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: What time is it, really?
    From: Greg R_
    Date: 2008 Jul 17, 22:18 -0700

    --- Bill  wrote:
    
    > An arc minute of longitude would be nominally 1 nm at the
    > equator, but less if the vessel's AP is north or south of the
    > equator.
    
    Correct - which is why I said "up to 1 NM".
    
    > I have never experienced my ideal conditions.  They would include a
    > crisp horizon, clear sky, and a relatively stable (or predictable)
    > platform.
    
    So you're saying that your artificial horizon isn't "crisp", and the
    location you're shooting from isn't "stable"? Hmm... either you're
    shooting from here in SoCal (where the ground beneath us is known to be
    "unstable" at times), or your location is somehow otherwise
    "navigationally challenged"... ;-)
    
    --
    GregR
    
    
    --- Bill  wrote:
    
    >
    > Bill asked
    >
    > >> What time is it, really?
    > >
    > > I believe the musical group Chicago answered that question back in
    > the
    > > late '60s... ;-)
    >
    > And does anyone really care?  I do.
    > >
    > >> A while ago there was a thread on time and the affect of dropping
    > >> leap seconds on cel nav.
    > >
    > > Don't think I was on the list for that thread, but as I understand
    > it
    > > leap seconds are added to UTC as needed to keep it within 0.9
    > seconds
    > > of astronomical time.
    > >
    > > The rule that I remember from back when I was first learning celnav
    > was
    > > that your observation time had to be accurate within 4 seconds,
    > > otherwise your LOP could be off by up to 1 NM just from that error
    > > alone (I interpret that to mean +/- 2 seconds). So I would say that
    > > unless you need exceptional accuracy with your celnav sights you're
    > > probably OK just ignoring the leap seconds.
    >
    > As understand it, with an earth rotation of 15d per hour, 1 second
    > time
    > equals 0.25 arc minute.  It follows that 4 seconds time would equate
    > to 1
    > arc minute.  An arc minute of longitude would be nominally 1 nm at
    > the
    > equator, but less if the vessel's AP is north or south of the
    > equator.
    > Roughly 1' longitude * cos latitude = fraction of a nautical mile
    > (ignoring
    > oblateness).  For example, near an elevated pole 360d longitude could
    > be
    > under 1 nautical mile.
    >
    > And why--despite the "former" CTA's cavalier attitude towards
    > chronometers--would I care?  With an artificial horizon, my Astra,
    > and a 3.5
    > scope, I consider an intercept of 0!0 from an average of 5 or more
    > observations from a known GPS position lucky. 0!1 very good.  0!2
    > average.
    > 0!3 fair, and > 0!3 has me checking IC and sextant calibration.
    >
    > I figure an artificial horizon cuts IE and observation errors in
    > half, so it
    > gives me 0!0 to 0!6 (averaged-observations intercept) as goal to
    > shoot for
    > under ideal conditions.
    >
    > I have never experienced my ideal conditions.  They would include a
    > crisp
    > horizon, clear sky, and a relatively stable (or predictable)
    > platform. And
    > of course accurate UT1 time.  But if I ever do...
    >
    > Bill B
    >
    >
    > >
    >
    
    
    --~--~---------~--~----~------------~-------~--~----~
    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