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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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