Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Leap seconds
    From: Greg R_
    Date: 2008 Dec 30, 23:41 -0800

    --- On Tue, 12/30/08, glapook@PACBELL.NET  wrote:
    
    > 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. 
    
    Agreed....
    
    > 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. 
    
    Correct. 
    
    But the point I was trying to make (and it seems to elude at least a few 
    people on the list) was that practically speaking it doesn't really matter 
    whether the timescale we use in navigation tracks "solar time" (and by 
    inference, the Earth's rotation) as long as whatever timescale we use for 
    navigation is the same as the one in the almanac. Whether or not the leap 
    seconds are added or not is really irrelevant, *as long as* the time we use 
    in the field is the same as is used in the almanacs.
    
    > 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.
    
    Agreed as well, but what I'm saying is it doesn't really matter whether that 
    time used for the sight is even closely related to solar time at all (though 
    it would be convenient if it were), again as long as our timescale matches 
    that of the almanacs.
    
    > Hopes this helps.
    
    Well, I think I'm beating a dead horse at this point - guess my assertion was 
    a little too "abstract" for some of those on the list... ;-)
    
    > 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?
    
    Definitely sounds like fun - let's talk about it sometime after the new year 
    (we've got some schedule changes happening at work that will actually make 
    that more viable than it is right now). And I'm still hoping to get back 
    current again sometime early next year, so your timing is impeccable (though 
    I'll leave the discussion of what timescale that one runs on to those with 
    more available free time... ;-)).
    
    --
    GregR
    
    
    --- On Tue, 12/30/08, glapook@PACBELL.NET  wrote:
    
    > From: glapook@PACBELL.NET 
    > Subject: [NavList 6855] Re: [NavList /] Re: Leap seconds
    > To: "NavList" 
    > Date: Tuesday, December 30, 2008, 1:52 PM
    > 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 [6805]
    > 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 [6849]-
    > >
    > > > "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:
    > [6802]
    > > > 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
    > > > [6805], 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 NavList-unsubscribe@fer3.com
    -~----------~----~----~----~------~----~------~--~---
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site