NavList:

   

Reply

Hi folks great stuff here. A mid morning sun line advanced to to the LAN Lat 
and Long, the latter having been determined by equal AM and PM altitude pairs 
and averaging, can yield three LOPS forming a triangle yielding a running 
fix. I am sure that was the procedure back in the days.
Mike Burkes








George H wrote:
"1. It wasn't even quite due to get to Local Apparent Noon then, if only 2
seconds short of it.
2. Around noon, the altitude changes so slowly that it's a significant time
before any descent can be noticed, even by the most skilled navigator.
3. At that date in May, the Sun's declination is increasing Northwards by
about half a minute each hour. So at the moment of LAN, the Sun's altitude
will still be increasing at that rate, and it will reach its maximum
somewhat later than LAN, before any fall can even commence."


Why do you care to "call noon" at all? This is one of those navigational
traditions on commercial vessels (specifically) that people love very much
but its navigational significance is hard to fathom.



I am not sure of this question.  Do you mean, "why do you not just use a watch 
and calculate time of LAN and shoot it at that point instead of waiting for 
the sun to descend?"  or is it "Why do you bother to shoot LAN at all?"  To 
the former, I would say that it was the way I was taught.  The reason for 
this lesson is that if you are on a ship without reliable electronic 
navigation, and only have a 6 hour DR track line run from a starfix (if it 
wasn't cloudy), with only a sun line to help out, you are far from sure of 
your position at all.  Add to this that your chronometer may or may not be 
accurate, and you will have a hard time predicting when LAN is even to the 
minute.  You merely guess as to where you are based on your DR, calculate 
LAN, than watch for the sun to hang, and then call noon for better or worse. 
I was taught in school by the old timers that you really know that the sun 
isn't rising when you can see it falling.  While this will add error, it got 
us across the oceans in one piece and pass the USCG exams.  When I go home, I 
will have to pull out one of the USCG problems that ask you to solve for LAN, 
when they don't even give you a time of the sight.



My question to you Frank, and anyone else who cares to answer, is what methods 
do you use to observe LAN?  I am certainly open to better techniques.



Of course in this day in age, it would be better for me to note my GPS 
longitude, punch it into my computer, get time of LAN, and then shoot it at 
the exact time to get a more accurate position, even underway.



If the question here is why we would observe LAN at all.  The short answer is 
that we don't.  Ships will not typically use celestial navigation at all to 
fix position.  It is used solely to determine gyrocompass error at sea.  The 
reason it is still practiced, is that at least in the US, the US Coast Guard 
requires all mates to be tested on this material and even have a practical 
assessment done (ie go out and shoot a LAN) in order to obtain certain ocean 
licenses.  I enjoy Celnav, so I shoot them on occasion.  I am not fond of LAN 
because of the reasons I mentioned earlier.  It is difficult to judge the 
exact time of LAN by altitude, on a rolling, steaming, ship, but I typically 
get within 1 nm of my GPS latitude if I have a good horizon to work with.  
Are there errors due to the fact that I call noon at some point other then 
actual transit?  Of course there are, but this does not impede me from 
getting a LOP of acceptable accuracy during an ocean transit.

    Historically LAN was shot because it was fairly independent of an exact 
time piece.  LAN was usually advanced or retarded and crossed with an AM 
sunline advanced to noon.  This gave a "noon position" which was then entered 
into a mercator sailing for the "day's run."  The mileage was then compared 
to the "engine miles" and slip was calculated for the engineers.  This is all 
part of the "noon" slip and is still done today, albeit with GPS noon fixes 
and computer sailings.  There were of course errors in the traditional noon 
position due to various inaccuracies, but it certainly was better then a DR.  
In fact, the noon fix was always suspect, where Longitude, derived from a 
sunline crossed with LAN, usually had a bit of error.


George's points above are all right on the money, Jeremy.



The quoted 3 points were 100% valid.   George was incorrect in his conclusion 
where he stated that the error was in my Longitude; and to that, I countered 
that it was not Longitude, but my personal observation error.  I thought the 
sun was going down but was not, it was in fact still rising.  I further said 
that my trouble with LAN is that you cannot see it occur by altitude alone, 
which is what you point out in your problem below.  I have never liked 
shooting LAN, but in time, I have been able to get within 2' of my latitude 
by calling noon basically whenever I shoot with a decent horizon.



 So let me pose a
problem for YOU that might help you see what George is getting at:
It is around local apparent noon on May 20, 2008. I am in longitude 71d 58.0
West exactly and latitude 40d 00.0 North approximately (off the southern
coast of New England). I am sailing due south at ten knots. What is the
exact GMT when the Sun is on the local meridian (azimuth=180)? What is the
exact GMT when the Sun reaches its maximum altitude? These are not the same.
In addition, how much time elapses after that maximum altitude before the
Sun's altitude changes by one-quarter of a minute of arc? When would you
"call noon"?



