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  1. #521
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    Sep 2008
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    Default Re: Golden Globe Race

    Quote Originally Posted by oschonrock View Post
    Fair enough.

    If you can average 60nm error with something like that, that would be good going. In sea state, more error. But just to play for fun, it's fine.

    Guess I am a romantic in this respect. I find my sextant to be a "beautiful instrument", to be loved and cared for. It's accurate to around 0.2nm. If I can hold it still enough! In reality anything better than 3nm is very good on a small boat with moderate seastate.
    For Heath Robinson Navigation, I find this guy's trips fascinating:

    http://www.furledsails.com/article.php3?article=774

    There's a guy called Tristan Gooley who does similar on land:

    https://www.naturalnavigator.com/books-and-library/

  2. #522
    Join Date
    Mar 2007
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    Atlantic
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    18,849

    Default Re: Golden Globe Race

    Quote Originally Posted by Mark-1 View Post
    I'm not that fussed about accurate Astro Nav on the boat itself so not sure I need a Sextant. A few years back I got myself a book on Astro and made myself a "start angle measuring device" with a protractor and a plumb line and took fixes just for interest. Can't say I felt the need to get more accurate than that. (Best 12 miles out, worst 200 miles out.) If I ever feel the need to try it again, I'll do the same.



    57 is a lot more than I know! Plus, I can't always recognise planets if I haven't seen them for a while and end up checking Skymap.
    This is all easier than people think, you don't have to know them.

    Planets. Venus, Mars, Jupiter and Saturn are tabulated in the Nautical Almanac with a really neat diagram of their meridian passages to tell you which ones you can use.

    Stars. In air Nav tables 'Selected Stars' and at your EP you can discover the seven most appropriate at morning and evening twilights. You don't need to learn them and as or mentioned, on passage you get to use the same ones quite a bit......cloud permitting!

    It's just another aspect of sailing that needs a bit of appliance and lots of practice.

    Those on the race are sure getting a shed load of that.

  3. #523
    Join Date
    Dec 2018
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    15

    Default Re: Golden Globe Race

    Quote Originally Posted by Mark-1 View Post
    I hadn't appreciate that artificial horizons didn't work on small boats (In fact I thought I'd read accounts of people using them on small boats), which does seem a pretty serious limitation that I'd not been aware of.
    Depends on what you are looking to do.

    If you want to "navigate by the stars", then yes, you only have about 20mins twice per day to do it.

    If you want to determine your position by celestial navigation => Use the sun. It's even often perfectly visible through the clouds with the right sextant shades on.

    A "running fix of the sun" as THE most common type of astronav fix. 10am local => approx noon local, and optionally 2pm local.

    Bob's your uncle.

  4. #524
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    Sep 2008
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    Default Re: Golden Globe Race

    Quote Originally Posted by capnsensible View Post
    This is all easier than people think
    I don't think anyone's claiming that it's difficult (as long as you're not working it out from first principles and aren't bothered about too much accuracy). I think people were just saying that there are serious limitations on how effective it is. That's where I came in because there was a limitation I was unaware of - I naively thought that you had all night every night where if you saw an identifiable star or two you could some kind of a position (maybe just a Latitude) with some kind of artificial horizon.
    Last edited by Mark-1; 13-12-18 at 11:39.

  5. #525
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    Dec 2018
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    15

    Default Re: Golden Globe Race

    Quote Originally Posted by Mark-1 View Post
    I don't think anyone's claiming that it's difficult, with a decent book you can learn it in an evening, then it's just practice to refine the accuracy. I think people were just saying that there are serious limitations on how effective it is. That's were I came in because there was a limitation I was unaware of - I naively thought that you had all night every night where if you saw an identifiable star or two you could some kind of a position (maybe just a Latitude) with some kind of artificial horizon.
    Sorry Mark-1, I have to disagree with most of that:

    1. Yes, you can learn how to take a sight on the sofa and reduce it from a book. But to operate a sextant in reality takes quite a bit of practice. Most people I have seen try are 100s of miles out when they first try. The whole process of "bouncing up and down, steadying yourself and finding the horizon (tops of distant waves!) and then critically (!) rotating the sextant around the line of sight to ensure you are holding it vertically, while lowering the object right to the horizon -- repeat several times; once per wave".... That is actually hard! (But a fun skill to learn, IMHO)

    2. Serious limitations on how effective it is? No. It's very effective. The skill and the art is to know what sight to take when. Mostly that's a sun sight.

    It's very easy to identify what the sun is! It's always there during the day, when the horizon is crisp and you can read your timepiece easily at the critical moment. You can wait for it to peek out between the clouds.

    Note: I hope this is obvious, you always need at least 2 sights at different bearings to get a fix, ie 2 sun sights several hours apart or sun+moon, or several stars/planets.

    You just need to appreciate that, for the most part, it's not about stars. Starts are the romantic fun bit, which you do when the opportunity presents itself. You don't need them.

    You can navigate the whole planet and never do anything but sun sights. That's what most people do.
    Last edited by oschonrock; 13-12-18 at 12:13.

  6. #526
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    Sep 2008
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    2,418

    Default Re: Golden Globe Race

    Quote Originally Posted by oschonrock View Post
    1. Yes, you can learn how to take a sight on the sofa and reduce it from a book. But to operate a sextant in practice takes quite a bit of practice. Most people I have seen try are 100s of miles out when they first try.
    Yeah, from what I've read that takes thousands of fixes to get good, and even then people are still miles out so in that sense Captain sensible is wrong: It's very difficult, but I don't think that's what he meant, he meant the mechanics of doing it are not difficult, which is true.

