Cunliffe's star gazing book

[Skylark]

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.

..........So I repeat my assertion that noon-sights should be taught as an interesting historical quirk not part of the usual routine or syllabus.

I don't think there's any reference within this thread that not doing an MP is somehow poor practice

Sun MP and a Position Line are different; nothing special, special case or historical quirk with either

Sun MP simply uses sextant angle (as ZD) and declination.

A Position Line uses sextant angle and time to determine altitude and azimuth from a chosen position.


Regarding Long at MP, here's a quote from Bowditch "Two methods are available to determine LAN with a precision sufficient for determining longitude". He then goes on to describe the two methods in detail. :encouragement:

 

[Kukri]

+1.

Rule one for noon latitude sight - only turn the knob one way - clockwise viewed from you, i.e. increasing the observed altitude. You need to school yourself to do this and not to “follow the sun down”.

I like “The Sextant Simplified”, which doesn’t teach you anything about navigation but it teaches you about using and caring for a sextant. There are Ancient and Modern editions, both good.

 

[capnsensible]

+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.

Well the thing is I've used it quite a few times and it does actually work. As I mentioned already in this thread, the last time Idid this was last week and one of my crew on first go got our position to around eight miles from the gps numbers.

To claim it doesn't work kinda counters my practical experience that it does!

Plus of course we had the equation of time from a Google search!

 

[GHA]

Well the thing is I've used it quite a few times and it does actually work. As I mentioned already in this thread, the last time Idid this was last week and one of my crew on first go got our position to around eight miles from the gps numbers.

To claim it doesn't work kinda counters my practical experience that it does!

Plus of course we had the equation of time from a Google search!

So how did you get an accurate time when the sun was bang on local solar noon?

Wouldn't just a few seconds out give a bigger error in longitude than 8 miles?

 

[KellysEye]

I forgot to mention I used Asrocalc http://maintain.astrocalc.com/ The site is waiting for new software.

 

[capnsensible]

So how did you get an accurate time when the sun was bang on local solar noon?

Wouldn't just a few seconds out give a bigger error in longitude than 8 miles?

You will have taken a few sights as the sun is nearing its zenith. After you see the sun starting to dip, re set your sextant to one of the earlier readings and note the exact time it reaches it again. Half the difference will give you a good enough time.

I've used it a good few times. Of course it takes practice and it's not always reliable on a bumpy sea. But worth it. Takes a bit of discipline to keep at it when you only get the one go per day. However if you combine it with a traditional sun, run meridian run sun it can be accurate enough for most people.

Then of course you can start your dr run and consider where you are gonna be for your evening stars! And if you use Tim Bartletts excellent explanation of the planet noon chart in the Almanac, get a nice fat planet too! Even with a plastic sextant.......:encouragement:

 

[AndrewB]

Well the thing is I've used it quite a few times and it does actually work. As I mentioned already in this thread, the last time Idid this was last week and one of my crew on first go got our position to around eight miles from the gps numbers.

To claim it doesn't work kinda counters my practical experience that it does!

Plus of course we had the equation of time from a Google search!

I think the Davis leaflet is actually intended to provide advice for an emergency situation. You have taken to your liferaft with a sextant, watch, pencil, and a page torn from your atlas, but without any tables other than those in this leaflet which is tucked into the sextant case. In such a situation, knowing your position to within 20 or 30 miles is a lot better than having no idea at all!

If you can get accuracy to within 8 miles, then I am impressed, no, amazed. I certainly couldn't (though I used to be confident of getting a normal sun-sight position to within 3 miles). Like Kukri warns, I wouldn't be sure that I had a true noon-sight until the sun was just beginning to visibly subside - probably a good 2 minutes after the actual time of noon implying 30 miles out.

P.S. In the 2 minutes after noon the sun subsides about 0.1', or about 15% of the sun's visible diameter. It takes a well-practised sextant user to spot anything less than that.

 

[GHA]

You will have taken a few sights as the sun is nearing its zenith. After you see the sun starting to dip, re set your sextant to one of the earlier readings and note the exact time it reaches it again. Half the difference will give you a good enough time.

How good?

How did your guess at local noon tally up with the actual?

Presumably you have a current nautical almanac onboard, plenty online as well - https://www.nauticalalmanac.it/en/pd-eng-nau-alm/file/61-nau-alm-2020-pdf.html

Seems to be a little reluctance from a few experienced on here that longitude from a MP will be accurate enough to be any use. Need to see your notes for a few solar noons

 

[Skylark]

This is the method described within Bowditch.

