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  1. #41
    Join Date
    Jun 2007
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    Default Re: Cunliffe's star gazing book

    Quote Originally Posted by jdc View Post
    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.

  2. #42
    Join Date
    Jul 2008
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    East coast UK. Mostly. Sometimes the Philippines
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    10,457

    Default Re: Cunliffe's star gazing book

    +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.

  3. #43
    Join Date
    Mar 2007
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    Atlantic
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    21,851

    Default Re: Cunliffe's star gazing book

    Quote Originally Posted by AndrewB View Post
    +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!

  4. #44
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    Jun 2013
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    9,826

    Default Re: Cunliffe's star gazing book

    Quote Originally Posted by capnsensible View Post
    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?

  5. #45
    Join Date
    Jul 2006
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    Emsworth Hants
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    12,695

    Default Re: Cunliffe's star gazing book

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

  6. #46
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    Default Re: Cunliffe's star gazing book

    Quote Originally Posted by GHA View Post
    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.......

  7. #47
    Join Date
    Jun 2001
    Location
    Corfu - mostly
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    5,415

    Default Re: Cunliffe's star gazing book

    Quote Originally Posted by capnsensible View Post
    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.
    Last edited by AndrewB; 26-11-19 at 16:18.

  8. #48
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    Jun 2013
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    Default Re: Cunliffe's star gazing book

    Quote Originally Posted by capnsensible View Post
    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...-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

  9. #49
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    Jun 2007
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    Default Re: Cunliffe's star gazing book

    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

  10. #50
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    Atlantic
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    Default Re: Cunliffe's star gazing book

    Quote Originally Posted by GHA View Post
    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...-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........

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