A question for the traditional ocean navigators?

[alant]

Controversial? Not really. At best its just a running fix.
Observing the sun gets you a single position line except for a few very exceptional circumstances. The opportunity to cross with another body like the Moon. Or Venus does occur but tends to give a very similar position lines which don’t cross well.

Anachronism? To big a word for me. Were you ever a Chief Officer .You are correct there is no need to run to noon and do a Meridian Passage any two observations will give you a running fix, The closer the azimuth difference is to 90 the better.

The Noon Latitude has a few advantages the main one being its easy. KISS. There’s not much calculation involved and its convenient. The obvious disadvantage it doesn’t work when you can’t see the sun at noon.

Outdated, a matter of opinion.

What is your opinion, of using a noon sight GHA, as longitude?

 

[Cruiser2B]

Much more likely imho to give a substantial error (and what has led to my biggest errors in EP) has been when sailing fast, when one can easily cover 170 miles in 24 hours, so if you get no sight at all one day then the next opportunity for a sight could be more than 200 miles away, with associated greater circle of uncertainty. This is particularly so if using a trailing log as it skips a bit when going really fast.

You will be going Noon to Noon maybe 24, 48, 72 or more hours without an updated observation

There is a technique called a days work, I would use to calculate a DR over a long period. Its just a simple table of courses and speeds resolved into d lat D long and Departure then added up at the end.
Exactly like adding vectors.

Bowditch describes 'fix expansion' under Dead Reckoning (chap 7) - http://msi.nga.mil/NGAPortal/MSI.portal?_nfpb=true&_pageLabel=msi_portal_page_62&pubCode=0002

I've used a similar, though more complex technique normally used by submariners, called 'pool of errors' - but even at the furthest reaches of uncertainty, in the worst possible scenario, you are never so far away from the DR to affect an astro-fix.

 

[Simes]

I see little issue with this, GHA as a noon sight Longitude. The only real issue is finding the exact time of LAN, Equation of time is a little known table these days. Once you have the time of LAN you might as well use Arc to Time to establish Longitude.

At this point anyone who has read or seen Longitude the book or the movie will understand how John Harrison was using time to establish Longitude.
No, I see nothing wrong with this at all.

Simes

 

[AndrewB]

30 mins seems fairly accurate? What if you've sat out gale hove-to for a couple of days? Would you be able to maintain a DR to that accuracy under those conditions?

The worst out I ever was with an EP, back in the days when we depended on sextant navigation, was 80 miles. This was after being driven off-course by a three-day gale in Biscay. Back then we were pretty anal about keeping DR up to date if a sight wasn't possible. But on that occasion I failed to allow for the big embaying current during Biscay gales, as well as a skipper who insisted on keeping the yacht going even though the gale was dead against us and no-one could steer a decent course.

If you simply hove-to for a day or two, once you knew your boat estimating the drift wasn't too hard.

 

[Skylark]

I have found that the standard deviation (ie 1 sigma) of the position is 1.5 miles, with an apparently normal distribution.

I have found this both by observation, from which I have enough data points to say only that it's about 1.5 miles, and by convolving uniform (aka 'Top Hat') distributions from: sextant observation errors, sight reduction errors (atmospheric corrections mostly), ephemeris errors (principally due to truncation to 1' resolution), plotting errors (accuracy of drawing and plotting angles) and from the Walker trailing log (the accuracy of which I estimate to be about 1.5% of length run) which again gave me sigma = 1.5 miles, with a more or less normal 2-D distribution.

I've thought a lot about this and I'm intrigued with the use of standard deviation and more so with a 1 sigma value of 1.5m. Assuming that 6 sigma pretty much coves the whole population (give or take a few in a million), that suggests a LoP or a fix never more than 10 miles out. Is that the correct interpretation?

I've never before seen the application of statistics to sight reduction. Given the many variables of this measurement system, would a Gauge R&R be a better measure?

Is it purely academic or is there reason to apply statistics?

 

[TQA]

How accurate was my sextant nav.

Not very. I only had a plastic Ebbco and the little calculator which averaged your sights and then crunched the numbers.

Generally 5 to 10 miles but that allowed me to find my way around and make safe landfalls.

I was glad I got my first GPS before the Bahamas. Anybody else familiar with the island that has a red roofed house in front of the conspicuous stand of Casuarinas.

 

[AndrewB]

[deleted]

 

[Uricanejack]

What is your opinion, of using a noon sight GHA, as longitude?

Sorry Alant I got rather long winded and never really expressed an opinion. At the end I have described some methods as.
Good enough, Better and Best. A Chief Officer would tell me his stars were the best.

