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    Re: What time is it, really?
    From: Bill B
    Date: 2008 Jul 18, 01:04 -0400

    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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