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AZgazer
2004-May-08, 11:02 PM
I had been curious about the use of the term UT instead of GMT, which was the term I had always used/seen used.

So after doing a little nosing around I found out UT was an astronomical term for GMT and is now used almost exclusively in it's place. OK, that's easy enough, new term... cool.

Then I find that there are 2 types of UT, UTC and UT1. I understand the fundamental differences in them, but I can't think of an application. Does anyone know of one? Also I read that sometimes UT is denoted as ####z. Would that Z be the result of the military use of "Zulu" for GMT? Any incidents we reported to higher HQ were in both Local and Zulu time.

I am using this page as my source; http://aa.usno.navy.mil/faq/docs/UT.html

swansont
2004-May-09, 01:42 AM
I had been curious about the use of the term UT instead of GMT, which was the term I had always used/seen used.

So after doing a little nosing around I found out UT was an astronomical term for GMT and is now used almost exclusively in it's place. OK, that's easy enough, new term... cool.

Then I find that there are 2 types of UT, UTC and UT1. I understand the fundamental differences in them, but I can't think of an application. Does anyone know of one? Also I read that sometimes UT is denoted as ####z. Would that Z be the result of the military use of "Zulu" for GMT? Any incidents we reported to higher HQ were in both Local and Zulu time.

I am using this page as my source; http://aa.usno.navy.mil/faq/docs/UT.html

For many applications UTC and GMT are interchengeable, but are not calculated the same way; UTC is calculated and disseminated by the International Bureau of Weights and Measures (Bureau International des Poids et Mesures - BIPM) and not by the Observatory at Greenwich. UT1 is based on the earth's rotation, and we add (or subtract) leap seconds to UTC to keep it within 0.9 seconds of UT1. There are actually many versions of UTC, as every national standards lab has their own realization of it.

Oh, there's a bunch more, too. EAL is unsteered atomic time, and TAI is atomic time steered using various primary frequency standards, and differs by the number of leap seconds added since that convention was started in 1972. There are UT0 and UT2, which are also related to earth rotation (with certain correections used or not used, with respect to UT1).

Here (http://www.ucolick.org/~sla/leapsecs/timescales.html) is more than you ever wanted to know about timescales, and were rightfully afraid to ask.

ToSeek
2004-May-09, 01:59 AM
There's been some discussion as to whether or not to continue having leap seconds. Conveniently, the Earth's rotation has settled down a bit for the time being, so it hasn't been an issue for several years now.

AZgazer
2004-May-09, 03:15 AM
Here (http://www.ucolick.org/~sla/leapsecs/timescales.html) is more than you ever wanted to know about timescales, and were rightfully afraid to ask.

Be afraid, be very, very afraid. :) Now if I could just figure out which one my Citizen runs on I would never be late, my boss isn't buying the WST (Walt's Standard Time) anymore.

"But sir, you see I'm actually not late because your clock is running on EAL which is a 70s technology and was replaced with TAI in 97' this lack of updating has led to a variance of ~.5s per year and now means your clock is... Well I am not 10 minutes late, your clock is old!"

Thanks for the link swansont I will try and actually comprehend that page when I have a little more free time. :) (No pun intended.)

milli360
2004-May-09, 12:01 PM
Would that Z be the result of the military use of "Zulu" for GMT? Any incidents we reported to higher HQ were in both Local and Zulu time.
No, other way around--the use of "zulu" by the military is the result of that z. The time zones are labeled (http://www.aos.wisc.edu/~hopkins/aos100/z-time.htm) with twenty-five letters of the alphabet, skipping J, and the international date line divides one into two parts. From that link, you can see it's an odd ball way of assigning letters, but this webpage (http://squall.sfsu.edu/crws/map_info/UTC_info.html) suggests that greenwich was assigned Z because its the Zero meridian. This NASA webpage (http://www.grc.nasa.gov/WWW/MAEL/ag/zulu.htm) says they skip I and O, but they furnish a link at the bottom of the page that gets you to some nice pdf files of the USNO (http://aa.usno.navy.mil/faq/docs/world_tzones.html) that disagree.

There's been some discussion as to whether or not to continue having leap seconds. Conveniently, the Earth's rotation has settled down a bit for the time being, so it hasn't been an issue for several years now.
I wouldn't characterize it as settling down though--it's sped up enough in the past five or ten years to compensate for over a hundred years of slowing down, more or less.

cyswxman
2004-May-09, 02:13 PM
What time is it? :wink:

milli360
2004-May-10, 01:51 PM
This NASA webpage (http://www.grc.nasa.gov/WWW/MAEL/ag/zulu.htm) says they skip I and O, but they furnish a link at the bottom of the page that gets you to some nice pdf files of the USNO (http://aa.usno.navy.mil/faq/docs/world_tzones.html) that disagree.
I emailed Bruce Bream, the person in charge of that webpage, and he has changed the info. He says he got the original from a reference book from 1996.

AZgazer
2004-May-10, 04:26 PM
Would that Z be the result of the military use of "Zulu" for GMT? Any incidents we reported to higher HQ were in both Local and Zulu time.
No, other way around--the use of "zulu" by the military is the result of that z. The time zones are labeled (http://www.aos.wisc.edu/~hopkins/aos100/z-time.htm) with twenty-five letters of the alphabet, skipping J, and the international date line divides one into two parts. From that link, you can see it's an odd ball way of assigning letters, but this webpage (http://squall.sfsu.edu/crws/map_info/UTC_info.html) suggests that greenwich was assigned Z because its the Zero meridian.

Ah, makes a lot of sense. I had never seen the timezones represented Alphabetically, except our reporting in both local & Z (for Zulu). :)