NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: What time is it?
From: Derrick Young
Date: 2004 Nov 9, 18:24 -0500
From: Derrick Young
Date: 2004 Nov 9, 18:24 -0500
Another good question. Lets follow it up with another aspect. Why is Europe changing to DST one week before the US and back to standard time one week after the US? Why is Turkey's local time is 30 minutes different from the surrounding countries. Japan and Korea do not have DST at all (but because of the physical size of the countries, they don't really need it). I know that Europeans like to do things differently than we do here, but this causes additional confusion when your business spans multiple time zones. I support computers in 22 time zones at some 290 locations. I really have to pay attention to local times when I am calling (or being called by) someone to discuss issues/problems and resolutions. Many folks make reference to GMT and UTC as if they are the same, but they aren't quite the same. GMT is the time as established by the Royal Naval Observatory in Greenwich, England. GMT is sometimes termed Zulu or Zulu time (no longer in common use but I still read it in US Coast Guard and Navy manuals). If you have a problem remembering how the conversion goes between GMT/ZT and your local time zone (and you know your offset from Greenwich) look at the table located at: http://wwp.greenwichmeantime.com/info/time-zones.htm - this is a simple lookup that helps. It does not take into account the differences between DST and Standard Zone Time - for that look at http://wwp.greenwichmeantime.com/info/timezone.htm Now, to the question, GMT/ZT, UTC, IAT or UT1? GMT and Zulu - no issues just different names. UTC (Coordinated Universal Time) is based on IAT (International Atomic Time) and only differs by some integer number of seconds. Then we come to UT1 - the NAVIGATORS TIME. This is not based on the same set of atomic clocks as IAT or UTC, but is based on the rotational rate of the Earth - with all of it's irregularity and slowing down. Per international agreement, there can be no more than 0.9 seconds difference between UTC and UT1 - sorry - my watch does not measure that closely - I am lucky to remain in the right day! Currently the difference between UTC and IAT is -32 seconds (I can measure this but I am really pushing myself to do so!) But all of these differences are introduced by the various definitions of a "second". Some important dates (no need to remember them!) in this evolution are: 1884 - International Meridian Conference based on a mean solar day at Greenwich, England - this established the common Prime Meridian in Greenwich. 1956 - Ephemeris Time: Based on lunar observations 1958 - Atomic Time: Based on clock data from numerous countries - not all the same elements were used 1967 - Atomic Time: based on radiation patterns of the caesium element. 1972 - Co-ordinated Universal Time (UTC) introduced. 1984 - Dynamical time - based on motion of celestial bodies 1986 - UTC replaced GMT. Now here is the balance of this interesting tidbit of useless history. Based on solar time, there are only four days each year that have exactly 24 hours. These occur on or about December 25, April 15th (just knew that there had to be something positive about the US tax day!), June 14th and August 31st. The balance of the days are either longer or shorter. If you look at the differences between solar time and standard zone time, noon may arrive as much as 16.3 minutes early or 14.4666 minutes late. Since earth time is the ultimate time, the time police add or subtract leap seconds from UTC to bring the IAT back in line with reality. Need further confusion? I can add it - after all I am from the Government and I am here to help. derrick