It was not a method I ever used. At least not directly. I never compared a Sun Run Noon fix to a simple Noon Latitude and observed time converted to longitude. I just accepted the logic as taught that it was less accurate with out questioning it. So I can’t really say just how much difference it would make.
I certainly did the opposite, converting longitude to time to determine when Apparent Noon would occur.

I could say. If there was ever a significant difference between when I though apparent noon was and the time I found apparent noon to be. It would clue me in to suspect there was something wrong with my position, particularly my longitude. If lucky, just an arithmetical error I could fix. If not then I had screwed something else up.

When I first started we still used what was basically a Harrison’s Chronometer. By the time I sailed as a 3rd Mate they were rapidly being replaced by quartz chronometers.
I wound one a few times, Under the direct supervision of the 2nd Mate. By tradition it was his responsibility. It was quite a production. The 2nd Mate would come to the bridge at about the same time every morning and the Chronometer would be wound the same number of turn’s at the same time every day.

We usually did the error at the same time, or just before the winding, The Marconi Sahib would tune into the world service pipe it through on a speaker to the Bridge and we would do the time signal. The error would be noted in The Chronometer rate book along with the temperature.
The rate was the change in the Error over 24 hrs. By Knowing the rate we could come up with a very accurate time any time during the day and the next day if we missed a time signal.
The idea behind the daily winding was you always used the same bit of spring even though there was a fusee chain which was supposed to keep it constant.

In Port, it might get forgotten occasionally, Forgetting to wind the chronometer was a major transgression. The 2nd would be on the Captains shit list Just for doing it late. I don’t remember anyone I sailed with letting it stop. Any 2nd who did would be extremely unpopular. It must have happened often enough. It used to be a standard Orals Question How to restart a chronometer which had stopped. Unfortunately my memory has failed the question.
My recollection is a little vague I think they would run for about 3 or 4 days without being wound before stopping. 84 hrs rings a bell.

We still did the same procedure with a quartz chronometer. Without the winding.

In Theory with this precision time, You should be able to determine your position accurately at noon by measuring the maximum altitude of the sun and the precise time it reaches this altitude.

At apparent noon the suns LHA is 0 by comparing this to the suns GHA the Difference is your longitude.
Simple and a huge leap forward in Navigation.

Why bother with all the morning sights and running fixes they appear to be unnecessary?. It’s all about accuracy,

The suns Altitude changes fairly slowly near its apex. The Suns Longitude is changing very quickly. A minute ever 4 seconds.

Being able to tell precisely when the Max altitude occurred to the exact second was just not practical,
Taking the noon sight. It was just not obvious enough.
I would determine the time of apparent noon for my DR Longitude which was run up from on my intercept from my morning observation.
I would pre-calculate the expected altitude for meridian passage and set this on my sextant, Then just before the appointed time, Go and watch as the sun rose,
There would usually be the two of us, we would watch the sun rising until it stopped rising, stayed steady for a bit and when we agreed had it started to dip bellow the horizon. We would read of our sextant altitudes.
Timing the precise moment of the highest altitude just wasn’t accurate enough. To say we had timed it to within a second. Or even quite a few seconds.

I read the book Longitude about Harrison, I found it very interesting and it was new to me. I also enjoyed the TV Show.
The book was more about Harrison the man and his clock and later watch, which became the Chronometer.
The book did not go deeper into the history and development of Navigation. Leaving the story with. “ knowing the time meant you knew your Longitude“. To explain more detailed techniques just wasn’t necessary for the story.

The idea behind the Sun and Run to Noon is you can determine an accurate Longitude in the morning before noon. If you take your observation, when the sun is close to due east of you. Or an azimuth of near 090 . At this time as the sun is rising quickly, you can get a precise time for your observation. It was known as Longitude by Chronometer. The idea was used the accurate time to enable you to determine where the sun was and from this information calculate the longitude your position line crossed your DR latitude.
Although I did use method, to learn it and to practice it. I preferred and used Marc St Hillarie.

How long after Harrison’s clock was this method developed? I don’t know but it was quite soon. The first edition of the Britannica Encyclopaedia explains how it was done and I believe it was the method used by Cook. Many other famous British sailors.

Is a simple meridian passage and by using the time good enough? Yes.
If all you require is a check on your electronics,
Would it be good enough to find Antigua, Bermuda or the Azores? Yes. It will get you within sight of land.

A simple method of getting a better longitude observation close to noon improving the observed longitude is to take or set a sight of the sun shortly before apparent noon along with the time,
After taking your observation at apparent noon, set the same angle on your sextant and take observations until the sun is back on the horizon and take the time.
By comparing the two times you can use the time half way between as a time for noon longitude.
I reality this has two significant problems.
Your boat moved between observations so noon is not exactly halfway between the two observations
Also the sun moved between observations in particular its declination changed.
Its described in the instructions for a Davis sextant, I’m sure there are lots of sailor who got around just fine using them.

