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    Re: What time is it, really?
    From: Greg R_
    Date: 2008 Jul 18, 01:33 -0700

    --- glapook@pacbell.net wrote:
    
    > The earth completes approximately 366 rotations per year, it
    > completes one rotation in 23 hours-56 minutes-3.9 seconds.
    
    OK, that's one "extra" day than the annual allotment of 365, so I go
    back to my original question about why we don't need leap-days every
    year instead of every 4 years or so (something sticks in my mind about
    it being 365 1/4 rotations/year(?) - which would jibe with 1 leap-day
    every 4 years or so).
    
    --
    GregR
    
    
    
    --- glapook@pacbell.net wrote:
    
    > 
    > The earth completes approximately 366 rotations per year, it
    > completes
    > one rotation in 23 hours-56 minutes-3.9 seconds.
    > 
    > gl
    > 
    > On Jul 18, 12:18 am, "Greg R."  wrote:
    > > --- "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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