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» Online Classroom   »   » Public Discussion of Cel Nav   » Hawaii by Sextant - Appendix 3

Author Topic: Hawaii by Sextant - Appendix 3
Jerold Frakes

 - posted April 22, 2019 02:02 PM      Profile for Jerold Frakes           Edit/Delete Post 
I’m not sure if I’m missing something about how to identify the triad of best choices in the radar charts of twilight skies. I can more or less identify them from the list at the bottom of the page, based on altitude and azimuth, but I suspect that there is a better way than my simply guessing.
From: Buffalo NY
David Burch

 - posted April 22, 2019 04:06 PM      Profile for David Burch           Edit/Delete Post 
There are two standard ways to predict the best sights. for stars alone, we could use Pub 249 vol 1, but this resource, unlike vol 2 and 3, is dated. one issue every 5 years good for 8 years. We no longer have Pub 249 v 1 for epoch 1980, so that solution is not useable here.

The other solution is manually make a list of the stars either by sight reduction, computation, or using the star finder. Then once you have a list of all the candidates, we manually choose the top triads of bodies, stars and planets, and maybe the moon if it happens to be there.

The choice is dominated most by the relative angles of the bodies. We want the three that are the closest to 120º apart, being above about 15º and below about 75º. Then we want brighter rather than faint, and then if we have the choice we would choose bodies of about the same height in the sky... reasons for these choices are covered in our textbook Celestial Navigation.

in HBS Appendix 3 we have listed the body heights and bearings and show them plotted out as a "radar" image with us in the center below bodies that are 90º high, with bodies on the horizon being on the circumference of the radar screen.

These data are provided so readers can confirm the choices we made underway... best or not best, as well as get practice in making such choices with real skies on a real voyage.

The alternative method, which i do not list in the "standard" methods, is to use our StarPilot calculator or phone or PC program which solves for the best triads of bodies automatically as well as telling you how well they each meet the specific quality criteria mentioned above... and indeed you can adjust these criteria to meet your own preferences. This is a luxury solution, and we definitely recommend knowing how to do this manually before calling upon such an aid.

From: Starpath, Seattle, WA
Jerold Frakes

 - posted April 22, 2019 04:55 PM      Profile for Jerold Frakes           Edit/Delete Post 
Thanks, David. I understood all that, but I was just not sure how to identify the three designated bodies on the radar plot. On this particular one it seems perhaps Mirfak, Moon, Vega (clockwise from 2 PM), but I find no actual identification of the three bodies in the book. Am I looking in the wrong place?
From: Buffalo NY
David Burch

 - posted April 22, 2019 05:20 PM      Profile for David Burch           Edit/Delete Post 
You can ID the bodies with a protractor centered on the middle, with 000 at the top of the page, then each ring is 15º of elevation.

I think there is one of these plots for each set of star sights taken on the voyage. This one is for Problem 9 and you will see there that we actually used Vega, Venus, and Polaris.

These plots are just to show what the sky looked like and what we might have taken compared to what we actually took on the voyage. Sometimes we have to consider where the sails are set so we can't see the best ones. Or we ran out of time, or part of the sky was obscured... or we just did not take the best. I must admit that i have not actually compared what these Appendix 3 predictions call for relative to actual sights taken. That would be a good exercise.

There are no specific exercises in using this data in the book. It is mostly background to make this a more realistic exercise... or at least one that is well specified.

From: Starpath, Seattle, WA

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