Up to 30 minutes before estimated MP, take a series of Hs and time, increase frequency as MP approaches. Continue taking sights for a similar time after MP.

Plot the results and draw a fair curve.

Draw a series of horizontal lines, constant Hs, at least 3. Note the times where the line intersects the curve, one ascending the other descending.

Average the 2 times. Repeat with the other horizontal lines and average the, at least 3, times. This will be a reasonable accurate time of MP.

What is the purpose of taking a fix? If sailing mid ocean it's to ensure that the vessel isn't standing into danger and to shape the course. In these circumstances, I believe that Long from MP by this method is sufficient to meet the objectives of navigation :encouragement:

 

[capnsensible]

How good?

How did your guess at local noon tally up with the actual?

Presumably you have a current nautical almanac onboard, plenty online as well - https://www.nauticalalmanac.it/en/pd-eng-nau-alm/file/61-nau-alm-2020-pdf.html

Seems to be a little reluctance from a few experienced on here that longitude from a MP will be accurate enough to be any use. Need to see your notes for a few solar noons

All I have left to suggest is that you try it a couple of dozen times........

 

[GHA]

All I have left to suggest is that you try it a couple of dozen times........

No mention of how close to local noon you were getting?

 

[capnsensible]

No mention of how close to local noon you were getting?

Well I have mentioned the eight mile thing a few times....... That's close enough for me. Remember it's the other way round, you measure in time your local noon and then use conversion of arc to time in reverse. As the sun moves it's longtitude in relative terms by one degree every four minutes, you can do it in your head.

 

[GHA]

Well I have mentioned the eight mile thing a few times....... That's close enough for me. Remember it's the other way round, you measure in time your local noon and then use conversion of arc to time in reverse. As the sun moves it's longtitude in relative terms by one degree every four minutes, you can do it in your head.

was the 8Nm in lat or long - or both?

A one of data point means nothing though, maybe he just got lucky.

Just tried it, not a great success but not really a surprise, no horizon so just picked a bit of solid stuff nearby so probably masses of parallax. And something up with the last sights as well. And on and off cloudy. Only 120Nm out

You'd do great on countdown if you can do all that base 60 maths in your head.

Did you forget to add/subtract the eqn of time in your head calcs?

Still not convinced it's worth the bother other than a test to see how close you can get. Which is reason enough to have a play anyway

 

[capnsensible]

Both.

cmon adding 13 minute to noon isn't countdown stuff nor is knowing the sun moves one degree every four minutes....

The sums for correcting sextant for index error, dip and altitude, simple.
True altitude off 89 60. Easy. Deciding to add or subtract declination...you know roughly where yo are.... Simple.

Hey presto, latitude. Conversion of arc to time? Simple. Longtitude. Good enough for an ocean and good enough for the standard plotting sheet. Don't let anyone fool you that Astro is hard.

Someone on here said a while back that your first 1000 sights won't be as good as the second1000. A league I will never be in!

 

[capnsensible]

Plus as has also been said on here, what, are you too busy to get it mid ocean? :encouragement:

 

[scotty123]

Correction for peeps who want to actually learn astro. Its a crap book.....waaaay too complicated. Puts makey learnies off. In my experience.

+1

However, there are people who want to 'learn' it & those who just want to 'do' it, Cunliffe is perfect for the latter.

 

[Kukri]

Mary Pera wrote the first book demystifying astro for yotties using the then new Air Tables, and started a genre of publishing called "Astro for Yotties - do it My Way"

Before her there was Claud Worth in "Yacht Navigation and Voyaging" which favours the explain everything approach
Most subsequent books have followed one or the other.

 

[lw395]

Learning to pass YM-Ocean seemed to me to be largely about memorising funny names for angles.
Then learning a handle turning process to generate a position line.
Breathe in Look something up
Breathe out add something to it

IT's possible to pass the course wihout having the faintest idea what any of it means.
The Cunliffe book is, as I recall good for learning to pass the course and will allow you to turn the handle and produce an answer.
I think there's very little luggage in, everything in there is on the syllabus.

Books are much cheaper than courses, so I went for reading/buying a few different books to get different insights.

Unfortunately, names of little angles and corrections is exactly the kind of thing that I forget if I haven't used it for 6 months, and I did the course ten years ago.
I'm more of a 'understand the big principle and work out the details for yourself' kind of learner.

 

[scotty123]

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).

Doing an astro plan, before attempting a star sight helps, using the period of twilight, when few stars out + horizon.
After that its bang, masses of them & only gifted sextantors can manage.

 

[scotty123]

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.

Worthwhile looking at the US Bowditch, also available online, they use an averaging method iirc, to obtain 'noon'.