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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Hawaii By Sextant: Problem #1

Author Topic: Hawaii By Sextant: Problem #1

 - posted April 02, 2020 02:02 PM      Profile for Ian           Edit/Delete Post 
I have been earnestly working through the contents of Hawaii By Sextant (using this 'quarantine' as a simulated cruise in more ways that one [Smile] and didn't get far before I found a discrepancy between what I'd learned from the Starpath Celestial Navigation book, and the solutions in HBS.

In the very first reduction, the DR Latitude is 46 30N. The Assumed Latitude could really be 46 or 47N, but the two yield a very different LOP, which, when combined with the LL Sun sight from later that day, yield very different fixes. What was the reasoning for using 46N?

As an aspiring tactical sailboat navigator, how can I ensure I make the right choice in this one particular case? Ironically, I find the process of determining the assumed Longitude to be very straightforward, and the exercises in Chapter 11 of the Celestial Navigation book helped significantly.

Thanks in advance! Ian.

From: San Diego, CA

 - posted April 03, 2020 09:44 AM      Profile for Ian           Edit/Delete Post 
I created a form like the USN 3530/1 to compare the reduction for using 46N and 47N. I will plot and take a photo of that, too

From: San Diego, CA
David Burch

 - posted April 03, 2020 09:51 AM      Profile for David Burch           Edit/Delete Post 
Looks very good. I think that is similar to the Davis forms, but a notable improvement!

You are of course welcome to use our Starpath forms for this. The forms are online as pdfs to print. The instructions are in a commercial book we include with our course:


From: Starpath, Seattle, WA
David Burch

 - posted April 03, 2020 09:57 AM      Profile for David Burch           Edit/Delete Post 
Ian, i apologize. I missed the original question here. You should get the same fix using either lat for the a-Lat. There can be some minor difference if the a values are longer than ~60' or indeed any significant line segment is longer than about 60 nmi.

How much different (nmi) is the fix using different a-Lat?

From: Starpath, Seattle, WA

 - posted April 03, 2020 04:09 PM      Profile for Ian           Edit/Delete Post 
Well...it turns out, it does NOT make a difference. I guess I just had to map it out. My previous effort did not result in such an accurate fix (here is was 45 53N and 128 06W)

From: San Diego, CA

 - posted April 09, 2020 04:06 PM      Profile for Ian           Edit/Delete Post 
Here's the plot with the azimuth for both the 47N and 46N a LATs plotted. As you can see, they are the same line! This is also a practical example of how the LOP (for a remote enough GP) does approximate a straight line.

From: San Diego, CA
David Burch

 - posted April 09, 2020 06:34 PM      Profile for David Burch           Edit/Delete Post 
Is this a problem from HBS? and if so, which problem and which sights? Thanks
From: Starpath, Seattle, WA

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