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    Re: What time is it, really?
    From: Gary LaPook
    Date: 2008 Jul 17, 23:36 -0700
    Gary writes:

    It's actually 15.041º per hour (15º 2.5') approximately 361º per solar day.

    gl


    Bill wrote:

    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.

    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. <g> But if I ever do...
    
    Bill B
    
    
    
    
      


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