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Let me suggest, that if Doug had the time, he could start taking sights, say
5 minutes before Meridian Passage, record his first reading, and if he wants
a series of readings, until Meridian Passage, record that, and then preset
his sextant to the first altitude he observed, and take the time when the
sun reached that altitude on the downside after passage. Averaging those two
times should allow him to solve for long.

That's more time than he wanted to allocate to this, but it shouldn't it
overcome the objections relative to getting long from a noon sight.

Joel Jacobs


----- Original Message -----
From: "Noyce, Bill" 
To: 
Sent: Thursday, January 22, 2004 4:07 PM
Subject: Re: Timed Noon sights for position


> Doug writes:
> -------------------
> One gets the estimate of latitude at LAN from the reduced noon sight.One
> now has an idea of ones' latitude.Are you going to trust a position line
> 40 hrs old to get an idea of the longitude?Yes,one can advance the
> line.But,in reality,how accurate will the results be from this r-fix?I
> was taught and practiced that the optimal time to advance or retard any
> line was 3-6 hrs. So,one is in the position in this scenario not having
> the "warm,fuzzy" feeling of confidence reguarding the ships'position. At
> the least one can use the r-fix and the noon GHA of the DR or EP to get
> a better idea of where  the position is or if some gross error exists in
> one of the parts.
> -------------------
>
> I'm still confused.  Suppose the procedure you suggest does give
> a reliable latitude (I have some questions about that, but don't
> want to discuss them until I'm sure I understand the procedure.)
> It looks to me as if you must then choose among some of the following
> alternatives for a longitude:
>
> 1. The longitude of the EP or DR position.
> 2. The longitude at which the (estimated) course line crosses the
>    latitude determined by LAN sight.
> 3. A longitude at which the (estimated) distance-made-good circle
>    crosses the latitude determined by LAN sight.
>
> #1 is equivalent to advancing your old fix to a new EP or DR and
> then finding the nearest position that matches the latitude
> determined by the LAN sight, since longitude lines are perpendicular
> to latitude lines.
>
> None of these methods gains any information about longitude from
> the sight.  If there is some information that your procedure
> provides, I've missed it.
>
> In your example, what would have been the resulting fix if you
> were actually 30 miles further east at the time of your sight?
> Here's how I figure it, using Hc from the USNO web site:
> EP 32*21'36"N,118*43'45"W (True position 31*19'55"N,118*13'47"W)
> GMT 12:04:26 15 Jan 04 (I assume you make the sight at the same
> time, based on your EP.)
> Ho (from USNO web) 36*32.1  Latitude computes as 32*20.0"
> As far as I can see, the longitude computations are exactly the
> same as in your original post -- the fact that the true position
> is off by 30 miles makes no changes to the longitude derived from
> this observation.  If this is true, in what way is the sight
> providing longitude information?
>
>         -- Bill

   

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