Cunliffe's star gazing book

Sandy said:
Totally agree, I have a first degree in engineering so lots of applyed maths and was totally confused by her book. It might suit somebody who likes pure maths?
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Yeah, there are obviously some fans. But I can only really repeat the feedback from the dozens of people who we have taught over the years who simply don't get on with it. Actually, not a single one!
 
Sandy said:
Totally agree, I have a first degree in engineering so lots of applyed maths and was totally confused by her book. It might suit somebody who likes pure maths?
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I also have a degree in engineering. There is no pure maths in the content. There isn't really much applied maths. In the Foreword, the former director of the RIN describes the book as "admirably concise". Perhaps this is the issue; there isn't much by way of explanation, rather, she dives in with worked examples.

There is nothing really difficult about taking and reducing a sextant sun sight to a Position Line. The procedure is simply to compare the angle measured by the sextant with the same angle calculated from the exact same time as recording the sight. Most of the sight reduction process is "look-up", either in a Nautical Almanac or a Table of Trigonometry (AP3270/aka Air Tables). There is very little maths but many people stumble with what was as a child, mental arithmetic. Adding and subtracting degrees, minutes and decimals; sign convention for West and East etc.

Due to the concise nature of Mary Blewit, it is a very good book as an aide memoire or for revision. I've said in a post above that it's not the best text book but it doesn't warrant this negativity.

Celestial navigation is not the easiest self-study course. I haven't taught dozens over the years but from those I have it's rewarding to witness the Eureka moment when the concept is grasped. Most people seem to need a helping hand to get over the first hurdle of understanding but from that point on can progress well.

Many people use Templates to help the sight reduction process. I don't use them myself but, of course, I teach their use. It's quite insightful to see if students use the template or can reduce the sight/time without one :)
 
davidjackson said:
I also have a degree in engineering. There is no pure maths in the content. There isn't really much applied maths. In the Foreword, the former director of the RIN describes the book as "admirably concise". Perhaps this is the issue; there isn't much by way of explanation, rather, she dives in with worked examples.

There is nothing really difficult about taking and reducing a sextant sun sight to a Position Line. The procedure is simply to compare the angle measured by the sextant with the same angle calculated from the exact same time as recording the sight. Most of the sight reduction process is "look-up", either in a Nautical Almanac or a Table of Trigonometry (AP3270/aka Air Tables). There is very little maths but many people stumble with what was as a child, mental arithmetic. Adding and subtracting degrees, minutes and decimals; sign convention for West and East etc.

Due to the concise nature of Mary Blewit, it is a very good book as an aide memoire or for revision. I've said in a post above that it's not the best text book but it doesn't warrant this negativity.

Celestial navigation is not the easiest self-study course. I haven't taught dozens over the years but from those I have it's rewarding to witness the Eureka moment when the concept is grasped. Most people seem to need a helping hand to get over the first hurdle of understanding but from that point on can progress well.

Many people use Templates to help the sight reduction process. I don't use them myself but, of course, I teach their use. It's quite insightful to see if students use the template or can reduce the sight/time without one :)
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Thanks for an interesting post David.

It sounds like it is a bit of a "Marmite" book. I was lucky enough to do Navigation at school and was introduced to Mary's book at the age of 15. I picked it up again at the age of 45 and quickly put it back down. It's a bit like some maths teachers, some are on your wavelength others appear to be from a parallel universe.

In the next few years I am planning to submit a passage for the RYA Yachtmater Ocean ticket and know a) I need to produce some celestial nav workings and b) am actually interested in the subject. I've had several discussions with Ocean qualified navigators to discuss what works for them and several have suggested other books and YouTube channels.

As I finish working full time in 2020 I'll have the time to play with all sort of celestial nav and will get back to you with what works for me.
 
Sandy said:
...It might suit somebody who likes pure maths?
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Nope. It obscures the underlying simplicity and makes it all too complex. In fact I believe they all do to some extent as they put over emphasis on the calculations; I suspect because none of the authors actually understand them.

I think the best is to use a computer program on some trivial thing like a cheap smart phone or ipad and not worry at all about the calculations nor even looking up ephemeris until actually taking a sight and plotting it becomes absolutely second nature. Then, if one can be bothered, one can start to look up the GHA and Declination - checking what one has looked up agrees with the computer, which gives confidence. Then progress to using pre-computed sight reductions, still checking the Computed Altitude and Azimuth against what the computer says.

