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Thanks Joel,

I see your point vis a vis the relative consistency of the bubble horizon,
however I should point out that it is not as consistent as you might
believe; in fact it is very fickle at the best of times. Even on land, when
taking bubble observations, the average error is likely to be in the realm
of 2 minutes of arc or more. Anything less is fortuitous and icing on the
cake. Now consider taking these observations in a moving aircraft. It is
painful. A bubble at sea is, in my experience, of dubious value so I won't
discuss it.

I agree that the horizon at sea is variable (how could I disagree?) and that
Greenaway did not take this into account because, after all, he was talking
about air navigation.  Nevertheless, I still see the value of the two times
two star shots if you can pull them off. But now you've got me to thinking
(I am not wholly obstinate and set in my ways) when you discussed -- for
perfectly valid and sensible reasons -- taking three shots instead of two.
In this, I also agree with you. In fact I like to try for four shots of each
(I get carried away when I get a good opportunity for a star shot and a
perfect sea horizon is for me, the same as a red flag for a bull, except I
grab my sextant instead of charging at it).

Nevertheless, I more or less subscribe to the idea of taking full advantage
of the limited time afforded by that magic period of time where star and
horizon are visible simultaneously, by concentrating on just two stars
rather than spreading myself out too thin.

Having said all of this, I have to acknowledge two things:

1) The bulk of my experience in astro-nav, pertains to land-based
operations, however, in my own defence, I have used this technique at sea
and successfully.

2) I acknowledge that Joel and many others on this list have a level of
experience and knowledge that vastly outclasses my own, so I am not arguing
against you but simply expressing what works for me.

Oh yes, Joel, I am getting more proficient at that star finder. It is a very
neat piece of gear once one gets the hang of it.

Robert



----- Original Message -----
From: "Joel Jacobs" 
To: 
Sent: Sunday, April 04, 2004 9:37 AM
Subject: Re: The cocked hat


> This is in response to Robert Enro.
>
> I have a few problems with Robert's recommending RCAF Navigator's Keith
> Greenway's two sight, two average fix method for use at sea.
>
> IMO, it is subject to the following deficiencies. Greenway is taking
sights
> from an aircraft using a bubble sextant so that the instrument provides
the
> same (bubble) horizon no matter in which direction he is shooting. You
don't
> have that at sea when using a traditional sextant. As everyone knows, the
> horizon at sea's definition varies with the direction you are looking due
to
> differences in light, sea conditions and cloud cover. Therefore, you have
> variables with which Greenway is not confronted since his horizon is a
> constant.
>
> The other issue is taking only two sights. I think that's a bad idea
because
> an error in one can only be halved when combined with the other. In most
> things, where you are trying to have a definitive outcome it is always
wiser
> to use an odd number so that you don't have an even split where one group
> can cancel the other out. I don't understand how Greenway concludes that
an
> error in a two sight fix is obvious. IMO, that it is impossible to judge
> which of the two parallel sights is in error. So what do you do? He
suggests
> that you go back and shoot it over again. That is not always possible, an
if
> it is, because of the significant time lapse, you have  a running fix, not
a
> fix. All something to think about.
>
> Robert, with that nifty and very unusual star finder we discussed off
group,
> I suggest you predetermine the alt and az of the bodies beforehand. Set
you
> sextant's to same (adjust for IC, REF, HE) accordingly, then adjust for
> variation and deviation, and note the adjusted compass bearing to the body
> for each object, and just aim along that line. You should have the target
in
> view. Some minor adjustment's and you have your sight,
>
> I preferred taking a minimum of three near simultaneous sights of the same
> body, and then moved on to the next. The planning took into account
> diminishing or increasing light conditions, and where I had to position
> myself for steadiness etc., as I moved from one target to the next. I
based
> this on where I would have the best horizon. If possible, I always used at
> least three bodies. Two sight fixes would more often be shooting an early
or
> late rising planet or moon and crossing it with the sun. Averaging was
done
> with a calculator that had the ability to manage time using Sexagsimals,
> base 60 numbering system.
>
> Let me add, this was more a matter of self-entertainment, because as I've
> said before, when you're out in the middle of the ocean, with no hazards
at
> hand, all that precision is unnecessary. But it was fun.
>
> Joel Jacobs
>

   

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