# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: What time is it, really?
From: Greg R_
Date: 2008 Jul 17, 22:18 -0700

```--- Bill  wrote:

> 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.

Correct - which is why I said "up to 1 NM".

> I have never experienced my ideal conditions.  They would include a
> crisp horizon, clear sky, and a relatively stable (or predictable)
> platform.

So you're saying that your artificial horizon isn't "crisp", and the
location you're shooting from isn't "stable"? Hmm... either you're
shooting from here in SoCal (where the ground beneath us is known to be
"unstable" at times), or your location is somehow otherwise

--
GregR

--- Bill  wrote:

>
>
> >> 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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