Let's see if my math is correct:

  The sun is on the meridian at 16h 44m 25s

The max HC (70d 09.4') occurs between 16h 43m 30s and 16h 45m 27s at the given 
latitude.  During this time declination changed less the 0.1 minutes of arc.  
However, since we are steaming south, the sun will continue to rise due to 
the change in latitude and will peak around 16h 46m at 70d 09.6.  The sun 
will have descended 0.2 minutes of arc by 16h 47m 00s.  I cannot say when I 
would "call noon" because I am not actively shooting this sight.  I would 
call it when my eye perceived the body to be descending.  The factors 
affecting this could be position, roll, pitch, sextant not perpendicular and 
parallel, imperfect horizon, etc.



Let us assume we are on Lat 40N at the exact time of LAN, but I called noon 
when the sun had descended about 0.2' of arc.  This would be 16h 47m 00s, 2m 
35s late.  My latitude would be 39-59.6'N due to the vessel's movement.  If I 
was fairly certain of my Longitude, and wanted a more accurate Lat, I would 
calculate an ex-meridian sight which would give a correction of C=at^2.  "A" 
according to Bowditch is 4.1 giving us a correction of 0.45 minutes to Ho of 
70d 09.4'.  If my Ho was perfect then applying the 0.45 to the Ho would 
yield, after calculation, a latitude of 39-59.55'N.  If I didn't do the 
ex-meridian and just reduced a simple LAN, I would have a half mile error.  
Even if I had a 1.3 mile error due to my sight (like in my example) we are 
still talking less than 2 nm, which is not an uncommon error in celnav at 
sea.



In my example, i did perform a 2 second ex-meridian calculation and even with 
the very high "A" correction factor, had an error of a few feet, and 
therefore I disregarded it.



.


And you wrote:
"Peter is correct in saying that you can shoot a number of sights over
say 15-20 minutes and smooth a curve to determine not only latitude,
but also longitude by LAN.  This method is in fact outlined in The
American Practical Navigator 1995 edition.  It is horribly impractical
at sea, and still questionable as far as Longitude goes, but it will
certainly give you a slightly better Latitude number."

Why do you say 'horribly impractical'? Do you mean because it would have you
busy for half an hour or more when you could be doing other work (which
would indeed be an issue on a commercial vessel, but then again, what are
you doing playing with a sextant in the first place?? ) or is there
another reason?



Spending a half hour of time shooting a large number of sights to obtain a 
latitude LOP so that the error is less then a pencil line on a plotting sheet 
while the captain is breathing down my neck for the noon slip is certainly 
"horribly impractical" to me.  On a sailboat it might be different.  In this 
case accuracy is traded off for speed, and the margin of error is within 
acceptable limits for merchant sailors.



 As for longitude, the biggest problem with using a series of
sights around noon is that they are not really symmetrical unless the
observer is motionless and the date is near one of the solstices.



We advance a AM sunline and get a noon R.Fix which gives us a suitably 
accurate Longitude for our purposes.



But these
issues can be corrected without a whole lot of trouble and you will then
have a longitude, too. The longitude would not be as accurate as the
latitude but not too bad either. It depends on the details (as Bill noted in
another post).



Exactly, and since even a "slow" ship is traveling in some direction (usually 
changing both latitude and longitude) at 13 knots, and in my case 18 knots ( 
0.3 nm of latitude per minute on N/S tracks), the math involved to correct 
each of 20 or so sights over 20-30 minutes would require quite a program to 
correct the changes of latitude and Ho due to the varying position of the 
ship at each sight.  Add to this that the ship is never a steady shooting 
platform; and often navigating at times far from the solstices, this method 
is not practical for determining longitude at sea from a merchant ship, which 
is all I am concerned about.  The time  spent taking that many sights added 
to data entry and plotting would make a star fix look like child's play.  I 
will leave such techniques to others.



As an aside Frank, I see that you are in Groton, CT.  I live in Cutchogue, NY; 
about half the way up the North Fork of LI.  It's too bad the conference in 
Mystic isn't a month later, or I'd be attending.  I do hope you do such 
meetings in the future as I'd like to talk shop.



Jeremy





________________________________

Wondering what's for Dinner Tonight? Get new twists on family favorites at AOL Food.



_________________________________________________________________
Keep your kids safer online with Windows Live Family Safety.
http://www.windowslive.com/family_safety/overview.html?ocid=TXT_TAGLM_WL_Refresh_family_safety_052008

--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.navlist.net
To post, email NavList@navlist.net
To , email NavList-@navlist.net
-~----------~----~----~----~------~----~------~--~---

   

Reply