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Hi George the advantage of making the altitudes exactly equal is the basic 
premise of the noon sight i.e. you follow the sun up 10 or more minutes 
before estimated LAN let's say about 5 AM sites, note the altitudes and 
times, observe maximum altitude via the hang of the sun for Latitude, wait 
for altitude decrease, set sextant to the same AM altitudes, wait for limb 
contacts and note times. Of course in ideal conditions you will have 5 pairs 
of equal AM-PM altitudes. LAN will be the total of the AM-PM times of each 
pair divided by 2. Since Long. is to be found timing has to be precise. 
Hopefully you will see the 5 LANs in close agreement of times maybe seconds 
difference and you look for those that are in close agreement, maybe only 3 
out of 5 and average them, convert LAN GMT to arc and hence Long. Some of the 
classic texts omit this procedure and the associated AM-PM curve. HTH.
Mike Burkes   
"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."
>
> Well, yes, but you can get three such LOPs, providing a fix which can give
> latitude and longitude, whether the AM and PM altitudes are equal or not. So
> what is the advantage of making the altitudes exactly equal?
>
> George.
>
> contact George Huxtable at george@huxtable.u-net.com
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>
>
>
>
>
>
>
>
>
> 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
>
>
>
>
>
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