    Quote Originally Posted by oschonrock View Post
    2. Serious limitations on how effective it is? No. It's very effective.
    To discuss that we'd need to quantify "Serious limitations" and "effective". It doesn't work in cloud, or at night. You could regard that as "serious limitations". Equally you could say that people managed with nothing but Sun and Stars for centuries so it has no serious limitations. I shouldn't have used a vague term.

  7. #527
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    Mar 2007
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    Atlantic
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    Default Re: Golden Globe Race

    Quote Originally Posted by Mark-1 View Post
    Yeah, from what I've read that takes thousands of fixes to get good, and even then people are still miles out so in that sense Captain sensible is wrong: It's very difficult, but I don't think that's what he meant, he meant the mechanics of doing it are not difficult, which is true.



    To discuss that we'd need to quantify "Serious limitations" and "effective". It doesn't work in cloud, or at night. You could regard that as "serious limitations". Equally you could say that people managed with nothing but Sun and Stars for centuries so it has no serious limitations. I shouldn't have used a vague term.
    It doesn't take thousands to be good enough. That's not what I've read, it's what I and others can do! But anyway, the only true way to be convinced is to do it. And really, it's not difficult.

    Thing is, like everything else, you have to want to.

  8. #528
    Join Date
    Dec 2018
    Posts
    15

    Default Re: Golden Globe Race

    Quote Originally Posted by Mark-1 View Post
    Yeah, from what I've read that takes thousands of fixes to get good, and even then people are still miles out so in that sense Captain sensible is wrong: It's very difficult, but I don't think that's what he meant, he meant the mechanics of doing it are not difficult, which is true.

    To discuss that we'd need to quantify "Serious limitations" and "effective". It doesn't work in cloud, or at night. You could regard that as "serious limitations". Equally you could say that people managed with nothing but Sun and Stars for centuries so it has no serious limitations. I shouldn't have used a vague term.
    1000s of sights? I think that's an exaggeration. Personally I got consistently within 5nm after 10 attempts (each attempt is always multiple sights, you never take just one). I then had to practice more in a range of conditions and points of sail. The "problem" is that you can't learn it on the couch, nor can you learn it on a short coastal sail, with land everywhere, really. You need space.

    On accuracy: If you work at it, and the conditions are not stupid, you can get within 3nm consistently, often better if you are skilled and using a decent sextant. I did most of my early astonav just after GPS became available, so we would often cross check it. I was often within 1nm and rarely more than 3nm out.

    The key thing to understand is: How accurate do you need to be when you are using astronav? Actually in 99% of cases, you really don't care if you are 10nm out. Or even 20nm. It just doesn't matter. Just because today with modern GPS, we can know our position in the middle of the southern ocean down the nearest meter, does not actually make that useful. Why is it not useful? Because it does not change the decisions we would make. We are going East or eastish anyway, so who cares? --- Ironically the southern no go zones in the GGR are sort of an exception, see my earlier post!

    What about landfall? Well yes, good question. You make landfalls when you can see what you are steering at! Not based on sextant observations. You use astro to get you close, until you can see something, then you use terrestrial features. Sometimes this means heaving to 30nm out to wait for daylight...so be it.

    In fact there are some (many?) people who scare me when they use GPS. Because they will blindly follow the damn device when really close to land and will not use other normal terrestrial techniques. Bad things happen! Mostly not because the GPS is inaccurate, but because the operator has made some mistake, or failed to zoom in on their vector chart plotter...etc...etc... The old techniques require much more discipline. That discipline is being slowly lost and that causes serious accidents - even for very experienced sailors (Volvo Ocean Race anyone?).

    So does astronav have "serious limitations" in terms of:

    - less accurate than GPS => yes, although not significantly
    - less positions per day than GPS => YES!!!! Astro is one per day, cloud permitting, GPS is one per second.
    - more effort than GPS => YES!!!

    Does astronav have "serious limitations" in terms or circumnavigating the planet?

    - NO! None at all - you just need to learn how, put a bit more effort in and use disciplines which you should be using anyway.

    Fair?
    Last edited by oschonrock; 13-12-18 at 13:56.

  9. #529
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    Sep 2008
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    Default Re: Golden Globe Race

    I got an answer to one of my one and a half questions and Google filled in the other, so I'm happy. I'm going to pass on the debate about the definitions of "difficult", "accurate" and "limitations" in different contexts. I'd find such a debate difficult, that's one of my limitations. I'm being accurate about that.

    Thanks all, fair winds.
    Last edited by Mark-1; 14-12-18 at 21:11.

  10. #530
    Join Date
    Dec 2018
    Posts
    15

    Default Re: Golden Globe Race

    Quote Originally Posted by Mark-1 View Post
    I got and answer to one of one and a half questions and Google filled in the other. I'm going to pass on the debate about the definitions of "difficult", "accurate" and "limitations" in different contexts. I'd find such a debate difficult, that's one of my limitations. I'm being accurate about that.

    Thanks all, fair winds.
    Fair winds indeed.

    Sorry, I never noticed we were having a debate.

    capnsensible & I were just clarifying facts based on our experience. Mainly for the benefit of the other readers in the forum.

    ;-)

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