Celestial Navigation—Sextant Upgrade

[John Stone]

During my last trip to the Caribbean I spent a couple weeks relearning and improving my celestial navigation skills. You can read about it here:https://farreachvoyages.net/2019/05/21/ ... beginning/.
And here:https://farreachvoyages.net/2019/05/27/ ... tar-shots/

I had some struggles on the voyage home last June getting accurate star sights. I was sometimes two to three nautical miles off according to the GPS. I felt I should have been consistently closer to a one mile error. I was almost solely focused on star shots because one gets a true resection position fix with multiple stars unlike a sun LOP. Anyway, part of the challenge for me was a near continuous very thin layer of high cirrus clouds throughout the voyage. Another challenge is my low-light vision is not as good as it once was. There was nothing I could do about the clouds (and it only takes the thinnest whisp of cirrus to seriously degrade the light from a star or worse, obscure it completely) or my less youthful eyes. But, there was a third factor making the star shots difficult which I could address—the sextant’s horizon mirror.

My Casssens & Plath sextant was made in 1980. But, it did not come with the traditional split horizon mirror with half clear glass and the other half a silvered reflective mirror. Instead, it came with a then new kind of horizon mirror called a whole horizon mirror. A whole horizon mirror is all clear glass with an opaque reflective coating that allows the user to see the horizon through the entire mirror superimposing the celestial body onto the clear glass. Basically, it gives you twice the field of view. When observing the sun, moon, or bright planets it works great. But, many of the navigational stars are not terribly bright to begin with and even less so when observing them using a sextant with a whole horizon mirror which by the nature of the design causes the observed body to appear less “bright.” The traditional mirror captures more of the star’s brightness making it more distinct which means it’s easier to see, hold in the index mirror, bring to the horizon, and accurately capture in a fading dusk or brightening dawn horizon. The whole horizon mirror was apparently a great advantage to navigators that relied almost exclusively on sun or moon shots but for those navigators keen on star shots...not so much.

I read that a Cassens and Plath sextant can be converted from a whole horizon mirror to a traditional split mirror. So, to find out what was in the art of the possible I decided to call an expert on sextants—Ridge White of “Robert White Instruments” in Marblehead, MA. Ridge was very helpful, knew exactly what I needed, and lucky for me, had a traditional split mirror in stock that fit my sextant. He said it was a simple one-for-one replacement I could perform myself. It arrived Monday and I was able to quickly make the switch sitting at the breakfast table.

It took about 20 min to readjust the sextant for perpendicularity and index error. As described in Bauer’s excellent book, I used a pair of dominos to make the former adjustment and a star shot off my back porch for the latter.

I’ll do so more reading and practice shots and calculations this winter and spring. I’d like to get comfortable and confident enough to rely completely on celestial navigation during my next offshore voyage. Along with more practice I think the split mirror will help.

[Rollergirl]

Have you considered a sight tube instead of the monocular you have now? The monocular is great for sun & moon shots, but a pinpoint of light (stars) magnified 3 or 5 or 10 times is still a pinpoint. The disadvantage of a monocular is the loss of light and contrast in an already marginal situation. The optically empty sight tube has no losses while shielding your eye from extraneous light.

Here's a sample: https://www.celestaire.com/product/zero ... ight-tube/

Good luck
Bill

[John Stone]

Hi Bill
Thanks for the suggestion. I have read about using an unmagnified sight tube for stars. But I don’t think it would work well for the sun. So, I’d have to switch back and forth. Does not seem very convenient...but perhaps it could work. I guess if it was really rough that might be just the thing.

Many sextants apparently have as their standard scope a 6x telescope...just for the sun and a smaller power for stars. My telescope is 4X. I have read that 3.5-4X is ideal for sun and stars on a small boat—one scope for both types of observations. Big ships are stable enough for the 6X to be useful. Must take pretty steady hands to make a 6X telescope work well on a small boat.

I’m going to try the split mirror and see how that works. In fact, probably the most important thing for me to improve accuracy is not the sextant but just practice getting the actual shot when the boat is rolling around—you know, the 5000 thousand hour rule. I think about a guy like John Guzzwell sailing his 20’ Triekka around the world. Or Daniel Hays sailing Sparrow, a 25’ Vertue around Cape Horn. So, I figure I ought to be able to pull this off in moderate conditions with more practice. Time will tell.

Thanks again.

[tartansailor]

What method do you use to reduce your sites?

[John Stone]

What method do you use to reduce your sites?

HO 249. “Sight Reduction for Air Navigation.”

[tartansailor]

Hi John,
249! That's one we don't have.
Taught CN in the Northern New Jersey Power Squadron back in the early '90s.
The nautical Almanac Site Reduction Method was rigor de jours, then to say the least.
I believe the Brits still use that method. It's a fascinating subject. I have volumes 229,
214, 208, and 260 which is great because it gives the azimuths of the Sun!
My little CD 25 is a wet boat especially in the lower Delaware Bay, so I use the
Calculator Method below:
sin Hc = (cos LHA x cos Lat x cos Dec) + (sin Lat x sin Dec)
cos Zc = (sin Dec - (sin Lat x sin Hc)) + ( cos Lat x cos Hc)
This all came crashing down on my head 3 years ago when I lost the sight
in my right eye due to a Bobcat mishap.
Oh, did you know that Alcor and Mizar are binaries? The ancient Arabs used them as
a test for eyesight.
If you're ever in the Delaware Bay, give a shout and we can take some sites together.
Happy New Year.
Dick

[John Stone]

Hi John,
249! That's one we don't have.
Taught CN in the Northern New Jersey Power Squadron back in the early '90s.
The nautical Almanac Site Reduction Method was rigor de jours, then to say the least.
I believe the Brits still use that method. It's a fascinating subject. I have volumes 229,
214, 208, and 260 which is great because it gives the azimuths of the Sun!
My little CD 25 is a wet boat especially in the lower Delaware Bay, so I use the
Calculator Method below:
sin Hc = (cos LHA x cos Lat x cos Dec) + (sin Lat x sin Dec)
cos Zc = (sin Dec - (sin Lat x sin Hc)) + ( cos Lat x cos Hc)
This all came crashing down on my head 3 years ago when I lost the sight
in my right eye due to a Bobcat mishap.
Oh, did you know that Alcor and Mizar are binaries? The ancient Arabs used them as
a test for eyesight.
If you're ever in the Delaware Bay, give a shout and we can take some sites together.
Happy New Year.
Dick

Hi Dick,
Well you certainly have all the references and the different methods. I have read about how to make the reduction with a scientific calculator. Seems simple enough once you learn the keystrokes. Do you prefer that method?

I am right brained so a bit less math capable than those gifted with numbers...but that is not to say I am not fascinated by math just not naturally great at it.

I have enjoyed 249. Pretty straight forward and relatively fast. Less flipping back and forth in the tables than 229. I think almost all professional nav schools now teach 229. I understand it to be a bit more accurate but I think 249 is more accurate than you can be on a small boat in the open ocean.

I also really like the way the stars are set up in Volume I for HO 249. In the end, I think any method works pretty well if you know it and then practice it.

Take some sights together would be fun.