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  1. #81
    Join Date
    Nov 2009
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    Somewhere in the Atacama
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    410

    Default Re: Cunliffe's star gazing book

    Why are either of you messing around with the equation of time when right next to it ( in the almanac ) you can lift the time of merpass ( at Greenwich) straight off the page? Saves one line of work... eliminates one chance of error......

    Time to arc in the head? In the example 02:54:53.... 3 hours = 45º then just subtract the small bits.... 5 minutes = 1º 15' ..... 7 seconds = 1.75' ...add those two.... 1º 16.75' and subtract it from 45º .... peasy....

  2. #82
    Join Date
    Nov 2009
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    Somewhere in the Atacama
    Posts
    410

    Default Re: Cunliffe's star gazing book

    And also... just missed noon because of a bit of cloud? Just use the Ex-Meridian tables in your Nories......

  3. #83
    Join Date
    Mar 2007
    Location
    Atlantic
    Posts
    21,782

    Default Re: Cunliffe's star gazing book

    Quote Originally Posted by Frank Holden View Post
    Why are either of you messing around with the equation of time when right next to it ( in the almanac ) you can lift the time of merpass ( at Greenwich) straight off the page? Saves one line of work... eliminates one chance of error......

    Time to arc in the head? In the example 02:54:53.... 3 hours = 45º then just subtract the small bits.... 5 minutes = 1º 15' ..... 7 seconds = 1.75' ...add those two.... 1º 16.75' and subtract it from 45º .... peasy....
    What, like I said in post #75?

    But many thanks for clarifying the head sums. It's kinda like I do it breaking it down.

  4. #84
    Join Date
    Jun 2013
    Location
    Hopefully somewhere warm
    Posts
    9,817

    Default Re: Cunliffe's star gazing book

    Doh! Almost missed it. And the fag packet python code needs some work....
    But sort of in the right ballpark.
    Wild guessing on a good day you'd be lucky to get to within a minute or so time for apparent noon, maybe 15/20Nm? Though python has some sophisticated smoothing modules for plotting, maybe it could do better than mere humans on a boat with a pencil.

    amanhã será melhor.

  5. #85
    Join Date
    Dec 2007
    Location
    Boat: Falmouth. Work: Cambridge
    Posts
    1,402

    Default Re: Cunliffe's star gazing book

    I feel rather vindicated in my diatribe against over emphasis of noon sights by certain authors.

    Teaching method 1:
    Take a sight in mid morning when the sun is bears a bit S of E (say 120)
    take another around local mid day when the sun bears about S, and
    one in the afternoon when the sun bears a bit S of W (say 240)
    The time when you do these isn't critical to an hour or so, and if you miss one it really doesn't matter.
    Use the same arithmetic / maths in all 3.

    Teaching method 2:
    Take a sight in mid morning when the sun is bears a bit S of E (say 120)
    Towards local mid day estimate your actual longitude,
    then using the equation of time - digression to explain it, and the analemma - or using a special table, only available in certain almanacs,
    calculate when you should start to make loads of observations - at least 5 and probably 10.
    If you don't know your longitude then make even more observations over a longer time.
    Plot these observed altitudes against time, using paper and pencil, or python or matlab etc, as you please.
    And take another sight in the afternoon when the sun bears a bit S of W (say 240).
    The times when you do 1 and 3 aren't critical but the second has to be spot on. Tant-pis if it's cloudy, you're in the middle of reefing, catching fish, cooking, etc.
    Use one arithmetic / maths method for 1 and 3, and quite a different method for number2 thus keeping dementia at bay through much exercise of mental arithmetic.

    Conceptually simpler to teach method 2? Really?

    (Apologies to those people lucky enough to be sailing in the Southern hemisphere)

  6. #86
    Join Date
    Jun 2007
    Location
    Home: North West, Boat: The Clyde
    Posts
    3,764

    Default Re: Cunliffe's star gazing book

    Quote Originally Posted by GHA View Post
    ........ Though python has some sophisticated smoothing modules for plotting, maybe it could do better than mere humans on a boat with a pencil.

    amanhã será melhor.
    As a mere human I believe that an ascending then descending sun curve should be smooth

    Quote Originally Posted by jdc View Post
    I feel rather vindicated in my diatribe against over emphasis of noon sights by certain authors.
    I’m pleased that you feel vindicated but I still don’t see that anyone has suggested “over emphasis of noon sights” However. I don’t understand why a mathematician would turn his back on the very straightforward opportunity to measure latitude

  7. #87
    Join Date
    Jun 2013
    Location
    Hopefully somewhere warm
    Posts
    9,817

    Default Re: Cunliffe's star gazing book

    Quote Originally Posted by davidjackson View Post
    As a mere human I believe that an ascending then descending sun curve should be smooth
    If the mere human can take accurate sights to within a few seconds of angle it would be

    And a vote to start with a noon sight when learning, not much maths involved, much easier to visualize what's going on and a very good chance you'll be motivated for more having got a latitude fix close to where you are on day one

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