Best? The Full Sun Run, Two or more observations.

 

[alant]

Sorry Alant I got rather long winded and never really expressed an opinion. At the end I have described some methods as.
Good enough, Better and Best. A Chief Officer would tell me his stars were the best.

It was not a method I ever used. At least not directly. I never compared a Sun Run Noon fix to a simple Noon Latitude and observed time converted to longitude. I just accepted the logic as taught that it was less accurate with out questioning it. So I can’t really say just how much difference it would make.
I certainly did the opposite, converting longitude to time to determine when Apparent Noon would occur.

I could say. If there was ever a significant difference between when I though apparent noon was and the time I found apparent noon to be. It would clue me in to suspect there was something wrong with my position, particularly my longitude. If lucky, just an arithmetical error I could fix. If not then I had screwed something else up.

When I first started we still used what was basically a Harrison’s Chronometer. By the time I sailed as a 3rd Mate they were rapidly being replaced by quartz chronometers.
I wound one a few times, Under the direct supervision of the 2nd Mate. By tradition it was his responsibility. It was quite a production. The 2nd Mate would come to the bridge at about the same time every morning and the Chronometer would be wound the same number of turn’s at the same time every day.

We usually did the error at the same time, or just before the winding, The Marconi Sahib would tune into the world service pipe it through on a speaker to the Bridge and we would do the time signal. The error would be noted in The Chronometer rate book along with the temperature.
The rate was the change in the Error over 24 hrs. By Knowing the rate we could come up with a very accurate time any time during the day and the next day if we missed a time signal.
The idea behind the daily winding was you always used the same bit of spring even though there was a fusee chain which was supposed to keep it constant.

In Port, it might get forgotten occasionally, Forgetting to wind the chronometer was a major transgression. The 2nd would be on the Captains shit list Just for doing it late. I don’t remember anyone I sailed with letting it stop. Any 2nd who did would be extremely unpopular. It must have happened often enough. It used to be a standard Orals Question How to restart a chronometer which had stopped. Unfortunately my memory has failed the question.
My recollection is a little vague I think they would run for about 3 or 4 days without being wound before stopping. 84 hrs rings a bell.

We still did the same procedure with a quartz chronometer. Without the winding.

In Theory with this precision time, You should be able to determine your position accurately at noon by measuring the maximum altitude of the sun and the precise time it reaches this altitude.

At apparent noon the suns LHA is 0 by comparing this to the suns GHA the Difference is your longitude.
Simple and a huge leap forward in Navigation.

Why bother with all the morning sights and running fixes they appear to be unnecessary?. It’s all about accuracy,

The suns Altitude changes fairly slowly near its apex. The Suns Longitude is changing very quickly. A minute ever 4 seconds.

Being able to tell precisely when the Max altitude occurred to the exact second was just not practical,
Taking the noon sight. It was just not obvious enough.
I would determine the time of apparent noon for my DR Longitude which was run up from on my intercept from my morning observation.
I would pre-calculate the expected altitude for meridian passage and set this on my sextant, Then just before the appointed time, Go and watch as the sun rose,
There would usually be the two of us, we would watch the sun rising until it stopped rising, stayed steady for a bit and when we agreed had it started to dip bellow the horizon. We would read of our sextant altitudes.
Timing the precise moment of the highest altitude just wasn’t accurate enough. To say we had timed it to within a second. Or even quite a few seconds.

I read the book Longitude about Harrison, I found it very interesting and it was new to me. I also enjoyed the TV Show.
The book was more about Harrison the man and his clock and later watch, which became the Chronometer.
The book did not go deeper into the history and development of Navigation. Leaving the story with. “ knowing the time meant you knew your Longitude“. To explain more detailed techniques just wasn’t necessary for the story.

The idea behind the Sun and Run to Noon is you can determine an accurate Longitude in the morning before noon. If you take your observation, when the sun is close to due east of you. Or an azimuth of near 090 . At this time as the sun is rising quickly, you can get a precise time for your observation. It was known as Longitude by Chronometer. The idea was used the accurate time to enable you to determine where the sun was and from this information calculate the longitude your position line crossed your DR latitude.
Although I did use method, to learn it and to practice it. I preferred and used Marc St Hillarie.

How long after Harrison’s clock was this method developed? I don’t know but it was quite soon. The first edition of the Britannica Encyclopaedia explains how it was done and I believe it was the method used by Cook. Many other famous British sailors.

