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--- 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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