That way the computer program becomes the most important teaching aid.
 
jdc said:
Nope. It obscures the underlying simplicity and makes it all too complex. In fact I believe they all do to some extent as they put over emphasis on the calculations; I suspect because none of the authors actually understand them.

I think the best is to use a computer program on some trivial thing like a cheap smart phone or ipad and not worry at all about the calculations nor even looking up ephemeris until actually taking a sight and plotting it becomes absolutely second nature. Then, if one can be bothered, one can start to look up the GHA and Declination - checking what one has looked up agrees with the computer, which gives confidence. Then progress to using pre-computed sight reductions, still checking the Computed Altitude and Azimuth against what the computer says.

That way the computer program becomes the most important teaching aid.
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Having spent my working life working with some very big computers I am planning not to get too involved with them when I stop full time work. Being a simple engineer I usually took to diagrams on bits of paper to describe what I needed the coders to do on the computers... To quote one of my maths lectures, "Does it look right", has always been a good starting point, using a computer wipes out looking at the maths so I would have no idea if it looked right or not! Please note that if I don't get it right in the day job undertakers are going to have a very profitable time!
 
Sandy,

With your “pencil and paper diagram” mentality I assure you that you will have no trouble grasping the concept of sight reduction. With your background there’s absolutely no reason to delegate the task to electrickery.

One to one is the ideal student-pupil ratio for this course. Do you know any approachable, helpful, friendly, flexible, knowledgeable, experienced, informative, affordable, like-minded retired engineers, qualified to teach it :)
 
Sandy said:
..Being a simple engineer I usually took to diagrams on bits of paper to describe what I needed the coders to do on the computers...
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Well, it's only a calculator, so not exactly a main-frame I meant! And the program I use I wrote myself.

I don't believe however that the trigonometry is tractable, and certainly is not to the accuracy required, without numeric methods; graphs and lines simply won't cut it. Using haversines etc are obsolete methods now and should no longer be taught as part of astro-nav imho. My essential point is that it's absurd to conflate repetitive, mindless, calculation and the human skill of taking an accurate sight and using it to determine one's position. Yet this is what most books do. Expanding on tis point, I have always felt that the concept of a 'noon sight' to be obsolete; it's emphasised by those who are afraid of doing the sight reduction due to the bore of the arithmetic.
 
Noon sight?

It's perfectly brilliant. I showed someone how to do it in half an hour last week. On first go his position was 8 miles out. In relative terms, excellent. It's not just latitude, you get arc to time longitude as well.

Maths, who needs complicated. It's a simple bit of adding up and taking away. Calculators.... Duh.

Get yourself a Davies sextant. Read the accompanying very small handbook and hey presto!

Don't believe anyone who poses that it's difficult. Ask people who have taken sights on ocean passages how simple it is.....:encouragement:
 
jdc said:
I have always felt that the concept of a 'noon sight' to be obsolete; it's emphasised by those who are afraid of doing the sight reduction due to the bore of the arithmetic.
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That's an interesting assertion. :) Why would you not take the opportunity for a relatively simple to derive measure of latitude?

As to trigonometry, the easiest way to derive DR from last fix is to use Plane Sailing methodology. Use Quadrant Notation for bearing plus a simple Sine and Cosine calculation. I still have my 4-figure Log Tables from the 1970s but I'll admit to using a pocket calculator :) This method is also idea for the "run" between forenoon and afternoon sights as it's more accurate than using the Plotting Sheet. (The Traverse Table in Nories does the same calculations but as a look-up).

Most people learn sight reduction using the Intercept Method, Marcq St Hilaire so Haversines not needed.
 
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I did an overnight channel crossing with chum who is a keen astronomer. OK I have a small boat, but we both had problems actually taking the sight. For me it was always a problem to identify the correct object in the sky using the sextant and that was with a keen astronomer at my side to point it out to me. It's hardly maths. Mind you youngsters these days have little experience of using tables. I did Logs in school and made extensive use of pre-prepared tables in my engineering degree. (Scientific Calculators were just coming in and most of us used Log Log Sliderules; I can't get a replacement cursor for mine).
 
davidjackson said:
That's an interesting assertion. :) Why would you not take the opportunity for a relatively simple to derive measure of latitude?

