

3. time zones, daylight saving time, and universal time

4. Manugistics run-time [was: STSC run-time]

5. GMT time vs. local time

6. Comparing system time and log file time

7. converting time in a string to a time field

8. Create Time / date or Modified Time / date of a txt file

9. Run-time & Design-time window size problem

10. time to time with different dates...

11. real time dispaly of time.

12. TIME&DATE in HTML shows TIME

 

 
Powered by phpBB® Forum Software