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    Re: What time is it, really?
    From: Greg R_
    Date: 2008 Jul 18, 00:18 -0700

    --- "Gary J. LaPook"  wrote:
    
    > It's actually 15.041� per hour (15� 2.5') approximately 361� per
    > solar day.
    
    Hmmm... not questioning your math, but if the Earth rotates 1� beyond a
    complete rotation every day, wouldn't we need to add leap days every
    year (i.e. 365� "extra" rotation in a year = 1 extra day + 5� "left
    over"), instead of approx. every 4? Seems like it should be closer to
    something like 360.25�/day (?).
    
    Then again, it's late and I'm not thinking clearly on this one....
    
    --
    GregR
    
    
    --- "Gary J. LaPook"  wrote:
    
    > Gary writes:
    > 
    > It's actually 15.041� per hour (15� 2.5') approximately 361� 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.  But if I ever do...
    > >
    > >Bill B
    > >
    > >
    > >>
    > >
    > >  
    > >
    > 
    > 
    > > 
    > 
    
    
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