As to trigonometry, the easiest way to derive DR from last fix is to use Plane Sailing methodology. Use Quadrant Notation for bearing plus a simple Sine and Cosine calculation. I still have my 4-figure Log Tables from the 1970s but I'll admit to using a pocket calculator :) This method is also idea for the "run" between forenoon and afternoon sights as it's more accurate than using the Plotting Sheet. (The Traverse Table in Nories does the same calculations but as a look-up).

Most people learn sight reduction using the Intercept Method, Marcq St Hilaire so Haversines not needed.
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Yes I know all that, but the problem with noon sights is, imho and in the context of this thread, that they are treated as something special rather than a special case. As if not doing a sight exactly at noon is somehow poor practice.

In my experience (several oceans crossings using nothing but the Ebco special and home calculated and printed tables) it's rather often not possible to get a decent noon sight. Usually cloudy (when not actually raining) and one simply doesn't have the 10 mins or so of clear weather and benign conditions at exactly the right moment to take 5 or so altitudes around local noon. So I've given up: of the 3 - 5 sights one might do in 24hrs, occasionally saving a bit of calculation on one of them seems a nugatory benefit. So I repeat my assertion that noon-sights should be taught as an interesting historical quirk not part of the usual routine or syllabus.

PS: I am a mathematician, so no wonder my views are aslant!
 
Sounds like bit of an unlucky few trips!

Shame because it's a great step in understanding the basics...... none of the other tables, no worries about GHA or LHA. a great building block for the next bits.

Simple arithmetic, not maths. Plus as I mentioned, you get not just latitude but longtitude as well. Have a look at the Davies sextant PDFs I posted. Easy.

Mind you I'm well aware of those clouds at the awkward moment, know what you mean there. But it's far from quirky and the cornerstone of the navigators day.
 
capnsensible said:
Plus as I mentioned, you get not just latitude but longtitude as well..
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How do you actually do that? Given that the sun's movement above the horizon will appear to stop for minute or so at local noon, even with a little graph it seems unlikely to be accurate - what sort of accuracy do you get?

Heard of people taking a sight an hour or so before noon then check the exact time an hour after when the sun is at the same height and work out local noon from that. Hardly long from a noon sight anymore though.
 
capnsensible said:
Avalook at page 11 of the link I posted, hope that helps. :encouragement:
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Ta. So 'Heard of people taking a sight an hour or so before noon then check the exact time an hour after when the sun is at the same height and work out local noon from that.'.

Not really a noon sight anymore though.

What sort of long accuracy do you get? Would need double figures sights compared to the gps to know probably. Doubtful it will be that accurate though.. but guessing there..
 
capnsensible said:
Avalook at page 11 of the link I posted, hope that helps. :encouragement:
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That pamphlet in your link is extremely good, 5 minutes reading the reverent sections and stuff I did in the 80's has just been refreshed. Thanks for posting.
 
GHA said:
How do you actually do that? Given that the sun's movement above the horizon will appear to stop for minute or so at local noon, even with a little graph it seems unlikely to be accurate - what sort of accuracy do you get?
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+1. Sorry CaptainSensible, no matter what your Davis leaflet says, this is NOT a practical method of determining longitude at sea to any sort of accuracy. The implication of being up to 2 minutes out is 30 miles of inaccuracy. And you've got to take into account the 'equation of time'; that the time of local noon runs fast or slow at different times of the year by up to 15 minutes from 'mean time' (to be fair, the leaflet explains this).

GHA's idea of taking the average of the two times when the sun's height is the same, going up and going down, only works if the boat isn't moving, or at least doesn't change longitude between the two sights. It still has to be adjusted for the 'equation of time'.

P.S. This reminds me of the suggestion made here several years ago, that by measuring the angle between the sun and moon one could determine longitude without needing to know the time. True in theory, and indeed this was the method used by land-based surveyors for 18thC charts prior to chronometers. But on a moving yacht with an ordinary sextant - no. The one time I tried, I found my position somewhere in central Kazakhstan.
 
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