Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site hou3c.UUCP Path: utzoo!linus!security!genrad!grkermit!masscomp!clyde!burl!hou3c!seismo!wcwells%ucbopal.CC@Berkeley.ARPA From: wcwells%ucbopal.CC@Berkeley.ARPA Newsgroups: net.mail.headers Subject: Re: Several questions/comments on time zones Message-ID: <8401310026.AA10569@ucbopal.CC.Berkeley.ARPA> Date: Mon, 30-Jan-84 19:26:51 EST Article-I.D.: ucbopal.8401310026.AA10569 Posted: Mon Jan 30 19:26:51 1984 Date-Received: Thu, 2-Feb-84 01:08:49 EST Sender: ka@hou3c.UUCP (Kenneth Almquist) Lines: 74 It looks like UT (or maybe UTC) = GMT = GCT = Z. Here is a reply I received from David J. Bryant via Mark Horton: ------- Date: Sun, 29 Jan 84 18:56:43 est; From: mark@cbosgd.UUCP (Mark Horton) To: wcwells@ucbopal; Subject: star gazers time Here is an answer to your star gazers questions Date: Sun, 29 Jan 84 18:21:08 est; From: djb (David J. Bryant) To: mark; Subject: Re: can you please answer the "star gazers" question in this? Astronomers time stamp observations and events according to Universal Time (abbreviated UT). This is based on atomic clocks and mathematical standards according to definition of the International Astromical Union (I.A.U.). Further, Greenwich Mean Time (GMT) is defined to be the same as UT, and applies to the standard meridian (longitude 0 degrees). Greenwich Civil Time (GCT) is synonymous with GMT. WWV and WBS (in England) broadcast UT (which is therefore also GMT, and for the military, Z ("Zulu") time. This is a worldwide standard reference, and probably the best for your purposes. (Elaine says that CompuServe used UT for ALL their record keeping, login accounting, etc.) There are other time schemes as follows: Apparent Time - The time indicated by a sundial. Because the Sun's speed across the sky is not uniform, this varies throughout the year, and is useless for time reckoning. Mean Time - To avoid the non-uniformities of Apparent Time, a fictitious "mean sun" is invoked; this moves across the sky at a constant rate throughout the year. Mean Time, like Apparent Time, is purely a function of your longitude, and so is too variable for much civil use. Civil Time - This is what the clock on the wall is set to. It is based on the concept of Time Zones, Daylight Saving Time, etc. Each time zone is homed on a central meridian, and the mean time for that meridian is used as the civil time throughout the time zone. Other adjustments (Daylight Saving Time) apply as required. Sideral Time - Unlike all the above, which is based on the 24 hour solar day, Sideral Time is based on the 23h 56min "star" day. There are numerous flavors of Sideral Time that compensate for variations, but I don't see any real reason to go into much detail. This is not a time scheme you want to use. Interestingly, all time schemes can be converted to any other time scheme, provided you specify latitude, date, etc. If you would like a more thorough, informative description of these (and other) time systems, let me know. I think this should be enough to answer your question about "star gazing" time. Personally, I encourage you to use UT, and to follow other standard astronomical (I.A.U.) conventions to deal with issues associated with time/date stamping, since astronomers have already worked out these problems. David ------- Now why UT and UTC? I suspect that UT is the time based on the atomic clock from a specific point in time and that UTC is UT which has been leap second adjusted. Bill Wells wcwells@Berkeley.ARPA