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--- Anabasis75@aol.com wrote:

> I will be interested to see where your notes take us in this one.

I'm puzzled too, since my method seems to work for my own local area.
My guess is it probably has something to do with doing corrections for
eastern longitudes (something that I've only done when learning CelNav
lots of years ago). I'll take a look my worksheet and post my
methodology that got those results. 

> I can't explain this, as the sun did set basically when I said it 
> would, around 6:35 PM and shot at 7 PM ZT

I'm sure you're right, since you were at that location and able to get
your shots at those times.

> LAT is Local apparent time (like as in LAN).  

Gotcha, I was going to e-mail you that was what I'd guessed it meant,
so thanks for adding to my navigation lexicon.

> However, the Sunrise and sunset are in Local MEAN time (LMT), not 
> Zone time or UTC.

Agreed, but since they're referenced on Greenwich that would also mean
they're valid for UT/Z time, right? Or am I not remembering something
correctly?

> It is the way we interpolate things, and you're right, you'd better 
> be ready long before that lol.

Especially since we're talking about the time for Nautical Twilight... 
;-)

> I shoot one each and throw out the ones that don't work.

That seems to be the most expeditious way of doing it, if you take
multiple sights of the same star which one(s) do you trust as being
accurate? I've seen an example (I think from a David Burch article)
where even if you're averaging sights, one bad one can throw off the
entire process (and seems like with only a few sights to average there
was even some ambiguity as to which one(s) was the bad one).

> With the computer, the whole process takes about 15 minutes. 

Which software are you using out there in the "real world"?

> Considerably longer when using tables.  

Yeah, I haven't used tables/manual plotting in years (other than just
recently for your problems posted to the list).

> 229 is just too slow with multiple LOP's

I've never figured out why 229 is LHA-based (instead of being
latitude-based as 249 is). Since you're going to use the same assumed
latitudes for a fix (or I guess maybe nearby latitudes, if a lot of
time passes between fixes) why should you have to flip to a different
page in the tables for each fix? 

Might have something to do with the computer program used to generate
the tables, but I'd also assume that the output could be setup in
whatever format was desired by the end-user.

--
GregR






--- Anabasis75@aol.com wrote:

> --- Anabasis  wrote:
> 
> > Your methodology  is very strange to me for the sunset numbers, and
> 
> > gives you the wrong  answers. 
> 
> Interesting, because it seems to work for me (at least here in  the
> western longitudes) - I've actually been doing some  sunset/twilight
> calculations lately for practice, and they seem to follow the  "real
> world" situation pretty accurately.
> 
> I'll have to check this from  home tomorrow (I'm at work and don't
> have
> my notes in front of me), but I did  run my numbers through Navigator
> and it agreed with me (within a few tenths  on some of the
> calculations,
> of course).
> 
> -----------------------------------
> -JCA I can't explain this, as the sun did set basically when I said
> it  
> would, around 6:35 PM and shot at 7 PM ZT.  I too ran it through my 
> computer.  I 
> hope I didn't give incorrect basic information.  Still  you have ZT
> of sunset 
> well after 8 PM which is definitely incorrect.
>  
> 
> 
> >  I got 1907 for NT, but I’m not going to quibble.  What is wrong 
> is
> > that this is the Time in LAT 
> 
> I'm not sure what "LAT" is,  but the times in the NA are UT/Z unless
> I
> don't remember correctly  (?).
> 
> I made an error here.  LAT is Local apparent time (like as in  LAN). 
> 
> However, the Sunrise and sunset are in Local MEAN time (LMT), not 
> Zone time or UTC.  
> This is the time based on the "average" sun and varies  from Zone
> Time based 
> on your difference in Longitude from the Zone Meridian (in  my case
> 150 deg 
> East.  This is where that Arc to time stuff comes in.   Since I am
> west of my 
> zone meridian, my ZT of sunset etc will be later then the  NA tables
> will tell 
> you.
> --------------------
>  
> Hopefully the navigator would be ready to shoot well before then,  so
> our minute of time difference shouldn't matter...   ;-)
> 
> 
> ------------------------
> -JCA It is the way we interpolate things, and you're right, you'd
> better be  
> ready long before that lol.
> 
> 
> >  What is wrong is that this is the Time in LAT which must be 
> > converted  to ZT before use.  
> 
> Again, not knowing what "LAT" is I can't  discuss this one way or
> another, but as I understand it all of the times on  the NA page
> (sunrise/sunset/twilights/meridian passage/GHA/etc.) are all in 
> UT/Z.
> 
> > These times are only good on the Zone reference meridians, 
> including 
> > Greenwich (divisible evenly by 15).
> 
> Correct, and the  arc-to-time conversion changes that time to the
> UT/Z
> time at the given DR  longitude position. Applying the ZD correction
> brings the time back to  local zone time (for the convenience of the
> navigator, but not used in the  LHA Aries calculation).
> 
> > Here is where we get really  different.  The correction from LAT
> > to ZT should be fairly small  (<1 hr) if we are keeping the correct
> > zone description.
> 
> Aha  - I think the difference here is from the different way we we
> (apparently)  calculate local times for suneset/twilights.
> 
> > The trouble with the  second is that you have to apply the
> correction
> > in the correct  direction which can be confusing, especially if you
> > are in East  Longitude and are from the Western Hemisphere.
> 
> Correct - and since  events happen earlier in eastern longitudes
> you'd
> subtract the (absolute  value of) the ZD from the UT/Z time given in
> the
> NA to change the time to  the local time zone.
> 
> > I got 146 degrees since we add 22’ longitude  to 34.7’ GHA to
> get 56.7
> 
> > which is close to 60, and we carry the  one to 145 to equal 146.
> 
> I'll have to look at my worksheet tomorrow to  see how I arrived at
> that
> LHA, but I'm reasonably sure that I remembered to  add 1 whole degree
> to
> it.
> 
> > I also use Pub 249 Vol 1 but Epoch  2010 (available online).
> 
> Aha, wasn't aware that 2010 was out yet  (though it might not be in
> printed form yet) - thanks for the  info.
> 
> > Obviously since my LHA is different from yours, there will  be
> > slightly different results:
> 
> Agreed.
> 
> >>  correction for precession and nutation
> > This is correct, but not  necessary for precalculation as we
> > only need an approximate Hs and Zn  to find the body.
> 
> Ah, seems like you mentioned getting Hc and Zn -  thought you'd meant
> to
> get values for the final sight reduction, but you're  correct that an
> approximate Hs/Zn will work to get the body into the  sextant's field
> of
> view.
> 
> > Also, I was taught to only apply this  correction in off-epoch
> years
> > to the actual fix instead of the LOP’s  (unless there is just one
> LOP 
> > of course).
> 
> According to the  instructions in my copy of Pub. 249, the correction
> can be applied to  either the LOP or the fix itself (and personally
> I'd
> rather move just the  final fix than the several LOPs used to
> determine
> it) - less time, and less  prone to errors.
> 
> > The point of pre-calculating stars at sea is so  that you can go
> out
> > and just point the sextant in the correct Azimuth,  then the star
> just
> > pops into view.  A tiny adjustment on the  micrometer drum and you
> > take your sight.  
> 
> I've tried to  do that from my front/back yard in recent weeks (using
> an
> approximate  eye-level on the neighbor's houses) but so far I've only
> been able to make  it work for a couple of the selected stars (there
> isn't much light  pollution where I live, but it's still not the same
> as
> being out on the  open water away from civilization).
> 
> > I can shoot a round of stars  in under 10 minutes, and am just
> usually
> 
> > waiting for the dimmer  ones to appear.  For this round, I
> > shot 5 bodies in just over 7  minutes.
> 
> How many shots of each star do you usually take? I've heard 
> conflicting
> philosophies on this - one says to take ~3 of each star, the  other
> says
> just to take one of all of them (assuming they're all in view)  and
> any
> errors will be obvious when the resulting LOP doesn't land 
> reasonably
> close to where the others cross.
> 
> I shoot one each and throw out the ones that don't work.  In my line 
> of 
> work, I am reasonably confident in my accuracy of shooting to give me
> a  precise 
> fix most of the times.  In fact, I "feel" when I have a bad  shot.  I
> can't 
> explain it, but I know instinctively.  I also "feel"  when I've
> nailed one.  In 
> any case, I don't have time to waste, especially  in the Tropics
> where the 
> horizon fades fast in the PM, to shoot 2-3 shots of  each star.  I
> shoot one and 
> move on, then put a fix down.  With the  computer, the whole process
> takes 
> about 15 minutes.  Considerably longer  when using tables.  Although
> with star 
> fixes, I tend to use 249 if I am  using Tables.  229 is just too slow
> with 
> multiple LOP's, but I have done  many of them in school.
>  
> I will be interested to see where your notes take us in this one.
>  
> Jeremy
> 
> 
> 
> 
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> 


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