A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Leap seconds
From: Gary LaPook
Date: 2008 Dec 30, 13:52 -0800
From: Gary LaPook
Date: 2008 Dec 30, 13:52 -0800
Greg, what George is saying is that as long as you can be sure that the time scale used to compute almanac data will stay within one second of solar time (GMT, UT1) you can do the computations and print the almanac. Since atomic time cannot meet this standard due to the unpredictability of the rotation of the earth (which necessitates the occasional leap second) that atomic time is not a good choice for astronomical use. The location of celestial bodes, when viewed from earth, is dependent of the orientation of the earth's coordinate system at he time the sight taken, not on some absolute time scale. Hopes this helps. On another subject, I heard from Mike Burke that he is interested in taking some celestial sights in flight, would you like to get up in a plane and give it a try also, I have lots of sextants? gl On Dec 30, 9:25�am, "Greg R."
wrote: > --- On Tue, 12/30/08, George Huxtable wrote: > > > Geoffrey Kolbe's intervention has been particularly helpful in > > clarifying matters. > > Indeed. He says "not tightly coupled", and I say "disconnected". Your argument would be what exactly? > > > Greg R has not since repeated that statement, but continues > > to defend it. > > Why would I need to "repeat it" (ad infinitum...). Did it not register with you the first time I said it? > > > It allows almanacs to be printed some years in advance of their > > predictions, without needing to know whether, or when, leap-seconds > > will need to be introduced in the interim. > > Which was my original point (and I think you're agreeing with it here, though you're probably refusing to admit it) - �that whether or not we use leap-seconds is totally irrelevant as long as our timescale agrees with the one that's published in the almanacs. > > -- > GregR > > --- On Tue, 12/30/08, George Huxtable wrote: > > > From: George Huxtable > > Subject: [NavList 6851] Re: [NavList /] Re: Leap seconds > > To: NavList@fer3.com > > Date: Tuesday, December 30, 2008, 3:10 AM > > Greg R wrote- > > > "George > > > "I don't know if you're being pedantic here, > > or if we have an honest failure > > to communicate (or maybe a little of both...). " > > > Yes, I am being pedantic. On a subject such as this, so > > easy to > > misunderstand, we have to be precise about our words. Greg > > R and I are > > completely failing to communicate properly. If we can't > > agree about this > > initial matter, there's no point in further discussion, > > so let's have > > another go. Geoffrey Kolbe's intervention has been > > particularly helpful in > > clarifying matters. > > > The words in question came from a posting  by Greg R, > > in which he > > wrote- "Besides, the almanacs have been on UT since > > when - mid 70s? (and > > thus pretty much "disconnected" from "sun > > time")." > > > Greg R has not since repeated that statement, but continues > > to defend it. > > > He continues, in - > > > "But I think we can both agree that the current > > timescale that most of us > > navigators use (i.e. UT) is based on an atomic standard, > > right?" > > > No, we can't agree on that. Greg R's phraseing is > > woolly. What does he mean > > by "based on"? Yes, the STARTING POINT is the > > counting of seconds, of Atomic > > Time, unrelated to the rotation of the Earth. But then (and > > this is the > > crucial bit) that count of seconds is SERIOUSLY MODIFIED, > > by the insertion > > of an unpredictable leap-second here and there, to create > > UT which IS then > > closely in step with the rotation of the Earth, within less > > than a second. > > That's the time-scale that almanacs work to. It allows > > almanacs to be > > printed some years in advance of their predictions, without > > needing to know > > whether, or when, leap-seconds will need to be introduced > > in the interim. > > > George. > > > =========================================== > > > --- On Mon, 12/29/08, George Huxtable > > wrote: > > > > From: George Huxtable > > > Subject: [NavList 6848] Re: [NavList /] Re: Leap > > seconds > > > To: NavList@fer3.com > > > Date: Monday, December 29, 2008, 11:00 AM > > > Greg R wrote- > > > > ----- Original Message ----- > > > From: "Greg R." > > > > > To: > > > Sent: Monday, December 29, 2008 12:09 AM > > > Subject: [NavList /] Re: Leap seconds. was:  > > Longest > > > year since 1992 > > > > You're still missing my point, which was (perhaps > > > obliquely) related to > > > whether we need leap seconds inserted into the UT > > scale or > > > not. > > > > What I'm trying to say (and I really don't > > know how > > > to re-word this to make > > > it any clearer) is that whatever time scale the > > navigation > > > community agrees > > > on (either with or without "leap seconds"), > > as > > > long as the time that we use > > > in the field (from a chronometer, radio time signal, > > > "atomic" watch, > > > whatever...) matches the time used in the various > > almanacs > > > it's *totally > > > irrelevant* whether or not that time matches the > > actual > > > "sun time" that's > > > been used for centuries. > > > > In other words, if the almanac shows a time of > > 16:00:00 for > > > noon at > > > Greenwich on a given day (granted that's a huge > > > difference from UT, but it > > > works for an example), as long as my local timepiece > > also > > > matches that time > > > scale I shouldn't have any problem working a sun > > sight > > > based on that (any > > > more that I would with a timepiece set to UT as is the > > > current practice). > > > > Granted it would be nice if our navigation timescale > > were > > > reasonably close > > > to "sun time", but as a practical matter > > whether > > > it does or not is really > > > irrelevant to solving the navigation problem. > > > > Make more sense now? > > > > ========================= > > > > What I questioned was Greg R's statement in > > , as > > > follows- > > > > "Besides, the almanacs have been on UT since when > > - > > > mid 70s? (and thus > > > pretty much "disconnected" from "sun > > > time"). " > > > > And I pointed out that the almanacs have remained > > closely > > > "connected" with > > > "sun time", precisely because of the > > insertions > > > of those leap seconds, by > > > which UT is made to correspond closely with the > > rotation of > > > the Earth.. > > > > I wonder what it was that happened, in the mid 70s, > > > according to Greg R, to > > > bring that about the "disconnection" he > > refers > > > to? In one way or another, > > > the Nautical Almanac has followed mean Sun time at > > > Greenwich, within a > > > fraction of a second, since 1834, and continues to do > > so. > > > > Greg R �has complained that "I think you > > > misinterpreted what I was trying to > > > say", and now "You're still missing my > > > point". > > > > But he hasn't withdrawn those words that I quoted, > > and > > > questioned, above, > > > about the almanacs being "disconnected" from > > > "sun time". And I don't know > > > how to interpret those words any differently than the > > way I > > > did. > > > > If we can find a way to agree about this minor matter, > > > perhaps we can go on > > > to discuss leap seconds more fruitfully. > > > > George. > > > > contact George Huxtable, at �geo...@hux.me.uk > > > or at +44 1865 820222 (from UK, 01865 820222) > > > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 > > 5HX, UK. > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavListfirstname.lastname@example.org -~----------~----~----~----~------~----~------~--~---