NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Navigation exercise
From: Jeremy C
Date: 2008 May 21, 05:05 EDT
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.
George's points above are all right on the money, Jeremy.
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?? <g>) or is there
another reason?
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From: Jeremy C
Date: 2008 May 21, 05:05 EDT
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"?
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?? <g>) 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.
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).
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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