Is a simple meridian passage and by using the time good enough? Yes.
If all you require is a check on your electronics,
Would it be good enough to find Antigua, Bermuda or the Azores? Yes. It will get you within sight of land.

A simple method of getting a better longitude observation close to noon improving the observed longitude is to take or set a sight of the sun shortly before apparent noon along with the time,
After taking your observation at apparent noon, set the same angle on your sextant and take observations until the sun is back on the horizon and take the time.
By comparing the two times you can use the time half way between as a time for noon longitude.
I reality this has two significant problems.
Your boat moved between observations so noon is not exactly halfway between the two observations
Also the sun moved between observations in particular its declination changed.
Its described in the instructions for a Davis sextant, I’m sure there are lots of sailor who got around just fine using them.

Best? The Full Sun Run, Two or more observations.

Thanks for that, great explanation from the real world of experience.

I believe the US Navy, use 3 set sextant observations before & after the apparent noon, using those to obtain the noon time.

 

[Babylon]

Uricanejack, thanks for an evocative recounting of how things used to happen!

I'm still studying the essentials of celestial-nav but have got enough grip on the theory to follow this thread.

With regard to Alant's reference to the US Navy, and presumably the RN and other navies, is celestial-nav still taught and practiced at all, or does everyone just rely on the Teasmade?

 

[jdc]

I've thought a lot about this and I'm intrigued with the use of standard deviation and more so with a 1 sigma value of 1.5m. Assuming that 6 sigma pretty much coves the whole population (give or take a few in a million), that suggests a LoP or a fix never more than 10 miles out. Is that the correct interpretation?

I've never before seen the application of statistics to sight reduction. Given the many variables of this measurement system, would a Gauge R&R be a better measure?

Is it purely academic or is there reason to apply statistics?

It's purely academic I fear! I was responding to a specific question about how accurate is it, and since I had some stats to hand, answered in a statistical way. However it's not a bad discipline to think in terms of probabilities rather than certainties.

As for are all sights therefore within 10 miles, (the 2D cumulative distribution formula is P = 1 - exp(r²/2/sigma²)), I think that a sight is very unlikely to be this far out without one knowing in advance that something was different from the reference conditions under which one had established that 1 sigma= 1.5 miles.

This may sound a bit trite (it's accurate unless not accurate...), but actually the point is that the accumulation of small errors, each of which certainly don't have a normal distribution, all of similar magnitude, do in aggregate give rise to a normal distribution whereas a much worse sight will have a single dominant cause of uncertainty, with distribution determined almost entirely by that single cause.

Examples of the single cause would be an unclear horizon (or Fata Morgana which I've found can give 15 miles error), or very rough seas and no opportunity to average altitude readings. One would be aware of these, and so take them into account.

 

[Cruiser2B]

With regard to Alant's reference to the US Navy, and presumably the RN and other navies, is celestial-nav still taught and practiced at all, or does everyone just rely on the Teasmade?

I can only speak for the RCN, which follows the RN model - when I was an instructor, I was assigned to revamp the Astro-nav syllabus; when I went through the supporting documentation, I found that Astro had been officially removed as a training requirement for watchkeepers some 10 years before and was only to be taught on the Navigating Officer's course. The time allotted for the training had also been trimmed to make room for priority training (ethics, anti-harassment, etc). In the end, the syllabus I produced concentrated on the use of the sextant (as it could be used terrestrially), with a quick overview of celestial navigation. This was 15 years ago. I imagine there are some keen young officers who teach themselves out of interest, and there are probably some traditionalist skippers who insist on the jr O's doing morning stars, but the majority have never swung a sextant. I wouldn't be surprised if it was the same in the RN and USN.

 

[alant]

I can only speak for the RCN, which follows the RN model - when I was an instructor, I was assigned to revamp the Astro-nav syllabus; when I went through the supporting documentation, I found that Astro had been officially removed as a training requirement for watchkeepers some 10 years before and was only to be taught on the Navigating Officer's course. The time allotted for the training had also been trimmed to make room for priority training (ethics, anti-harassment, etc). In the end, the syllabus I produced concentrated on the use of the sextant (as it could be used terrestrially), with a quick overview of celestial navigation. This was 15 years ago. I imagine there are some keen young officers who teach themselves out of interest, and there are probably some traditionalist skippers who insist on the jr O's doing morning stars, but the majority have never swung a sextant. I wouldn't be surprised if it was the same in the RN and USN.

IIRC, the RN cadets at Dartmouth, do an RYA Day Skipper course.
Without the leap frogging directly to Yacht master, as proposed by many on this forum!